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Solid-state thermomagnetic modules operating based on the Nernst–Ettingshausen effects are an alternative to conventional solid-state thermoelectric modules. These modules are appropriate for low-temperature applications where the thermoelectric modules are not efficient. Here, we briefly discuss the application, performance, similarities, and differences of thermoelectric and thermomagnetic materials and modules. We review thermomagnetic module design, Nernst coefficient measurement techniques, and theoretical advances, emphasizing the Nernst effect and factors influencing its response in semimetals such as carrier compensation, Fermi surface, mobility, phonon drag, and Berry curvature. The main objective is to summarize the materials design criteria to achieve high thermomagnetic performance to accelerate thermomagnetic materials discovery. [ABSTRACT FROM AUTHOR]

Transition metal oxides (TMOs) demonstrate a broad spectrum of properties encompassing electronic correlations, anisotropic transport, magnetism, and optical behavior. The anisotropy arises from both intrinsic crystal symmetry and extrinsic factors like epitaxial strain and structural asymmetry at TMO interfaces. Weiss and Neel's work has elucidated anisotropic magnetic behavior in antiferromagnetic (AFM) materials. AFM TMOs exhibit unique magnetotransport behavior, including weak antilocalization (WAL) and anisotropic magnetoresistance (AMR). Understanding the magnetic structure and band topology in AFM perovskites and their interfaces enables the tailored design of materials for spintronics and energy conversion. In few interfaces lacking inversion symmetry, Rashba spin–orbit coupling (SOC) induces WAL, a quantum correction in conductivity in a two-dimensional electronic system. Electron accumulation and charge transfer across 3d, 5d transition metal-based perovskite interfaces affect WAL and AMR, as observed in 3d/3d and 3d/5d AFM heterostructures, respectively. Advancements in spintronics rely on exploring spin-dependent transport anisotropy. This review focuses on various scattering mechanisms, categorized as extrinsic and intrinsic, in anisotropic transport, particularly in 3d/5d AFM superlattices. The WAL scattering mechanism depends on both intrinsic factors related to Rashba SOC-induced band topology and extrinsic sources like spin impurities and lattice ions. Moreover, the investigation into AMR mechanisms involves the application of impurity-based extrinsic scattering models, which are aligned with the Rashba and Dresselhauss models on Fermi surfaces. This exploration specifically targets the interface of two-band insulators, exemplified by LaAlO3/SrTiO3 and LaVO3/KTaO3. Furthermore, this model achieves comprehensive coverage, extending its applicability to 3d/5d AFM heterostructures like LaMnO3/SrIrO3 and CaMnO3/CaIrO3. Additionally, the intrinsic scattering mechanism tied to Berry phase effects related to band topology is studied, focusing on the CaMnO3/CaIrO3 superlattice. Despite manipulation challenges stemming from reduced stray fields, AFM materials show potential in interface physics and applications within the realm of spintronics. [ABSTRACT FROM AUTHOR]

Investigating high-performance and stable spintronics devices has been a research hotspot in recent years. In this paper, we employed first-principles methods and Monte Carlo (MC) simulations to explore the structure, electronic, and magnetic properties of monolayer NbSe2, as well as its behavior under carrier concentration modulation. The research on the electronic structure reveals that by introducing an appropriate amount of holes, the material can undergo a transition from metal to a half-metal state, achieving 100% high spin polarization. Investigation of magnetic crystalline anisotropy shows that the magnetic crystal anisotropy energy of 1210 μeV in out-of-plane is beneficial to maintain ferromagnetic order at high temperatures. In addition, doping with suitable carriers can effectively enhance or strengthen the ferromagnetic coupling in NbSe2 so that the magnetization easy axis is shifted. This reveals the potential application prospects of NbSe2 in electronically controlled spintronic devices. Analysis of the Fermi surface shows that both holes and electron doping increase the Fermi velocity of the material. The effect of hole doping is particularly significant, indicating its potential application in Fermi velocity engineering. Under the theoretical framework of the extended two-dimensional Ising model, based on MC simulation, the Curie temperature (TC) of NbSe2 is predicted to be 162 K. The effects of carrier concentration and the magnetic field on the magnetic and thermal properties of monolayer NbSe2 are simulated. The results show that appropriately increasing the hole doping concentration and magnetic field is conducive to obtaining ferromagnetic half-metallic materials with TC higher than room temperature, which provides theoretical support for experimental preparation. [ABSTRACT FROM AUTHOR]

Hybrid-order topological insulators combine first- and higher-order topological properties and host topological boundary states with codimension one and more than one in different bandgaps. A Weyl semimetal (WSM) can possess two types of Weyl points: one class of Weyl points terminates the Fermi arc surface states, while another class of Weyl points not only launch Fermi arc surface states but also hinge arc states, exhibiting the hybrid-order topology. Here, we propose a hybrid-order WSM by stacking two-dimensional rhomboid lattices based on chiral nearest-neighbor and double-helix next-nearest interlayer couplings. The first type of Weyl point that only truncates the Fermi arc surface states exists at the crossing of any two-fold degeneracy of two adjacent bands, and the second type of Weyl point that connects the hinge arc states only appears at the crossing of the two middle bands. Our findings enrich the classification of topological semimetals in condensed matter physics. [ABSTRACT FROM AUTHOR]

The family of BiS2-based superconductors has attracted considerable attention since their discovery in 2012 due to the unique structural and electronic properties of these materials. Several experimental and theoretical studies have been performed to explore the basic properties and the underlying mechanism for superconductivity. In this review, we discuss the current understanding of pairing symmetry in BiS2-based superconductors and particularly the role of point-contact spectroscopy in unravelling the mechanism underlying the superconducting state. We also review experimental results obtained with different techniques including angle-resolved photoemission spectroscopy, scanning tunnelling spectroscopy, specific heat measurements, and nuclear magnetic resonance spectroscopy. The integration of experimental results and theoretical predictions sheds light on the complex interplay between electronic correlations, spin fluctuations, and Fermi surface topology in determining the coupling mechanism. Finally, we highlight recent advances and future directions in the field of BiS2-based superconductors, underlining the potential technological applications. [ABSTRACT FROM AUTHOR]

The two-dimensional electron gas (2DEG) at the LaAlO /SrTiO (LAO/STO) interface exhibits gate-tunable superconductivity with a dome-like shape of critical temperature as a function of electron concentration. This behavior has not been unambiguously explained yet. Here, we develop a microscopic model based on the Schrödinger–Poisson approach to determine the electronic structure of the LAO/STO 2DEG, which we then apply to study the principal characteristics of the superconducting phase within the real-space pairing mean-field approach. For the electron concentrations reported in the experiment, we successfully reproduce the dome-like shape of the superconducting gap. According to our analysis such behavior results from the interplay between the Fermi surface topology and the gap symmetry, with the dominant extended s-wave contribution. Similarly as in the experimental report, we observe a bifurcation effect in the superconducting gap dependence on the electron density when the 2DEG is electrostatically doped either with the top gate or the bottom gate. Our findings explains the dome-shaped phase diagram of the considered heterostucture with good agreement with the experimental data which, in turn, strongly suggest the appearance of the extended s-wave symmetry of the gap in 2DEG at the LAO/STO interface. [ABSTRACT FROM AUTHOR]

The low energy manifold for fermions at finite density is the Fermi surface. I describe renormalization group (RG) in which modes encountered on approaching the Fermi surface are systematically integrated out. The fixed point is described by strictly marginal coupling functions identified as the Landau parameters and marginally relevant coupling functions describing the BCS instability in various angular momentum channels. [ABSTRACT FROM AUTHOR]

An experimental study found superconductivity in bilayer phase of La3Ni2O7, with the highest superconducting transition temperature (Tc) ∼ 80 K under pressure. Recently, some reports claimed that there exists a competitive monolayer-trilayer structural phase in La3Ni2O7 compounds. We perform the first-principles calculations and find that bilayer phase of La3Ni2O7 is energetically favorable under pressure. Although extensive studies have been done to investigate the electronic correlation and potential superconducting pairing mechanism in bilayer phase of La3Ni2O7, the phonon properties and electron-phonon coupling (EPC) in the high-pressure I4/mmm phase of La3Ni2O7 are not reported. Using the density functional theory (DFT) combined with Wannier interpolation technique, we study the phonon properties and EPC in bilayer phase of La3Ni2O7 under 29.5 GPa. Our findings reveal that EPC is insufficient to explain the observed superconducting Tc ∼ 80 K. And the calculated Fermi surface nesting may explain the experimentally observed charge density wave (CDW) transition in bilayer phase of La3Ni2O7. Our calculations substantiate that bilayer phase of La3Ni2O7 is an unconventional superconductor. [ABSTRACT FROM AUTHOR]

While recent experiments confirm the existence of hundreds of topological electronic materials, only a few exhibit the coexistence of superconductivity (SC) and a topological electronic state. These compounds attract significant attention in forefront research because of the potential for the existence of topological SC, paving the way for future technological advancements. SrSn4 is known for exhibiting unusual SC below the transition temperature (TC) of 4.8 K. Recent theory predicts a topological electronic state in this compound, which is yet to be confirmed by experiments. Systematic and detailed studies of the magnetotransport properties of SrSn4 and its Fermi surface characterizations are also absent. For the first time, a quantum oscillation study reveals a nontrivial 흅‐Berry phase, very light effective mass, and high quantum mobility of charge carriers in SrSn4. Magnetotransport experiment unveils large linear transverse magnetoresistance (TMR) of more than 1200% at 5 K and 14 T. Angle‐dependent transport experiments detect anisotropic and fourfold symmetric TMR, with the maximum value (≈2000%) occurring when the angle between the magnetic field and the crystallographic b‐axis is 45°. The results suggest that SrSn4 is the first topological material with SC above the boiling point of helium that displays such high magnetoresistance. [ABSTRACT FROM AUTHOR]

Topological semimetals represent a novel class of quantum materials displaying non‐trivial topological states that host Dirac/Weyl fermions. The intersection of Dirac/Weyl points gives rise to essential properties in a wide range of innovative transport phenomena, including extreme magnetoresistance, high mobilities, weak antilocalization, electron hydrodynamics, and various electro‐optical phenomena. In this study, the electronic, transport, phonon scattering, and interrelationships are explored in single crystals of the topological semimetal HfAs2. It reveals a weak antilocalization effect at low temperatures with high carrier density, which is attributed to perfectly compensated topological bulk and surface states. The angle‐resolved photoemission spectroscopy (ARPES) results show anisotropic Fermi surfaces and surface states indicative of the topological semimetal, further confirmed by first‐principle density functional theory (DFT) calculations. Moreover, the lattice dynamics in HfAs2 are investigated both with the Raman scattering and density functional theory. The phonon dispersion, density of states, lattice thermal conductivity, and the phonon lifetimes are computed to support the experimental findings. The softening of phonons, the broadening of Raman modes, and the reduction of phonon lifetimes with temperature suggest the enhancement of phonon anharmonicity in this new topological material, which is crucial for boosting the thermoelectric performance of topological semimetals. [ABSTRACT FROM AUTHOR]

A density functional theory based calculations are performed to investigate structural, hydrogen storage, mechanical, electronic and thermodynamical properties of lead-free hydride perovskites XGeH 3 (X=Cs, Fr). The optimized lattice constant and hence volume against the minimum energy is obtained and found to be 4.176 Å and 4.184 Å for CsGeH 3 and FrGeH 3 respectively. The gravimetric and volumetric hydrogen storage capacity of 1.43% and 1.00%, 72.81 (g.H 2 / L) and 73.21 (g.H 2 / L) for CsGeH 3 and FrGeH 3 respectively. The density of states and electronic band structure predicted the metallic nature of both hydrides which was affirmed by the Fermi Surfaces. The calculation of Poisson's, Pugh's ratio and Cauchy's pressure suggested their brittle nature. The Debye temperature decreases with the increase of temperature while volume and entropy increase by increasing applied temperature. The present study shows the potential of lead-free hydride perovskites in hydrogen storage applications due to rather suitable gravimetric hydrogen capacity and high stability. • Hydrogen storage capacity of XGeH 3 (X = Cs, Fr) are investigated. • Gravimetric and volumetric hydrogen storage capacity of hydrides are calculated. • XGeH 3 (X = Cs, Fr) hydrides exhibit mechanical and thermodynamical stability. • Both compounds show metallic nature with 0 eV band gap. • Mechanical properties predict brittle nature of XGeH 3 (X = Cs, Fr). [ABSTRACT FROM AUTHOR]

We study the transport properties of underdoped trilayer cuprate HgBa2Ca2Cu3O8+δ with doping level p = 0.10–0.12 in magnetic field up to 88 T. We report for the first time in a cuprate superconductor a dramatic change of the quantum oscillation spectrum versus temperature, which is accompanied by a sign change of the Hall effect below T ≈10 K. Based on numerical simulations, we infer a Fermi surface reconstruction in the inner plane from an antiferromagnetic state (hole pockets) to a biaxial charge density wave state (electron pockets). We show that both orders compete and share the same hotspots of the Fermi surface, and we discuss our result in the context of spin-fermion models. [ABSTRACT FROM AUTHOR]

Starting from the Bogolubov–de Gennes equations for superconductor with an arbitrary anisotropic Fermi surface we derive the Andreev-type theory accounting for the quasiparticle trajectory interference and corresponding nonquasiclassical quantization effects. The resulting Andreev equations are applied for the analysis of the subgap quasiparticle states localized at the pinned vortex cores. The normal reflection of electrons and holes at the defects is shown to result in the qualitative transformation of the subgap spectra and formation of new types of bound quasiparticle states. We focus on the effect of the Fermi surface anisotropy on the quasiparticle spectral properties including opening of soft and hard minigaps in the spectrum and spatial behavior of the local density of states around pinned vortex cores. [ABSTRACT FROM AUTHOR]

We present a comprehensive investigation of quantum oscillations (QOs) in the strongly-correlated Falicov-Kimball model (FKM). The FKM is a particularly suitable platform for probing the non-Fermi liquid (NFL) state devoid of quasiparticles, affording exact Monte Carlo simulation across all parameter spaces. In the high-correlation regime, we report the presence of prominent QOs in magnetoresistance and electron density at low temperatures within the phase separation state. The frequency behavior of these oscillations uncovers a transition in the Fermi surface as electron density diminishes, switching from hole-like to electron-like. Both types of Fermi surfaces are found to conform to the Onsager relation, establishing a connection between QOs frequency and Fermi surface area. Upon exploring the temperature dependence of QOs amplitude, we discern a strong alignment with the Lifshitz-Kosevich (LK) theory, provided the effective mass is suitably renormalized. Notwithstanding, the substantial enhancement of the overall effective mass results in a notable suppression of the QOs amplitude within the examined temperature scope, a finding inconsistent with Fermi liquid predictions. For the most part, the effective mass diminishes as the temperature increases, but an unusual increase is observed at the proximity of the second-order phase transition instigated by thermal effects. As the transition ensues, the regular QOs disappear, replaced by irregular ones in the NFL state under a high magnetic field. We also uncover significant QOs in the insulating charge density wave state under weak interactions ( 0 < U < 1 ), a phenomenon we elucidate through analytical calculations. Our findings shed light on the critical role of quasiparticles in the manifestation of QOs, enabling further understanding of their function in this context. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) half-metallic magnets are expected to achieve breakthroughs in the field of spintronics and quantum computation due to their 100% spin polarization and non-dissipative ballistic transport characteristics. However, the current availability of practical applications for these materials is limited. In this paper, a new 2D material NiVS6As2 is predicted based on density functional theory (DFT). It has a ferromagnetic ground state with a high Curie temperature, TC = 150 K and half-semi-metal (HSM) band structure protected by a structural symmetry degeneracy point on the Fermi surface, which can be controlled by an electric field and strain. When an out-of-plane electric field is applied, NiVS6As2 shows a Chern insulator with a quantum anomalous Hall (QAH) effect. These results expand the family of 2D magnetic semimetals, and provide new theoretical inspiration for understanding the protection of degenerate point symmetry. [ABSTRACT FROM AUTHOR]

Rich momentum-dependent electronic structure naturally arises in solids with long-range crystalline symmetry. Reliable and scalable quantum technologies rely on materials that are either not perfect crystals or non-crystalline, breaking translational symmetry. This poses the fundamental questions of whether coherent momentum-dependent electronic states can arise without long-range order, and how they can be characterized. Here we investigate Bi2Se3, which exists in crystalline, nanocrystalline, and amorphous forms, allowing direct comparisons between varying degrees of spatial ordering. Through angle-resolved photoemission spectroscopy, we show for the first time momentum-dependent band structure with Fermi surface repetitions in an amorphous solid. The experimental data is complemented by a model that accurately reproduces the vertical, dispersive features as well as the replication at higher momenta in the amorphous form. These results reveal that well-defined real-space length scales are sufficient to produce dispersive band structures, and that photoemission can expose the imprint of these length scales on the electronic structure. Recent studies showing dispersive surface states in an amorphous topological insulator has deepened the understanding of momentum-space electronic structure. Here the authors report dispersive electronic states and a distinct Fermi surface with Brillouin-zone-like repetitions in amorphous Bi2Se3. [ABSTRACT FROM AUTHOR]

Dispersion relations in a metal with a commensurate helical magnetic order are considered in the framework of one- and two-dimensional tight-binding models. The generalized Bloch theorem for translations combined with spin rotations, together with the Born–Karman periodic boundary conditions, leads to the appearance of multisheet dispersion curves (surfaces). It is demonstrated that the resulting band structure is topologically nontrivial, which can lead to a spin textured Fermi surface and cause transport anomalies. [ABSTRACT FROM AUTHOR]

Periodic lattice distortion, known as the charge density wave, is generally attributed to electron–phonon coupling. This correlation is expected to induce a pseudogap at the Fermi level in order to gain the required energy for stable lattice distortion. The transition metal dichalcogenide 1T-VSe2 also undergoes such a transition at 110 K. Here, we present detailed angle-resolved photoemission spectroscopy experiments to investigate the electronic structure in 1T-VSe2 across the structural transition. Previously reported warping of the electronic structure and the energy shift of a secondary peak near the Fermi level as the origin of the charge density wave phase are shown to be temperature independent and hence cannot be attributed to the structural transition. Our work reveals new states that were not resolved in previous studies. Earlier results can be explained by the different dispersion natures of these states and temperature-induced broadening. Only the overall size of the Fermi surface is found to change across the structural transition. These observations, quite different from the charge density wave scenario commonly considered for 1T-VSe2 and other transition metal dichalcogenides, bring fresh perspectives toward correctly describing structural transitions. Therefore, these new results can be applied to material families in which the origin of the structural transition has not been resolved. [ABSTRACT FROM AUTHOR]

The mixed-valence compound YbB12 displays paradoxical quantum oscillations in electrical resistivity and magnetic torque in a regime with a well-developed insulating charge gap and in the absence of an electronic Fermi surface. However, signatures of such unusual fermionic quasiparticles in other bulk thermodynamic observables have been missing. Here we report the observation of a series of sharp double-peak features in the specific heat as a function of the magnetic field. The measured Hall resistivity evolves smoothly across the field values at which the characteristic anomalies appear in the thermodynamic response and rules out the possibility of conventional electrons as their origin. Our observations of thermodynamic anomalies in a bulk three-dimensional electrical insulator provide the evidence for the presence of emergent dispersing fermionic excitations within the insulating bulk, which sets the stage for further investigation of electron fractionalization in other correlated mixed-valence compounds. Reports of quantum oscillations in the absence of an electronic Fermi surface in Kondo insulator candidates such as SmB6 and YbB12 have been under intense study. Here the authors report the thermodynamic properties of YbB12 in a magnetic field pointing to the presence of in-gap fermionic excitations. [ABSTRACT FROM AUTHOR]

Efficient detection of the magnetic state is a critical step towards useful antiferromagnet-based spintronic devices. Recently, finite tunneling magnetoresistance (TMR) has been demonstrated in tunnel junctions with antiferromagnetic electrodes, however, these studies have been mostly limited to junctions with two identical antiferromagnet (AFM) electrodes, where the matching of the spin-split Fermi surfaces played critical role. It remains unclear if AFMs can provide a finite net spin polarization, and hence be used as a spin polarizer or detector. In this work, we experimentally fabricate single-sided antiferromagnetic tunnel junctions consisting of one AFM electrode (Mn3Sn) and one ferromagnet (FM) electrode (CoFeB), where the spin polarized tunneling transport from AFM is detected by the FM layer. We observe a high TMR at cryogenic temperature (>100% at 10 K) in these asymmetric AFM tunnel junctions, suggesting a large effective spin polarization from Mn3Sn despite its nearly vanishing magnetization. The large TMR is consistent with recent theoretical studies where the broken symmetry in non-collinear AFMs is predicted to lift the spin degeneracy in the band structure. Our work provides strong evidence that spin polarized electrical transport can be achieved from AFMs. Antiferromagnets have both negligible stray fields, and fast dynamics, making them ideal for fast and densely packed spintronic devices. However, readout of the antiferromagnet state is challenging. Here, Chou et al detect the spin-polarized current emanating from a noncollinear AFM, Mn3Sn. [ABSTRACT FROM AUTHOR]

SrRuO3 is a prototypical transition metal oxide that hosts rich physical properties, including itinerant ferromagnetism, high conductivity, and intrinsic Hall effect originating in the Weyl points. Recently, high-quality SrRuO3 films with residual resistivity ratios of more than 50 have been reported to exhibit quantum oscillations at low temperatures in spite of their strong electron correlation. While the origin of the oscillations has been discussed in relation to Weyl orbits based on the Weyl semimetal band structure, so far experimentally reported results are consistent neither with each other nor with theoretically expected behavior, leaving the origin of the oscillations in SrRuO3 films still elusive. In this report, we have carefully evaluated the quantum oscillations observed in three-dimensional thick SrRuO3 films with a high residual resistivity ratio of RRR = 82. We reveal the coexistence of two oscillation components both derived from two-dimensional electronic states and with slightly different masses, suggesting the involvement of surface Fermi arc states formed between different Weyl point pairs. [ABSTRACT FROM AUTHOR]

We present a theory for charge-4e superconductivity as a leading low-temperature instability with a nontrivial d-wave symmetry. We show that in several microscopic models for the pair-density-wave (PDW) state, when the PDW wave vectors connect special parts of the Fermi surface, the predominant interaction is in the bosonic pairing channel mediated by exchanging low-energy fermions. This bosonic pairing interaction is repulsive in the s-wave channel but attractive in the d-wave one, leading to a d-wave charge-4e superconductor. By analyzing the Ginzburg-Landau free energy including higher-order fluctuation effects of PDW, we find that the charge-4e superconductivity emerges as a vestigial order of PDW, and sets in via a first-order transition. Both the gap amplitude and the transition temperature decay monotonically with increasing superfluid stiffness of the PDW order. Our work provides a microscopic mechanism of higher-charge condensates with unconventional ordering symmetry in strongly-correlated materials. [ABSTRACT FROM AUTHOR]

Specific properties, such as surface Fermi arcs, features of quantum oscillations and of various responses to a magnetic field, distinguish Dirac semimetals from ordinary materials. These properties are determined by Dirac points at which a contact of two electron-energy bands occurs and in the vicinity of which these bands disperse linearly in the quasimomentum. This work shows that almost the same properties are inherent in a wider class of materials in which the Dirac spectrum can have a noticeable gap comparable with the Fermi energy. In other words, the degeneracy of the bands at the point and their linear dispersion are not necessary for the existence of these properties. The only sufficient condition is the following: In the vicinity of such a quasi-Dirac point, the two close bands are well described by a two-band model that takes into account the strong spin-orbit interaction. To illustrate the results, the spectrum of ZrTe5 is considered. This spectrum contains a special quasi-Dirac point, similar to that in bismuth. Dirac semimetals are 3D materials where the conduction and valence bands meet at what are called Dirac points. The author shows that almost all the properties inherent in the Dirac semimetals are exhibited by a wider class of materials that need not have the gapless Dirac points. [ABSTRACT FROM AUTHOR]

In the present work, we numerically and experimentally study the Co 3 - x Ni x O 4 (spinel-like oxides) system. Using the perturbative density functional theory (p-DFT) method, we start the study from the homogeneous sample ( x = 0 ), obtaining the main electronic properties (band structure (BS), density of states (DOS), and Fermi surface (FS)). Subsequently, we doped (x) with Ni atoms in different proportions (0–7% respectively, taking 56 atoms as 100% and the percentage of doping, on this percentage). As we increase the doping (x ≠ 0) , we have found that the forbidden gap decreases and the Fermi energy (FE) decreases, causing the material to exhibit a transition phase for a particular doping value. In addition, we find that more bands are generated when the system is doped, which would be responsible for the phase transition. The data from the theoretical analysis carried out in this paper was compared with the experimental data of various widely accepted works. Some of the results, when compared with the information available from the experimental ones, show good agreement. [ABSTRACT FROM AUTHOR]

Solid state detectors composed of GeSn (germanium-tin) alloys offer improved capabilities compared with mercury cadmium telluride detectors. GeSn detectors may be smaller in size and weight, capable of operating with a noncryogenic detector, and provide increased sensitivity. Recent advances in nonequilibrium remote plasma-enhanced chemical vapor deposition growth enable GeSn crystalline growth with up to 10% Sn concentration, free of surface migration. Absorption spectroscopy combined with Tauc analysis results in 0.79, 0.73, 0.69, 0.59, 0.57, and 0.51 eV direct bandgap energies for GeSn samples with 0%, 2.7%, 4.6%, 6.6%, 7.1%, and 8.0% Sn, respectively. These absorption bandgap energies closely agree with density functional theory energies within ±0.05 eV. However, the rate of change of indirect bandgap narrowing as a function of Sn content is more diverse than a numerical result. The current research evidences that the indirect-to-direct transition crossover point occurs at a Sn content greater than 8%. From the analysis of the Urbach tail, the optical bandgap exhibits a potential structure disorder in the Urbach region. For example, this disorder may cause bandgap narrowing by more than 50% of the intrinsic bandgap energy in the highest Sn content (e.g., 8% Sn) sample. The surface Fermi level approximation validates p-type Fermi level pinning very close to the valence band maximum, often seen in highly doped semiconductors. [ABSTRACT FROM AUTHOR]

Two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are making impressive strides in a short duration compared to other candidates. However, to unlock their full potential for advanced logic transistors, attention must be given to improving the contacts or interfaces they form. One approach is to interface with a suitable low work function metal contact to allow the surface Fermi level (EF) movement toward intended directions, thereby augmenting the overall electrical performance. In this work, we implement physical characterization to understand the tin (Sn) contact interface on monolayer and bulk molybdenum disulfide (MoS2) via in situ x-ray photoelectron spectroscopy and ex situ atomic force microscopy. A Sn contact exhibited a van der Waals type weak interaction with the MoS2 bulk surface where no reaction between Sn and MoS2 is detected. In contrast, reaction products with Sn—S bonding are detected with a monolayer surface consistent with a covalentlike interface. Band alignment at the interface indicates that Sn deposition induces n-type properties in the bulk substrate, while EF of the monolayer remains pinned. In addition, the thermal stability of Sn on the same substrates is investigated in a sequential ultrahigh vacuum annealing treatment at 100, 200, 300, and 400 °C. Sn sublimated/desorbed from both substrates with increasing temperature, which is more prominent on the bulk substrate after annealing at 400 °C. Additionally, Sn significantly reduced the monolayer substrate and produced detectable interface reaction products at higher annealing temperatures. The findings can be strategized to resolve challenges with contact resistance that the device community is having with TMDs. [ABSTRACT FROM AUTHOR]

Materials with 100% spin polarisation, high-speed conduction electrons, high Curie temperatures, and ferromagnetic ground state are known for spintronics. Double perovskites (DPs) with these features are becoming popular nowadays. Herein, introducing another halide-based DP, we determined that Cs2KMnCl6 is a robust intrinsic 3d ferromagnetic half-metal with a 4 μB magnetic moment per unit cell. It exhibits the metallic band for the spin-up configuration, enabling their smooth transport, together with a massive gap of ∼4.51 eV for the spin-down states, thus protecting it from spin–flip transitions. The calculated structural and elastic stability criteria substantiate the stability of the cubic Cs2KMnCl6 double perovskite with a F m 3 ¯ m symmetry. The 3d states of Mn mainly contribute to the robust ferromagnetism above room temperature and show Tc ∼ 534 K. The different Fermi surface topologies within the Brillouin zone establish its conducting nature for up-spin channels and favor the spin-polarized transport. Moreover, the spin-polarized dielectric constant and absorption coefficient also evidenced the half-metallic characteristics. Therefore, our findings offer a novel DP system for investigating spintronic application. [ABSTRACT FROM AUTHOR]

Tuning interlayer interactions offer an alternative approach to access novel electronic structure and intriguing physical properties in layered materials. Here, the emergence of a new form of superconductivity in two‐dimensional (2D) binary phosphides by strengthening the interlayer coupling with lattice compression is reported. Electrical transport measurements show strong evidence of superconductivity in InP3 with the highest critical temperature (Tc) of 9.5 K at 45.1 GPa. Raman and X‐ray diffraction (XRD) measurements indicate that the interlayer interactions are dramatically modulated under compression, along with the deformation of local In–P bipyramid structure and reduction of the interlayer distances, which eventually results in the formation of In–P bonds between neighboring In–P bipyramids and a R3¯$\bar{3}$m to Cmcm structural transition. First‐principles density functional theory (DFT) calculations reveal that pressure enhances the interlayer interactions, which increases the density of states (DOS) near the Fermi surface (N(EF)) and strengthens the electron–phonon coupling. Consequently, this favors the occurrence of superconductivity in compressed InP3. This study not only introduces a new superconductivity phase with enhanced electron–phonon coupling in binary phosphides, but also provides a platform for exploring the pressure effect on interlayer interactions in material systems with corrugated layered structure. [ABSTRACT FROM AUTHOR]

Quantum materials are often characterized by a marked sensitivity to minute changes in their physical environment, a property that can lead to new functionalities and thereby, to novel applications. One such key property is the magneto-elastoresistance (MER), the change in magnetoresistance (MR) of a metal induced by uniaxial strain. Understanding and modeling this response can prove challenging, particularly in systems with complex Fermi surfaces. Here, we present a thorough analysis of the MER in the nearly compensated Dirac nodal-line semi-metal ZrSiSe. Small amounts of strain (0.27%) lead to large changes (7%) in the MR. Subsequent analysis reveals that the MER response is driven primarily by a change in transport mobility that varies linearly with the applied strain. This study showcases how the effect of strain tuning on the electrical properties can be both qualitatively and quantitatively understood. A complementary Shubnikov-de Haas oscillation study sheds light on the root of this change in quantum mobility. Moreover, we unambiguously show that the Fermi surface consists of distinct electron and hole pockets revealed in quantum oscillation measurements originating from magnetic breakdown. [ABSTRACT FROM AUTHOR]

Water oxidation of photosynthesis at the oxygen evolving complex (OEC) is driven by the polarization field induced by the photoelectric hole. By highlighting the role of the polarization field in reshaping the spin and orbit potentials, we reveal in this work the characteristics and underlying mechanism in the relatively simpler OEC evolutions within the states S 0 – S 2 prior to the water oxidation. The characteristic shifts of the density of states (DOS) of the electron donor Mn atom are observed in the vicinity of the Fermi surface to occur with the spin flips of Mn atoms and the change in the Mn oxidation states during the electron transfer. Notably, the spin flips of Mn atoms point to the resulting spin configuration of the next states. It is found that the electron transfer tends to stabilize the catalyst OEC itself, whereas the proton transfer pushes the evolution forward by preparing a new electron donor, demonstrating the proton-coupled electron transfer. Meanwhile, it shows that the Mn–O bonds around the candidate Mn atom of the electron donor undergo characteristic changes in the bond lengths during the electron transfer. These concomitant phenomena uncovered in first-principle calculations characterize the essential equilibrium of the OEC between the state evolution and stability that forms the groundwork of the dynamic OEC cycles. In particular, the characteristic undulation of the DOS around the Fermi level occurring at the proton-coupled electron transfer can be used to reveal crucial processes in a wide range of realistic systems. [ABSTRACT FROM AUTHOR]

Light-rare-earth-based RAlSi family serves as an exceptional platform for elucidating the unique properties of Weyl systems, featuring both types of Weyl fermions, with and without magnetism. In this investigation, we conducted a comparative analysis of the resistivity and magnetoresistance (MR) behaviors of single-crystal RAlSi (R = La, Ce, and Pr). A hump appears in the temperature-dependent resistivity curve of CeAlSi, attributed to the crystal-electric-field effect and showing insensitivity to the magnetic field. In nonmagnetic LaAlSi, a quasilinear MR response above the crossover field is linked to mobility fluctuations originating from slight inhomogeneity. Upon heating CeAlSi from 2 K to its critical temperature for ferromagnetic (FM) ordering (TC), a field-induced contraction in the twin Fermi surfaces was observed, likely contributing to negative MR peaks at TC. The robustness of Weyl nodes in FM RAlSi is demonstrated by the inherent broken space inversion symmetry, allowing them to change positions via violation of time-reversal symmetry. Our findings suggest the potential for magnetic modulation of the electronic structure in FM RAlSi, leading not only to shifts in Weyl nodes but also alterations in the Fermi surface. [ABSTRACT FROM AUTHOR]

In the underdoped n-type cuprate Nd2-xCexCuO4, long-range antiferromagnetic order reconstructs the Fermi surface, resulting in a putative antiferromagnetic metal with small Fermi pockets. Using angle-resolved photoemission spectroscopy, we observe an anomalous energy gap, an order of magnitude smaller than the antiferromagnetic gap, in a wide portion of the underdoped regime and smoothly connecting to the superconducting gap at optimal doping. After considering all the known ordering tendencies in tandem with the phase diagram, we hypothesize that the normal-state gap in the underdoped n-type cuprates originates from Cooper pairing. The high temperature scale of the normal-state gap raises the prospect of engineering higher transition temperatures in the n-type cuprates comparable to those of the p-type cuprates. [ABSTRACT FROM AUTHOR]

UTe2 is a promising candidate for spin-triplet superconductors, in which a paramagnetic normal state becomes superconducting due to spin fluctuations. Here, we theoretically show that electron correlation induces a dramatic change in the normal state fermiology with an emergent correlated Fermi surface (FS) driven by Kondo resonance at low temperatures. This emergent correlated FS can account for various unconventional superconducting properties in a unified way. In particular, the geometry of the correlated FS can naturally host topological superconductivity in the presence of odd-parity pairings, which become the leading instability due to strong ferromagnetic spin fluctuations. Moreover, two pairs of odd-parity channels appear as nearly degenerate solutions which may lead to time-reversal breaking multicomponent superconductivity. The resulting time-reversal-breaking superconducting state is a Weyl superconductor in which Weyl points migrate along the correlated FS as the relative magnitude of nearly degenerate pairing solutions varies. UTe2 is a promising candidate for spin-triplet unconventional topological superconductivity. The authors theoretically show that electron correlation induces a dramatic change in the normal state fermiology of this material with an emergent correlated Fermi surface driven by Kondo resonance at low temperatures, which can account for various unconventional superconducting properties in a unified way. [ABSTRACT FROM AUTHOR]

UTe2 is a promising candidate for spin-triplet superconductors, in which a paramagnetic normal state becomes superconducting due to spin fluctuations. Here, we theoretically show that electron correlation induces a dramatic change in the normal state fermiology with an emergent correlated Fermi surface (FS) driven by Kondo resonance at low temperatures. This emergent correlated FS can account for various unconventional superconducting properties in a unified way. In particular, the geometry of the correlated FS can naturally host topological superconductivity in the presence of odd-parity pairings, which become the leading instability due to strong ferromagnetic spin fluctuations. Moreover, two pairs of odd-parity channels appear as nearly degenerate solutions which may lead to time-reversal breaking multicomponent superconductivity. The resulting time-reversal-breaking superconducting state is a Weyl superconductor in which Weyl points migrate along the correlated FS as the relative magnitude of nearly degenerate pairing solutions varies. UTe2 is a promising candidate for spin-triplet unconventional topological superconductivity. The authors theoretically show that electron correlation induces a dramatic change in the normal state fermiology of this material with an emergent correlated Fermi surface driven by Kondo resonance at low temperatures, which can account for various unconventional superconducting properties in a unified way. [ABSTRACT FROM AUTHOR]

The recently discovered high-Tc superconductor La3Ni2O7 has sparked renewed interest in unconventional superconductivity. Here we study superconductivity in pressurized La3Ni2O7 based on a bilayer two-orbital t−J model, using the renormalized mean-field theory. Our results reveal a robust s±-wave pairing driven by the inter-layer d z 2 magnetic coupling, which exhibits a transition temperature within the same order of magnitude as the experimentally observed Tc ~ 80 K. We establish a comprehensive superconducting phase diagram in the doping plane. Notably, the La3Ni2O7 under pressure is found to be situated roughly in the optimal doping regime of the phase diagram. When the d x 2 − y 2 orbital becomes close to half-filling, d-wave and d + is pairing can emerge from the system. We discuss the interplay between Fermi surface topology and different pairing symmetries. The stability of the s±-wave pairing against Hund's coupling and other magnetic exchange couplings is discussed. [ABSTRACT FROM AUTHOR]

We report theoretical results for the electronic contribution to thermal and electrical transport for chiral superconductors belonging to even or odd-parity E1 and E2 representations of the tetragonal and hexagonal point groups. Chiral superconductors exhibit novel properties that depend on the topology of the order parameter and Fermi surface, and--as we highlight--the structure of the impurity potential. An anomalous thermal Hall effect is predicted and shown to be sensitive to the winding number, ], of the chiral order parameter via Andreev scattering that transfers angular momentum from the chiral condensate to excitations that scatter off the random potential. For heat transport in a chiral superconductor with isotropic impurity scattering, i.e., point-like impurities, a transverse heat current is obtained for v = ±1, but vanishes for |v| > 1. This is not a universal result. For finite-size impurities with radii of order or greater than the Fermi wavelength, R ≥ ħ/p[sub f], the thermal Hall conductivity is finite for chiral order with |v| ≥ 2, and determined by a specific Fermi-surface average of the differential cross-section for electron-impurity scattering. Our results also provide quantitative formulae for analyzing and interpreting thermal transport measurements for superconductors predicted to exhibit broken time-reversal and mirror symmetries. [ABSTRACT FROM AUTHOR]

Superconducting infinite-layer (IL) nickelates offer a new platform for investigating the long-standing problem of high-temperature superconductivity. Many models were proposed to understand the superconducting mechanism of nickelates based on the calculated electronic structure, and the multiple Fermi surfaces and multiple orbitals involved create complications and controversial conclusions. Over the past five years, the lack of direct measurements of the electronic structure has hindered the understanding of nickelate superconductors. Here we fill this gap by directly resolving the electronic structures of the parent compound LaNiO2 and superconducting La0.8Ca0.2NiO2 using angle-resolved photoemission spectroscopy. We find that their Fermi surfaces consist of a quasi-2D hole pocket and a 3D electron pocket at the Brillouin zone corner, whose volumes change upon Ca doping. The Fermi surface topology and band dispersion of the hole pocket closely resemble those observed in hole-doped cuprates. However, the cuprate-like band exhibits significantly higher hole doping in superconducting La0.8Ca0.2NiO2 compared to superconducting cuprates, highlighting the disparities in the electronic states of the superconducting phase. Our observations highlight the novel aspects of the IL nickelates, and pave the way toward the microscopic understanding of the IL nickelate family and its superconductivity. [ABSTRACT FROM AUTHOR]

The sensitive response of antiferromagnetic order to As ions height in pressure environment makes the reduction of As ions height beneficial for rapid suppression of antiferromagnetism. The analysis from lattice parameters and electron density indicates that As ions height has a significant impact on antiferromagnetic suppression. The interrelationship between As ions height and antiferromagnetic suppression makes it easier for uniaxial pressure along the c‐axis environments to suppress antiferromagnetism, cause Lifshitz transition compared and induce Fermi surface nesting than uniaxial pressure along the a‐axis (or b‐axis) environments, which can explain the fluctuation of superconducting critical pressure measurements in experiments with uneven pressure application from a microscopic perspective. [ABSTRACT FROM AUTHOR]

One‐third of monolayer of Sn adatoms on a Ge(111) substrate forms a 2D triangular lattice with one unpaired electron per site. The system presents a metal–insulator transition when decreasing the temperature and it is known to exhibit strong electron–phonon coupling at 120–150 K. Herein, a study of the electronic band structure for α‐Sn/Ge(111) between 150 and 5 K is reported. Both the experimental Fermi surfaces and the energy dispersions along high symmetry directions as a function of the temperature are presented. At 5 K it is observed a weakly or low‐dispersing spectral feature, exhibiting an extended gap in the reciprocal space. This feature is derived from the topmost occupied band, which is metallic at high temperature and which develops a kink associated with the strong electron–phonon coupling. The spectral evolution is partially explained with an increase of the electron–phonon coupling when decreasing the temperature. The increase of the electron–phonon coupling at low temperatures gives light into the new physics of this 2D system. The bandwidth is progressively reduced when reducing the temperature, enhancing the electronic correlation effects, and triggering the Mott transition. [ABSTRACT FROM AUTHOR]

Using first-principles calculations based on density-functional theory, this paper investigates the electronic structure and magnetic properties of the Heusler alloy Mn2LiGe bulk and its (001) surfaces. The Mn2LiGe bulk is demonstrated to be inverse Heusler alloy with space group of F43m and lattice constant of 5. 87 Å. A direct band gap with a width of 1. 1 eV near the Fermi surface of the spin-down channel is detected. The Mn2LiGe bulk possesses stable half-metallic and magnetism in the lattice constant range of 5. 55 - 6. 33 Å. In addition, six different surface structures of Mn2LiGe are constructed in this paper. The first layer atoms on the surface undergo different displacements, increasing the surface roughness. Due to surface effects, the magnetic properties of atoms on the surface are different compared to those in the bulk. Both the MnA MnA and MnB MnB surface structures have the highest magnetic moments, while the LiLi and the GeGe surface structures have the lowest magnetic moments. Electronic structure calculations show that the half-metallic band gap present in the bulk is destroyed in all six surface structures and the spin polarization is weakened in varying degrees. Only the LiLi surface structure maintains up to 99. 9% of the surface spin polarization, making the surface an excellent prospect for applications in spintronic devices. [ABSTRACT FROM AUTHOR]

Materials with large Rashba spin splitting (RSS) and quantum spin Hall effect (QSHE) are extremely important in spintronic device applications. Based on first-principles calculations, we present H–Pb–OH, a two-dimensional (2D) hexagonal lattice material capable of achieving complete coexistence of massive RSS and QSHE. The RSS phenomenon (αR = 3.66 eV Å) in the electronic band of H–Pb–OH occurs when the system's spatial inversion symmetry is broken and there is an intrinsic electric field present. Furthermore, biaxial stress can modulate the RSS of H–Pb–OH (−8 to 8%). By examining the electronic characteristics of H–Pb–OH, we discover that there is a significant bandgap (1.12 eV) around the Fermi surface, and the system exhibits a topological invariant Z2 = 1 due to the inversion of the s-p orbitals of the Pb atoms, indicating that H–Pb–OH is a 2D topological insulator with a large topological bandgap. Our findings provide an outstanding candidate material for investigating and implementing the coexistence of RSS and QSHE. [ABSTRACT FROM AUTHOR]

Utilizing density functional theory, the structural, electronic, vibrational, and thermophysical properties of L10 FePt, L12 Fe3Pt, and L12 FePt3 alloys are meticulously analyzed. Employing projected augmented wave pseudopotentials alongside the Perdew–Burke–Ernzerhof exchange‐correlation function, equilibrium lattice constants are computed, aligning closely with existing data, thus validating our approach. To ascertain the dynamical stability of these alloys, phonon frequencies and density of states across high symmetry directions of the Brillouin zone are computed, affirming their stability with positive phonon frequencies throughout. Furthermore, the electronic band structure, the total and projected density of states, electronic charge density, and Fermi surfaces of the alloys are delved. The thorough analysis of phonon dispersion curves, electronic band structures, and the density of states, charge densities, and Fermi surfaces provides conclusive insights into the properties and behavior of the alloys. In essence, comprehensive investigation offers valuable insights into the thermophysical properties of L10 FePt, L12 Fe3Pt, and L12 FePt3 alloys, spanning equilibrium lattice constants, phonon characteristics, and electronic properties. These findings significantly augment the understanding of the structural stability, phonon dynamics, and electronic behavior exhibited by these alloys. [ABSTRACT FROM AUTHOR]

Superconductivity of iron chalocogenides is strongly enhanced under applied pressure yet its underlying pairing mechanism remains elusive. Here, we present a quantum oscillations study up to 45 T in the high-Tc phase of tetragonal FeSe0.82S0.18 up to 22 kbar. Under applied pressure, the quasi-two-dimensional multi-band Fermi surface expands and the effective masses remain large, whereas the superconductivity displays a threefold enhancement. Comparing with chemical pressure tuning of FeSe1−xSx, the Fermi surface expands in a similar manner but the effective masses and Tc are suppressed. These differences may be attributed to the changes in the density of states influenced by the chalcogen height, which could promote stronger spin fluctuations pairing under pressure. Furthermore, our study also reveals unusual scattering and broadening of superconducting transitions in the high-pressure phase, indicating the presence of a complex pairing mechanism. [ABSTRACT FROM AUTHOR]

Superconductivity often emerges as a dome around a quantum critical point (QCP) where long-range order is suppressed to zero temperature, mostly in magnetically ordered materials. However, the emergence of superconductivity at charge-order QCPs remains shrouded in mystery, despite its relevance to high-temperature superconductors and other exotic phases of matter. Here, we present resistance measurements proving that a dome of superconductivity surrounds the putative charge-density-wave QCP in pristine samples of titanium diselenide tuned with hydrostatic pressure. In addition, our quantum oscillation measurements combined with electronic structure calculations show that superconductivity sets in precisely when large electron and hole pockets suddenly appear through an abrupt change of the Fermi surface topology, also known as a Lifshitz transition. Combined with the known repulsive interaction, this suggests that unconventional s± superconductivity is mediated by charge-density-wave fluctuations in titanium diselenide. These results highlight the importance of the electronic ground state and charge fluctuations in enabling unconventional superconductivity. [ABSTRACT FROM AUTHOR]

X-ray diffraction measurements of equiatomic B2-ordered FeV were performed in a diamond-anvil cell at room temperature at several pressure points up to 80 GPa that showed the cubic phase to be stable with no indication of structural phase transitions. Density functional theory at 0 K predicts Fermi surface nesting, an electronic topological transition, and a phonon dynamical instability within the experimentally investigated pressure range. Nevertheless, the instability is absent in phonon dispersion curves extracted from ab initio molecular dynamics simulations below the critical volume at temperatures as low as 10 K, indicating that thermal atomic displacements can frustrate the phase transition by renormalizing the phonon dispersion curves. Ferrimagnetism is critical for the stability of the cubic phase at low temperature, but thermal atomic displacements are enough to support the structure at and above the Néel temperature. [ABSTRACT FROM AUTHOR]

Reducing interface nonradiative recombination is important for realizing highly efficient perovskite solar cells. In this work, we develop a synergistic bimolecular interlayer (SBI) strategy via 4-methoxyphenylphosphonic acid (MPA) and 2-phenylethylammonium iodide (PEAI) to functionalize the perovskite interface. MPA induces an in-situ chemical reaction at the perovskite surface via forming strong P-O-Pb covalent bonds that diminish the surface defect density and upshift the surface Fermi level. PEAI further creates an additional negative surface dipole so that a more n-type perovskite surface is constructed, which enhances electron extraction at the top interface. With this cooperative surface treatment, we greatly minimize interface nonradiative recombination through both enhanced defect passivation and improved energetics. The resulting p-i-n device achieves a stabilized power conversion efficiency of 25.53% and one of the smallest nonradiative recombination induced Voc loss of only 59 mV reported to date. We also obtain a certified efficiency of 25.05%. This work sheds light on the synergistic interface engineering for further improvement of perovskite solar cells. Reducing interface nonradiative recombination is important for realizing highly efficient perovskite solar cells. Here, the authors employ a bimolecular interlayer to functionalize the perovskite interface, achieving cooperative surface treatment and certified power conversion efficiency of 25.05%. [ABSTRACT FROM AUTHOR]

K- and P-doped SrFe2As2 belong to hole doping and isovalent doping, respectively, and their effects on the band, Fermi surface, and partial density of states of Fe 3d and As 4p orbitals are also significantly different. However, these two types of doping consistently follow the rule that reducing the Fe ions height can quickly suppress antiferromagnetic order. Compared with the As-Fe-As angle, Fe-As bond length, and Fe-Fe distance, the decrease in Fe ions height more directly and quickly suppresses antiferromagnetic order, and the total density of states in the range of – 1.5 eV to ~ 1.5 eV exhibits similar properties in K and P doping in the process of 25% K- and 12.5% P-doped SrFe2As2. [ABSTRACT FROM AUTHOR]