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Weyl semimetal (WSM), a novel state of quantum matter, hosts Weyl fermions as emergent quasiparticles resulting from the breaking of either inversion or time-reversal symmetry. Magnetic WSMs that arise from broken time-reversal symmetry provide an exceptional platform to understand the interplay between magnetic order and Weyl physics, but only a few WSMs have been realized. Here, we identify CeAlSi as a new noncentrosymmetric magnetic WSM via angle-resolved photoemission spectroscopy (ARPES) and first-principles, density-functional theory based calculations. Our surface-sensitive vacuum ultraviolet ARPES data confirm the presence of surface Fermi arcs as the conclusive evidence for the existence of the Weyl semimetallic state in CeAlSi. We also observe bulk Weyl cones at the Fermi arc terminations in CeAlSi using bulk-sensitive soft-x-ray ARPES measurements. These results implicate CeAlSi as a unique platform for investigating exotic quantum phenomena resulting from the interaction of topology and magnetism.

The thermodynamic and physical properties of Helium (He) and Hydrogen (H) mixtures are crucial form an astrophysical perspective. Nowadays, it is well known that these two elements constitute roughly 95% of the matter in the solar system. The high-pressure equation of He state doped with H is calculated in the framework of density functional theory (DFT) for three crystallographic structures. Namely, the Body Centered Cubic (BCC), Face Centered Cubic (FCC), and hexagonal close packed (HCP) structures have been studied. The equations of state (EOS) are provided for the He0.90H0.10 and He0.70H0.30 mixtures and the energy of the same mixtures are calculated with the earlier structures. The band structure for pressures of 0, 75 and 150 GPa were estimated by our calculations for the presumed crystal structures in the case of pure He and He0.70H0.30 mixture for comparison. The k(x) - k(y) cuts of the Bloch Spectral Functions (BSF) are presented for the pure He and He0.70H0.30 mixture for the three structures to explore the impact of doping on electronic properties. In addition, electrical resistivity calculations of the mentioned structures were carried out for mixtures with 10, 15, 20 and 30% of H. At final stage, the enthalpy (Delta H) of these latest mixtures have been deduced for the three crystal structures.

The thermodynamic and physical properties of Helium (He) and Hydrogen (H) mixtures are crucial form an astrophysical perspective. Nowadays, it is well known that these two elements constitute roughly 95% of the matter in the solar system. The high-pressure equation of He state doped with H is calculated in the framework of density functional theory (DFT) for three crystallographic structures. Namely, the Body Centered Cubic (BCC), Face Centered Cubic (FCC), and hexagonal close packed (HCP) structures have been studied. The equations of state (EOS) are provided for the He0.90H0.10 and He0.70H0.30 mixtures and the energy of the same mixtures are calculated with the earlier structures. The band structure for pressures of 0, 75 and 150 GPa were estimated by our calculations for the presumed crystal structures in the case of pure He and He0.70H0.30 mixture for comparison. The k(x) - k(y) cuts of the Bloch Spectral Functions (BSF) are presented for the pure He and He0.70H0.30 mixture for the three structures to explore the impact of doping on electronic properties. In addition, electrical resistivity calculations of the mentioned structures were carried out for mixtures with 10, 15, 20 and 30% of H. At final stage, the enthalpy (Delta H) of these latest mixtures have been deduced for the three crystal structures., QC 20220304

Many charge density wave (CDW) systems exhibit q(T ) electron-hole modulations continuously varying with T and saturating upon cooling at an incommensurate value even if the maximum occurring in the electron-hole Lindhard response does not exhibit such a thermal shift. Using a simple RPA argument we show that the experimental q(T ) can be understood if the electron-phonon coupling (EPC) g(q), necessary to set coupled electronic and structural modulations, is momentum dependent. In this analysis, the sense of variation of q(T ) depends upon the sign of ∂g(q) ∂q and its amplitude of thermal variation is controlled by the electron-hole coherence length (or CDW rigidity) in the modulation direction. This model quantitatively accounts for the thermal dependence of q(T ) in the one-dimensional (1D) CDW system K0.3MoO3 (blue bronze) both in its CDW ground state and in its pretransitional CDW fluctuation regime. We suggest that such a general analysis can be extended to account for the q(T ) dependence observed in other 1D and 2D CDW systems such as the transition metal di- and trichalcogenides as well as the lanthanide and rare-earth tritellurides. Using a detailed analysis of the low frequency phonon spectrum of the blue bronze, we then propose a new scenario for the q dependent EPC, where g(q) is due to a momentum-dependent hybridization between the critical phonon branch bearing the Kohn anomaly and other low-lying phonon branches. This allows obtaining a sign of ∂g(q) ∂q in agreement with that deduced from the analysis of q(T ). Finally, we propose that similar hybridization effects could also be relevant for other 1D and 2D CDW systems exhibiting a thermally dependent modulation.

NbSe3 and monoclinic-TaS3 (m-TaS3) are quasi-1D metals containing three different types of chains and undergoing two different charge density wave Peierls transitions at ${T}_{{\mathrm{P}}_{1}}$ and ${T}_{{\mathrm{P}}_{2}}$ associated with type III and type I chains, respectively. The nature of these transitions is discussed on the basis of first-principles DFT calculation of their Fermi surface (FS) and electron–hole response function. Because of the stronger inter-chain interactions, the FS and electron–hole response function are considerably more complex for NbSe3 than m-TaS3; however a common scenario can be put forward to rationalize the results. The intra-chain inter-band nesting processes dominate the strongest response for both type I and type III chains of the two compounds. Two well-defined maxima of the electron–hole response for NbSe3 are found with the (0a*, 0c*) and (1/2a*, 1/2c*) transverse components at ${T}_{{\mathrm{P}}_{1}}$ and ${T}_{{\mathrm{P}}_{2}}$, respectively, whereas the second maximum is not observed for m-TaS3 at ${T}_{{\mathrm{P}}_{2}}$. Analysis of the different inter-chain coupling mechanisms leads to the conclusion that FS nesting effects are only relevant to set the transverse a* components in NbSe3. The strongest inter-chain Coulomb coupling mechanism must be taken into account for the transverse coupling along c* in NbSe3 and along both a* and c* for m-TaS3. Phonon spectrum calculations reveal the formation of a giant 2kF Kohn anomaly for m-TaS3. All these results support a weak coupling scenario for the Peierls transition of transition metal trichalcogenides.

A suite of density functional theory (DFT) approaches that provide accurate and reliable superconductivity parameters without post-calculation corrections is presented. The sensitivity of phonon dispersion (PD) outputs to slight variations in calculation and external parameters enables identification of the most electron-coupled phonon mode(s). These mode(s) are directly linked, in a range of superconductors, to specific band structure characteristics and to the response of electrons to deformation potentials. These approaches are applied successfully to MgB2 under a wide range of variable external conditions, including isotopic state, metal substitutions, pressure, and temperature. This approach has also been extended to other chemical systems with significantly different composition and crystal symmetry, such as YB6, H3S under pressure, B-doped diamond, B-doped silicon, and A15 compounds, to name a few. The effectiveness of these approaches that link both PDs and electronic band structures (EBSs) is largely dependent on reliable and accurate DFT results at the meV range that resolve Fermi surface (FS) detail in close reciprocal space proximity. Such a level of meV resolution is possible only by using extremely fine k-grids and large cutoff radii, much smaller and larger, respectively, to what has typically been used in DFT calculations reported in the literature.

Motivated by the recent discovery of superconductivity in infinite-layer (Sr,Nd)NiO2 films with Sr content x≃0.2 [D. Li, Nature (London) 572, 624 (2019)NATUAS0028-083610.1038/s41586-019-1496-5], we examine the effects of electron correlations and Sr doping on the electronic structure, Fermi-surface topology, and magnetic correlations in (Nd,Sr)NiO2 using a combination of dynamical mean-field theory of correlated electrons and band-structure methods. Our results reveal a remarkable orbital-selective renormalization of the Ni 3d bands, with m∗/m∼3 and 1.3 for the dx2-y2 and d3z2-r2 orbitals, respectively, that suggests orbital-dependent localization of the Ni 3d states. We find that upon hole doping, (Nd,Sr)NiO2 undergoes a Lifshitz transition of the Fermi surface which is accompanied by a change of magnetic correlations from three-dimensional (3D) Néel G-type (111) to quasi-2D C-type (110). We show that magnetic interactions in (Nd,Sr)NiO2 demonstrate an unanticipated frustration, which suppresses magnetic order, implying the importance of in-plane spin fluctuations to explain its superconductivity. Our results suggest that frustration is maximal for Sr doping x≃0.1-0.2, which is in agreement with an experimentally observed doping value Sr x≃0.2 of superconducting (Nd,Sr)NiO2. © 2020 American Physical Society.

We report an investigation of the combined structural and electronic properties of the bronze Na 1.5 ( PO 2 ) 4 ( WO 3 ) 20 . Its low-dimensional structure and possible large reconstruction of the Fermi surface due to charge density wave instability make this bulk material a natural superlattice with a reduced number of carriers and Fermi energy. Signatures of multilayered two-dimensional (2D) electron weak localization are consequently reported, with an enhanced influence of quantum oscillations. A crossover between these two antagonistic entities, previously observed only in genuine low-dimensional materials and devices, is shown to occur in a bulk crystal due to its hidden 2D nature.

Interactions between particles play an important role in quantum degenerate gases. In fact, the system’s behavior is usually largely determined by the strength, range, and symmetry of the interactions, even if very weak. For more than two decades after the first experimental realization of a Bose-Einstein condensate (BEC) in 1995, investigations of ultracold atomic gases have mainly considered contact-type interactions, which model the short-range van der Waals interactions between the atoms in terms of a single parameter, the s-wave scattering length. However, some atomic or molecules species possess a magnetic or electric dipole moment and additionally interact among each other via the dipole-dipole interaction (DDI), which is long-ranged and anisotropic. Signatures of the DDI have been first observed in a BEC of chromium atoms in 2005. In the last several years, a new class of atoms from the lanthanide series, e.g., dysprosium and erbium, with stronger dipolar properties, have come into play. This permits a more full-fledged study of dipolar effects in BECs. Furthermore, ultracold molecules and highly excited Rydberg atoms are other promising examples for futures studies of systems where dipolar interaction will be even more dominant, but they still need experimental efforts to achieve the desired quantum regime. Whereas the weak DDI in a chromium BEC is well described with the Gross-Pitaevskii equation, stronger dipolar interactions certainly necessitate a beyond-mean-field description within a Bogoliubov theory. In parallel to research on bosons, cold fermions also trigger great interest because they are actually often found in nature, e.g., within the electron gas of metals, as neutrons of heavy stars, or as quarks in plasmas as constituent elements of composite particles. Contrary to Bose systems, quantum degeneracy in experiments is much more difficult to achieve in Fermi systems, since s-wave scattering is absent between identical fermions, due to the Pauli, Interakcije izmedu ˇcestica igraju vaˇznu ulogu u razumevanju ponaˇsanja degeneri- - sanih kvantnih gasova, koje je u velikoj meri odredeno jaˇcinom, dometom i simetri- - jama interakcija, ˇcak i ako su veoma slabe. Tokom viˇse od dve decenije nakon prve eksperimentalne realizacije Boze-Ajnˇstajn kondenzatacije 1995. godine, istraˇzivanje ultrahladnih atomskih gasova je uglavnom bilo fokusirano na prouˇcavanje kontaktnih interakcija izmedu atoma, ˇsto je jednoparametarski model za kratkodometne van - der Valsove interakcije, koji kao parametar uzima u obzir samo duˇzinu rasejanja u simetriˇcnom (s) kanalu. Medutim, neke vrste atoma ili molekula poseduju magnetni - ili elektiˇcni dipolni moment, pa moramo da uzmemo u obzir i dipol-dipol interakcije (DDI) izmedu ˇcestica. DDI su dugodometne i anizotropne, i njihov uticaj je prvi put - eksperimentalno izmeren u kondenzovanom gasu atoma hroma 2005. godine. Nova klasa atoma iz grupe lantanoida, kao ˇsto su disprozijum i erbijum, sa jaˇcim dipolnim osobinama, postala je dostupna u poslednjih nekoliko godina. Ovo omogu´cava potpunije istraˇzivanje dipolnih efekata u Boze-Ajnˇstajn-kondenzovanim sistemima. Dalje, ultrahladni molekuli i visoko pobudeni Ridbergovi atomi predstavljaju druge mogu´ce - pravce za prouˇcavanje sistema u kojima ´ce dipolna interakcija biti joˇs znaˇcajnija, ali ovo zahteva dodatne eksperimentalne napore kako bi se dostigao ˇzeljeni kvantni reˇzim. Dok se slaba DDI izmedu atoma hroma u kondenzovanim sistemima moˇze dobro opisati - Gros-Pitaevski jednaˇcinom, prisustvo jake DDI zahteva uraˇcunavanje efekata koji nisu prisutni u teoriji srednjeg polja u Bogoljubovljevoj teoriji. Paralelno sa istraˇzivanjima bozona, hladni fermioni su takode izazvali veliko in- - teresovanje jer se ˇcesto sre´cu u prirodi, na primer u sistemima kao ˇsto je elektronski gas u metalu, u sistemima neutrona u masivnim zvezdama, ili kao kvarkovi koji ˇcine kvark-gluonsku plazmu u kompozitnim ˇcesticama. Za razliku od bozonsk

A simple approach for extracting the superconducting gap of materials from trendline fittings of key sections of their Electronic Band Structures (EBS), calculated using Density Functional Theory (DFT), is presented. The proposed approach works well for very different compositions, structure types and superconducting transition temperatures, indicating that it is robust, reliable and versatile. This approach provides many advantages in terms of required computational time and power, compared with similar information extracted from phonon dispersion calculations, making it accessible to wider DFT-computational chemistry and physics communities. The approach also identifies requirements for superconductivity and indirectly contributes towards understanding of superconductivity mechanisms.

The aim of this research is to develop an N -dimensional adaptive sampling algorithm to efficiently sample functions, meaning that with fewer samples the same accuracy is achieved compared to what homogeneously spaced samples would achieve. This algorithm is based on an existing Python package called Adaptive. The developed algorithm is applied to find and plot the Fermi surface of crystals with a higher resolution than homogeneous sampling would with the same number of points., Applied Mathematics | Applied Physics

Iron-based binary chalcogenide superconductors FeSe and FeS have attracted much recent attention due to their complex orbital-selective correlations and Cooper pairing, offering the minimal model system holding the key properties to understanding the physics of high-Tc superconductors. Here, using density functional plus dynamical mean-field theory method with full self-consistency over the charge density, we study the effect of electronic correlations on the electronic structure, magnetic properties, orbital-dependent band renormalizations, and Fermi surface of the tetragonal phase of bulk FeS. We perform a direct structural optimization of the P4/nmm crystal structure of paramagnetic FeS, minimizing the total energy of FeS with respect to the lattice constant a and the internal coordinate zS of atom S. Our results show an anomalous sensitivity of the electronic structure and magnetic properties of FeS (intrinsic to all Fe-based chalcogenide and pnictide superconductors) to fine details of its crystal structure, e.g., to a small variation of the chalcogen coordinate zS. Upon expansion of the lattice volume (which can be realized, e.g., in FeS1-xSex and FeS1-xTex), we observe a remarkable change of the electronic structure of FeS which is associated with a complete reconstruction of the Fermi-surface topology (Lifshitz transition). This behavior is ascribed to a correlation-induced shift of the Van Hove singularity associated with the Fe xy and xz/yz orbitals at the M point across the Fermi level. The Lifshitz phase transition is accompanied by a significant growth of local magnetic moments and emergence of strong orbital-selective correlations. It is seen as a pronounced anomaly ("kink") in the total energies upon expansion of the lattice, associated with a remarkable enhancement of compressibility. This behavior is accompanied by an orbital-dependent formation of local moments, a crossover from itinerant to localized orbital-selective moment behavior of the Fe 3d

Uniaxial pressure applied along a Ru-O-Ru bond direction induces an elliptical distortion of the largest Fermi surface of Sr2RuO4, eventually causing a Fermi surface topological transition, also known as a Lifshitz transition, into an open Fermi surface. There are various anomalies in low-temperature properties associated with this transition, including maxima in the superconducting critical temperature and in resistivity. In the present paper, we report refined measurements of the strain at which this transition occurs, employing apparatus in which the stress on the sample is measured, and resonant ultrasound measurement of the low-temperature elastic moduli. The Lifshitz transition is found to occur at a longitudinal strain Exx of (-0.44±0.06)×10-2, which corresponds to a B1g strain Exx-Eyy of (-0.66±0.09)×10-2. This is considerably smaller than the strain corresponding to a Lifshitz transition in density functional theory calculations, even if the spin-orbit coupling is taken into account. Using dynamical mean-field theory, we show that electronic correlations reduce the critical strain. It turns out that the orbital anisotropy of the local Coulomb interaction on the Ru site is, furthermore, important to bring this critical strain close to the experimental number and thus well into the experimentally accessible range of strains. © 2019 American Physical Society.

The aim of this research is to develop an N -dimensional adaptive sampling algorithm to efficiently sample functions, meaning that with fewer samples the same accuracy is achieved compared to what homogeneously spaced samples would achieve. This algorithm is based on an existing Python package called Adaptive. The developed algorithm is applied to find and plot the Fermi surface of crystals with a higher resolution than homogeneous sampling would with the same number of points., Applied Mathematics | Applied Physics

On the basis of first-principles DFT calculations the wave-vector and temperature dependencies of the Lindhard response function of the blue bronze K0.3MoO3 have been calculated. The kIF+ kIIF interband component of the response, which is responsible for the Peierls instability, has been quantitatively analyzed. It is found that (i) the electron-hole coherence length of this response determines the length scale of the experimental intrachain CDW correlations, and (ii) the intrachain q dependence of such a response also determines the shape of the Kohn anomaly experimentally measured. These findings provide compelling evidence that the Peierls transition of the blue bronze K0.3MoO3 follows the weak electron-phonon coupling scenario in the adiabatic approximation, something that had not yet been proved on the basis of first-principles calculations for a real material. It is proposed that the CDW interchain coupling occurs through a Coulomb coupling between dipolar CDWs. The nature of the phonon mode leading to the dipolar nature of the CDWs is also discussed, and the relevance of these results to rationalize the CDW instabilities in other oxides and bronzes is pointed out. These findings are also contrasted with recent results for other CDW materials like chalcogenides and tellurides.

Erworben im Rahmen der Schweizer Nationallizenzen (http://www.nationallizenzen.ch), We explore the strong variations of the electronic properties of copper-oxygen compounds across the doping phase diagram in a quantitative way. To this end we calculate the electronic Raman response on the basis of results from angle-resolved photoemission spectroscopy (ARPES). In the limits of our approximations we find agreement on the overdoped side and pronounced discrepancies at lower doping. In contrast to the successful approach for the transport properties at low energies, the Raman and the ARPES data cannot be reconciled by adding angle-dependent momentum scattering. We discuss possible routes towards an explanation of the suppression of spectral weight close to the (π, 0) points which sets in abruptly close to 21% doping.

A predictive tool for the design of new, higher temperature superconductors requires a simple, first-principles technique based on well-established axioms as embodied in Density Functional Theory (DFT), without post-calculation corrections or increased levels of complexity. We show that anomalies in the calculated phonon dispersions of compounds with AlBc for new compounds.

Using full-potential local-orbital minimum-basis along with spin-polarized relativistic Korringa-Kohn-Rostoker methods, we study the electronic, Fermi surface, Curie temperature and optical properties of Zr2CoAl alloy. The alloy with Li2AgSb and Cu2MnAl structures are compared in terms of magnetic properties, and the electronic structures in two structures are also discussed. According to the calculated electronic states, it finds that the Zr2CoAl with Li2AgSb structure is half-metallic ferromagnet with an integral magnetic moment of 2.00μB, meanwhile we also notice the d-d and p-d hybridizations are responsible for the formation of minority-spin gap, furthermore, the fat-bands are applied to discuss the mixture between d and p electrons in the vicinity of the Fermi level. The Fermi surfaces related to the valence bands are constructed, and it is found that the spin-up valence bands 26, 27 and 28 across the Fermi energy dominate the nature of electrons. By mapping the system onto a Heisenberg Hamiltonian, we obtain the exchange coupling parameters, and observe that the Zr(A)-Co(C) and Zr(A)-Zr(B) interactions provide a major contribution for exchange interactions. Based on the calculated exchange coupling parameters, the Curie temperature is estimated to be 287.86 K at equilibrium, and also the dependence of Curie temperature on lattice constant related to the tunable Curie temperature in Zr2CoAl alloy is studied. Finally, we report the optical properties of Zr2CoAl alloy, and present the photon energy dependence of the absorption, the optical conductivity and the loss function., QC 20170208

Using full-potential local-orbital minimum-basis along with spin-polarized relativistic Korringa-Kohn-Rostoker methods, we study the electronic, Fermi surface, Curie temperature and optical properties of Zr2CoAl alloy. The alloy with Li2AgSb and Cu2MnAl structures are compared in terms of magnetic properties, and the electronic structures in two structures are also discussed. According to the calculated electronic states, it finds that the Zr2CoAl with Li2AgSb structure is half-metallic ferromagnet with an integral magnetic moment of 2.00 mu(beta), meanwhile we also notice the d-d and p-d hybridizations are responsible for the formation of minority-spin gap, furthermore, the fat-bands are applied to discuss the mixture between d and p electrons in the vicinity of the Fermi level. The Fermi surfaces related to the valence bands are constructed, and it is found that the spin-up valence bands 26, 27 and 28 across the Fermi energy dominate the nature of electrons. By mapping the system onto a Heisenberg Hamiltonian, we obtain the exchange coupling parameters, and observe that the Zr(A)-Co(C) and Zr(A)-Zr(B) interactions provide a major contribution for exchange interactions. Based on the calculated exchange coupling parameters, the Curie temperature is estimated to be 287.86 K at equilibrium, and also the dependence of Curie temperature on lattice constant related to the tunable Curie temperature in Zr2CoAl alloy is studied. Finally, we report the optical properties of Zr2CoAl alloy, and present the photon energy dependence of the absorption, the optical conductivity and the loss function.

Band structure calculations relevant to bis-ethylenedithio-tetrathiafulvalene-based charge transfer salts containing tris(oxalato)metallate anions, with generic formula (BEDT-TTF)4A[M(C2O4)3]·Solv, where A is a monovalent anion, M is a trivalent cation and Solv is a solvent, suggest that their Fermi surface is liable to achieve networks of compensated orbits coupled by magnetic breakdown. Even though this picture accounts for quantum oscillations spectra of a number of these compounds, puzzling results can be noticed in several cases, pointing to the possibility of Fermi surface reconstruction at low temperature.

By means of first principles calculations, we investigate the nature of exchange coupling in ferromagnetic bcc Fe on a microscopic level. Analyzing the basic electronic structure reveals a drastic difference between the 3d orbitals of Eg and T2g symmetries. The latter ones define the shape of the Fermi surface, while the former ones form weakly interacting impurity levels. We demonstrate that, as a result of this, in Fe the T2g orbitals participate in exchange interactions, which are only weakly dependent on the configuration of the spin moments and thus can be classified as Heisenberg-like. These couplings are shown to be driven by Fermi surface nesting. In contrast, for the Eg states, the Heisenberg picture breaks down since the corresponding contribution to the exchange interactions is shown to strongly depend on the reference state they are extracted from. Our analysis of the nearest-neighbor coupling indicates that the interactions among Eg states are mainly proportional to the corresponding hopping integral and thus can be attributed to be of double-exchange origin. By making a comparison to other magnetic transition metals, we put the results of bcc Fe into context and argue that iron has a unique behavior when it comes to magnetic exchange interactions. © 2016 American Physical Society.

Using first-principles based density functional theory, we have investigated the structural instabilities in the austenite phases of Mn2NiX (X = Al, Ga, In, Sn) magnetic shape memory alloys. A complete softening is observed in the acoustic TA(2) branches for all the materials along [xi xi 0] directions leading to instability in the austenite structure which effectively stabilizes into martensitic structure. The reasons behind this softening are traced back to the repulsion from the optical T-2g branches and to the nesting features in the Fermi surfaces. The vibrational density of states, the force constants and the elastic moduli are also computed and analyzed, which reconfirm the underlying mechanism behind the instabilities. The results indicate that the phonon anomalies are related to the occurrence of possible pre-martensitic phases which can be quite complex.

The electronic structure and phase stability of paramagnetic FeSe is computed by using a combination of ab initio methods for calculating band structure and dynamical mean-field theory. Our results reveal a topological change (Lifshitz transition) of the Fermi surface upon a moderate expansion of the lattice. The Lifshitz transition is accompanied with a sharp increase of the local moments and results in an entire reconstruction of magnetic correlations from the in-plane magnetic wave vector, (π,π) to (π,0). We attribute this behavior to a correlation-induced shift of the van Hove singularity originating from the dxy and dxz/dyz bands at the M point across the Fermi level. We propose that superconductivity is strongly influenced, or even induced, by a van Hove singularity. © 2015 American Physical Society. © 2015 American Physical Society.

We review recent progress in single crystal growth and study of electronic properties in. Czocharalski pulling, using purified uranium metal and subsequent annealing under ultra-high vacuum, is successfully applied to this compound, and it yields the highest residual resistivity ratio. These high-quality single crystals allow us to investigate Fermi surfaces using quantum oscillation and to make detailed transport measurements at low temperature.

We review recent progress in single crystal growth and study of electronic properties in. Czocharalski pulling, using purified uranium metal and subsequent annealing under ultra-high vacuum, is successfully applied to this compound, and it yields the highest residual resistivity ratio. These high-quality single crystals allow us to investigate Fermi surfaces using quantum oscillation and to make detailed transport measurements at low temperature.

In Part I, the relationship between the topology of the Fermi surface and the entanglement entropy S is examined. Spinless fermionic systems on one and two dimensional lattices at fixed chemical potential are considered. The lattice is partitioned into sub-system of length L and environment, and the entanglement of the subsystem with the environment is calculated via the correlation matrix. S is plotted as a function of the next-nearest or next-next nearest neighbor hopping parameter, t. In 1 dimension, the entanglement entropy jumps at lifshitz transitions where the number of Fermi points changes. In 2 dimensions, a neck-collapsing transition is accompanied by a cusp in S, while the formation of electron or hole-like pockets coincides with a kink in the S as a function of the hopping parameter. The entanglement entropy as a function of subsystem length L is also examined. The leading order coefficient of the LlnL term in 2 dimensions was seen to agree well with the Widom conjecture. Of interest is the difference this coefficient and the coefficient of the term linear in L near the neck-collapsing point. The leading order term changes like |t-tc|1/2 whereas the first sub-leading term varies like |t-tc|1/3, where tc is the critical value of the hopping parameter at the transition. In Part II, we study the statistics of fractionalized excitations in a bosonic model which describes strongly interacting excitons in a N-band insulator. The elementary excitations of this system are strings, in a large N limit. A string is made of a series of bosons whose flavors are correlated such that the end points of a string carries a fractionalized flavor quantum number. When the tension of a string vanishes, the end points are deconfined. We determine the statistics of the fractionalized particles described by the end points of strings. We show that either bosons or Fermions can arise depending on the microscopic coup, Master of Science (MSc)

We present an experimental setup for laser-based angle-resolved time-of-flight photoemission. Using a picosecond pulsed laser, photons of energy 10.5 eV are generated through higher harmonic generation in xenon. The high repetition rate of the light source, variable between 0.2 and 8 MHz, enables high photoelectron count rates and short acquisition times. By using a time-of-flight analyzer with angle-resolving capabilities, electrons emitted from the sample within a circular cone of up to +/- 15 degrees can be collected. Hence, simultaneous acquisition of photoemission data for a complete area of the Brillouin zone is possible. The current photon energy enables bulk sensitive measurements, high angular resolution, and the resulting covered momentum space is large enough to enclose the entire Brillouin zone in cuprate high-T(c) superconductors. Fermi edge measurements on polycrystalline Au shows an energy resolution better than 5 meV. Data from a test measurement of the Au(111) surface state are presented along with measurements of the Fermi surface of the high-T(c) superconductor Bi(2)Sr(2)CaCu(2)O(8+delta) (Bi2212)., QC 20111107

type:text, Highly efficient spin- and angle-resolved photoelectron spectrometer named ESPRESSO (Efficient SPin REsolved SpectroScopy Observation) machine has been developed at the beamline BL-9B in Hiroshima Synchrotron Radiation Center. Combination of high-resolution hemispherical electron analyzer and the high-efficient spin detector based on very low energy electron diffraction by the ferromagnetic target makes the high-energy resolution and angular resolution compatible with spin- and angle-resolved photoemission (SARPES) measurement. 7.5 meV in energy and ±0.18° in angular resolution have been achieved with spin resolution. The ESPRESSO machine, combination of quick energy-band dispersion measurement and Fermi surface mapping by two-dimensional electron detector for the spin integrated ARPES and the high-efficient spin analysis by the efficient spin detector realizes the comprehensive investigation of spin electronic structure of materials.

Highly efficient spin- and angle-resolved photoelectron spectrometer named ESPRESSO (Efficient SPin REsolved SpectroScopy Observation) machine has been developed at the beamline BL-9B in Hiroshima Synchrotron Radiation Center. Combination of high-resolution hemispherical electron analyzer and the high-efficient spin detector based on very low energy electron diffraction by the ferromagnetic target makes the high-energy resolution and angular resolution compatible with spin- and angle-resolved photoemission (SARPES) measurement. 7.5 meV in energy and ±0.18° in angular resolution have been achieved with spin resolution. The ESPRESSO machine, combination of quick energy-band dispersion measurement and Fermi surface mapping by two-dimensional electron detector for the spin integrated ARPES and the high-efficient spin analysis by the efficient spin detector realizes the comprehensive investigation of spin electronic structure of materials.

Highly efficient spin- and angle-resolved photoelectron spectrometer named ESPRESSO (Efficient SPin REsolved SpectroScopy Observation) machine has been developed at the beamline BL-9B in Hiroshima Synchrotron Radiation Center. Combination of high-resolution hemispherical electron analyzer and the high-efficient spin detector based on very low energy electron diffraction by the ferromagnetic target makes the high-energy resolution and angular resolution compatible with spin- and angle-resolved photoemission (SARPES) measurement. 7.5 meV in energy and ±0.18° in angular resolution have been achieved with spin resolution. The ESPRESSO machine, combination of quick energy-band dispersion measurement and Fermi surface mapping by two-dimensional electron detector for the spin integrated ARPES and the high-efficient spin analysis by the efficient spin detector realizes the comprehensive investigation of spin electronic structure of materials.

The detailed electronic structures of CeRu2Si2 have been investigated for a nonmagnetic state and a magnetic state under magnetic fields by in LSDA+U method. It is found that the Si position in the crystal structure is an essential parameter to reproduce precise topology of the experimentally determined Fermi surfaces of non-f reference LaRu2Si2. As for CeRu2Si2, the LDA calculation Lids to predict the nonmagnetic round state, which is experimentally reported to be Mainly occupied by vertical bar j = 5/2.j(z) = +/- 5/2 > orbitals, and the Fermi surfaces. The nonmagnetic case solution of LSDA+U method greatly improves the electronic structure in the nonmagnetic CeRu2Si2. An LSDA+U method is also applied to investigate the electronic Structure under applied magnetic fields, then the change of the so-called large Fermi Surfaces to small Fermi surfaces is Successfully described.

The current driven magnetization dynamics of a helical spin-density wave is investigated. Expressions for calculating the spin-transfer torque of real systems from first-principles density-functional theory are presented. These expressions are used for calculating the spin-transfer torque for the spin spirals of Er and fcc Fe at two different lattice volumes. It is shown that the calculated torque induces a rigid rotation of the order parameter with respect to the spin spiral axis. The torque is found to depend on the wave vector of the spin spiral and the spin polarization of the Fermi surface states. The resulting dynamics of the spin spiral is also discussed.

Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic properties of substitutional refractory Ru1-xNixAl alloys. We have found that the elastic constants C-' and C-11 exhibit pronounced peculiarities near the concentration of about 40 at. % Ni, which we ascribe to electronic topological transitions. Our suggestion is supported by the Fermi surface calculations in the whole concentration range. Results of our calculations show that one can design Ru-Ni-Al alloys substituting Ru by Ni (up to 40 at. %) with almost invariable elastic constants and reduced density.

The Fermi surface (FS) of URu2Si2 in the paramagnetic phase has been reinvestigated via the measurement of the two-dimensional angular correlation of the positron annihilation radiation. Unlike previous measurements our experiment provides evidence that density-functional theory gives a fair qualitative description of the FS, showing a large contribution from the 5f electrons, although the Fermi volume of electron and hole pockets determined experimentally is smaller than the theoretical description. Furthermore, we propose a method to investigate the nature of the hidden order responsible for the 17.5 K transition.

Neutron scattering experiments of Cr/V(001) superlattices are discussed, which show that the incommensurate spin-density-wave (SDW) in thick Cr layers becomes suppressed when the V spacer layers are loaded with hydrogen. The hydrogen loading triggers a transition from the incommensurate SDW state to a commensurate antiferromagnetic state. Model Hamiltonian calculations are presented, which show that this transition is not connected with the nesting property of the Cr Fermi surface. Instead, the transition is a manifestation of the antiferromagnetic ground state of Cr, which is separated from the incommensurate SDW state by an energy barrier. Hydrogen is identified as an effective trigger for reducing the activation barrier, enabling the system to relax to the ground state.

Electronic structure calculations are employed in order to investigate the cohesive properties (lattice parameter, enthalpy of formation, and bulk modulus) of random Fe-Cr alloys as a function of composition and magnetic state, as well as to derive the chemical and magnetic exchange interactions of the constituent atoms. The calculations predict certain anomalies in the cohesive properties of ferromagnetic alloys at a concentration of about 7 at % Cr; these anomalies may be related to the changes in Fermi-surface topology that occur with composition in this alloy system. The obtained interatomic interactions are used as parameters in the configurational (Ising) and magnetic (Heisenberg) Hamiltonians for modeling finite-temperature thermodynamic properties of the alloys. We discuss the approximations and limitations of similar modeling approaches, investigate the origin of existing difficulties, and analyze possible ways of extending the theoretical models in order to capture the essential physics of interatomic interactions in the Fe-Cr or similar alloys where magnetism plays a crucial role in the phase stability., QC 20150622

Wehave performed ab initio calculations for the cubic inverse-perovskite Sc3EN(E=Al,Ga,In) systems to study their electronic band-structures and elastic properties.In this study, we used the accurate augmented plane waveplus local orbital method to find the equilibrium structural parametersand to compute the full elastic tensors. The obtained single-crystalelastic constants were used to quantify the stiffness of theSc-based ternary nitrides and to appraise their mechanical stability. Thesite-projected density of states, Fermi surfaces, and the charge-density plotshave also been used to analyze the chemical bonding betweenthe Sc6N cluster and the surrounding metallic lattice of eitherAl, Ga, or In atoms. Our calculations show that Sc3GaNhas the largest covalent Sc-N bonding-type character with the highestYoung, shear, and bulk moduli. Compared with the other twoisoelectronic systems, it also behaves as the most brittle materialwith a relatively large elastic anisotropy., Original Publication:Maurizio Mattesini, Martin Magnuson, Ferenc Tasnádi, Carina Höglund, Igor A. Abrikosov and Lars Hultman, Elastic properties and electrostructural correlations in ternary scandium-based cubic inverse perovskites: A first-principles study, 2009, Physical Review B. Condensed Matter and Materials Physics, (79), 125122.http://dx.doi.org/10.1103/PhysRevB.79.125122Copyright: American Physical Societyhttp://www.aps.org

Wehave performed ab initio calculations for the cubic inverse-perovskite Sc3EN(E=Al,Ga,In) systems to study their electronic band-structures and elastic properties.In this study, we used the accurate augmented plane waveplus local orbital method to find the equilibrium structural parametersand to compute the full elastic tensors. The obtained single-crystalelastic constants were used to quantify the stiffness of theSc-based ternary nitrides and to appraise their mechanical stability. Thesite-projected density of states, Fermi surfaces, and the charge-density plotshave also been used to analyze the chemical bonding betweenthe Sc6N cluster and the surrounding metallic lattice of eitherAl, Ga, or In atoms. Our calculations show that Sc3GaNhas the largest covalent Sc-N bonding-type character with the highestYoung, shear, and bulk moduli. Compared with the other twoisoelectronic systems, it also behaves as the most brittle materialwith a relatively large elastic anisotropy., Original Publication:Maurizio Mattesini, Martin Magnuson, Ferenc Tasnádi, Carina Höglund, Igor A. Abrikosov and Lars Hultman, Elastic properties and electrostructural correlations in ternary scandium-based cubic inverse perovskites: A first-principles study, 2009, Physical Review B. Condensed Matter and Materials Physics, (79), 125122.http://dx.doi.org/10.1103/PhysRevB.79.125122Copyright: American Physical Societyhttp://www.aps.org

Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic properties of substitutional refractory Ru1-xNixAl alloys. We have found that the elastic constants C- and C-11 exhibit pronounced peculiarities near the concentration of about 40 at. % Ni, which we ascribe to electronic topological transitions. Our suggestion is supported by the Fermi surface calculations in the whole concentration range. Results of our calculations show that one can design Ru-Ni-Al alloys substituting Ru by Ni (up to 40 at. %) with almost invariable elastic constants and reduced density., Original Publication:I D Bleskov, E A Smirnova, Yu Kh Vekilov, P A Korzhavyi, B Johansson, M Katsnelson, L Vitos, Igor Abrikosov and Eyvas Isaev, Ab initio calculations of elastic properties of Ru1-xNixAl superalloys, 2009, APPLIED PHYSICS LETTERS, (94), 16, 161901.http://dx.doi.org/10.1063/1.3120543Copyright: American Institute of Physicshttp://www.aip.org

Ab initio total energy calculations based on the exact muffin-tin orbitals method, combined with the coherent potential approximation, have been used to study the thermodynamical and elastic properties of substitutional refractory Ru1-xNixAl alloys. We have found that the elastic constants C- and C-11 exhibit pronounced peculiarities near the concentration of about 40 at. % Ni, which we ascribe to electronic topological transitions. Our suggestion is supported by the Fermi surface calculations in the whole concentration range. Results of our calculations show that one can design Ru-Ni-Al alloys substituting Ru by Ni (up to 40 at. %) with almost invariable elastic constants and reduced density., Original Publication:I D Bleskov, E A Smirnova, Yu Kh Vekilov, P A Korzhavyi, B Johansson, M Katsnelson, L Vitos, Igor Abrikosov and Eyvas Isaev, Ab initio calculations of elastic properties of Ru1-xNixAl superalloys, 2009, APPLIED PHYSICS LETTERS, (94), 16, 161901.http://dx.doi.org/10.1063/1.3120543Copyright: American Institute of Physicshttp://www.aip.org

We report measurements of the de Haas-van Alphen effect on a series of compounds, LaMIn5 (M=Co,Rh,Ir). The results show that each of the Co and Ir Fermi surfaces (FSs) exhibits some portions that are two dimensional and some portions that are three dimensional. The most two-dimensional character is exhibited in LaCoIn5, less two-dimensional behavior is seen in LaIrIn5, and no part of Fermi surface of LaRhIn5 is found to have a two-dimensional character. Thus the two dimensionality of portions of the FSs is largely determined by the d character of the energy bands while all of the effective masses remain ≤1.2. This fact has implications for the causes of the heavy fermion nature of superconductivity and magnetism in the Ce-based compounds having similar composition and structure. © 2009 The American Physical Society.

We report a high-field investigation (up to 45 T) of the metamagnetic transition in CeIrIn5 with resistivity and de Haas-van Alphen (dHvA) effect measurements in the temperature range of 0.03-1 K. As the magnetic field is increased the resistivity increases, reaches a maximum at the metamagnetic critical field, and falls precipitously for fields just above the transition, while the amplitude of all measurable dHvA frequencies are significantly attenuated near the metamagnetic critical field. However, the dHvA frequencies and cyclotron masses are not substantially altered by the transition. In the low-field state, the resistivity is observed to increase toward low temperatures in a singular fashion, a behavior that is rapidly suppressed above the transition. Instead, in the high-field state, the resistivity monotonically increases with temperature with a dependence that is more singular than the iconic Fermi-liquid, temperature-squared behavior. Both the damping of the dHvA amplitudes and the increased resistivity near the metamagnetic critical field indicate an increased scattering rate for charge carriers consistent with critical fluctuation scattering in proximity to a phase transition. The dHvA amplitudes do not uniformly recover above the critical field, with some holelike orbits being entirely suppressed at high fields. These changes, taken as a whole, suggest that the metamagnetic transition in CeIrIn5 is associated with the polarization and localization of the heaviest of quasiparticles on the holelike Fermi surface. © 2009 The American Physical Society.

The superconducting instability of the Fermi-liquid state is investigated by considering anisotropic electron-boson couplings. Both electron-electron interactions and anisotropic electron-boson couplings are treated with a renormalization group method that takes into account retardation effects. Considering a noninteracting circular Fermi surface, we find analytical solutions for the flow equations and derive a set of generalized Eliashberg equations. Electron-boson couplings with different momentum dependences are studied and we find superconducting instabilities of the metallic state with competition between order parameters of different symmetries. Numerical solutions for some couplings are given to illustrate the frequency dependence of the vertices at different coupling regimes. © 2009 The American Physical Society.

We have made measurements of the de Haas-van Alphen (dHvA) effect and the magnetic susceptibility of La1-x Smx B6 for x=0, 0.05, and 0.10 and Ce1-x Cax B6 for x=0 0.01, 0.03, and 0.05 in order to see changes in the Fermi surface (FS) volume with doping using lower valence substitutions for La and Ce. The FS volume decrease in La1-x Smx B6 is found to be directly proportional to the decrease in the number of electrons contributing to the boron-based conduction band by the decrease in valence of the divalent element. Two overlapping pieces of FS in the same portion of the Brillouin zone are seen to decrease in volume to the extent that they no longer overlap. In the case of Ce1-x Cax B6 the combination of increased disorder decreasing the mean-free path due to doping and heavy effective masses results in the damping of the dHvA oscillations and no signals are observed above x=0.05. Due to the mixed valence of the Ce atoms the volume decrease is not proportional to the percent valence change. Additional magnetic-susceptibility and magnetization measurements show the changing magnetic phase diagram in Ce1-x Cax B6. In the antiferroquadrapolar state of all of the Ce1-x Cax B6 samples the near-zero applied field magnetization is seen to increase with decreasing temperature terminating in a transition to the antiferromagnetic state. © 2009 The American Physical Society.

We investigate the electronic structures of the alloyed Bi1-x Sbx compounds based on first-principles calculations including spin-orbit coupling (SOC), and calculate the surface states of semi-infinite systems using maximally localized Wannier function. From the calculated results, we analyze the topological nature of Bi1-x Sbx, and found the followings: (1) pure Bi crystal is topologically trivial. (2) Topologically nontrivial phase can be realized by reducing the strength of SOC via Sb doping. (3) The indirect bulk band gap, which is crucial to realize the true bulk insulating phase, can be enhanced by uniaxial pressure along c axis. (4) The calculated surface states can be compared with experimental results, which confirms the topological nature. (5) We predict the spin-resolved Fermi surfaces and showed the vortex structures, which should be examined by future experiments. © 2009 The American Physical Society.

The tunneling conductance between a metal and a multiband s-wave superconductor with a thin layer of single-band s-wave superconductor sandwiched in between is examined in this paper. We show that an in-gap peak in conductance curve is found as a result of the formation of in-gap bound state between the single-band and multiband superconductor junctions if the phases of the superconducting order parameters of the multiband superconductor are frustrated. The implication of this result in determining the gap symmetry of the iron-based superconductors is discussed.

We investigate the electronic structures of the alloyed Bi1-x Sbx compounds based on first-principles calculations including spin-orbit coupling (SOC), and calculate the surface states of semi-infinite systems using maximally localized Wannier function. From the calculated results, we analyze the topological nature of Bi1-x Sbx, and found the followings: (1) pure Bi crystal is topologically trivial. (2) Topologically nontrivial phase can be realized by reducing the strength of SOC via Sb doping. (3) The indirect bulk band gap, which is crucial to realize the true bulk insulating phase, can be enhanced by uniaxial pressure along c axis. (4) The calculated surface states can be compared with experimental results, which confirms the topological nature. (5) We predict the spin-resolved Fermi surfaces and showed the vortex structures, which should be examined by future experiments. © 2009 The American Physical Society.

The tunneling conductance between a metal and a multiband s-wave superconductor with a thin layer of single-band s-wave superconductor sandwiched in between is examined in this paper. We show that an in-gap peak in conductance curve is found as a result of the formation of in-gap bound state between the single-band and multiband superconductor junctions if the phases of the superconducting order parameters of the multiband superconductor are frustrated. The implication of this result in determining the gap symmetry of the iron-based superconductors is discussed.