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Two-dimensional magnetic materials with tunable physical parameters are emerging as potential candidates for topological phenomena as well as applications in spintronics. The famous Mermin-Wagner theorem states that spontaneous spin symmetry cannot be broken at finite temperature in low dimensional magnetic systems which forbids the possibility of a transition to a long-range ordered state in a two-dimensional magnetic system at finite temperature. Though, there are some exceptions to Mermin-Wagner theorem in particular low dimensional magnetic systems with topologically ordered phase transitions. Here, we present an in-depth temperature dependent analysis for the bulk single crystals of two-dimensional (Mn 1-x Ni x ) 2 P 2 S 6 with x = 1, 0.7, 0.3, 0 using the Raman spectroscopy supported by first-principles calculations of the phonon frequencies. We observed multiple phase transitions with tunability as a function of doping associated with the short and long-range spin-spin correlations. First transition at ~ 150 K to ~ 170 K for x = 0 to x = 0.7, and second one from ~ 60 K to ~ 153 K. Quite interestingly, a third transition is observed at low temperature (much below their respective T N ) ~ 24 K to ~ 60 K and is attributed to the potential topological phase transition. These transitions are marked by the distinct changes observed in the temperature evolution of the phonon self-energy parameters, modes intensity and dynamic Raman susceptibility., Competing Interests: Declarations. Competing interests: The authors declare no competing interests., (© 2025. The Author(s).)

Topological superconductivity is a promising concept for generating fault-tolerant qubits. Early experimental studies looked at hybrid systems and doped intrinsic topological or superconducting materials at very low temperatures. However, higher critical temperatures are indispensable for technological exploitation. Recent angle-resolved photoemission spectroscopy results have revealed that superconductivity in the type-I Weyl semimetal-trigonal PtBi 2 (t-PtBi 2 )-is located at the Fermi-arc surface states, which renders the material a potential candidate for intrinsic topological superconductivity. Here we show, using scanning tunnelling microscopy and spectroscopy, that t-PtBi 2 presents surface superconductivity at elevated temperatures (5 K). The gap magnitude is elusive: it is spatially inhomogeneous and spans from 0 to 20 meV. In particular, the large gap value and the shape of the quasiparticle excitation spectrum resemble the phenomenology of high-T c superconductors. To our knowledge, this is the largest superconducting gap so far measured in a topological material. Moreover, we show that the superconducting state at 5 K persists in magnetic fields up to 12 T., Competing Interests: Competing interests The authors declare no competing interests., (© 2024. The Author(s).)

An essential ingredient for the production of Majorana fermions for use in quantum computing is topological superconductivity 1,2 . As bulk topological superconductors remain elusive, the most promising approaches exploit proximity-induced superconductivity 3 , making systems fragile and difficult to realize 4-7 . Due to their intrinsic topology 8 , Weyl semimetals are also potential candidates 1,2 , but have always been connected with bulk superconductivity, leaving the possibility of intrinsic superconductivity of their topological surface states, the Fermi arcs, practically without attention, even from the theory side. Here, by means of angle-resolved photoemission spectroscopy and ab initio calculations, we identify topological Fermi arcs on two opposing surfaces of the non-centrosymmetric Weyl material trigonal PtBi 2 (ref.  9 ). We show these states become superconducting at temperatures around 10 K. Remarkably, the corresponding coherence peaks appear as the strongest and sharpest excitations ever detected by photoemission from solids. Our findings indicate that superconductivity in PtBi 2 can occur exclusively at the surface, rendering it a possible platform to host Majorana modes in intrinsically topological superconductor-normal metal-superconductor Josephson junctions., (© 2024. The Author(s).)

An ultrasharp photoluminescence line intimately related to antiferromagnetic order has been found in NiPS_{3}, a correlated van der Waals material, opening prospects for magneto-optical coupling schemes and spintronic applications. Here we unambiguously clarify the singlet origin of this excitation, confirming its roots in the spin structure. Based on a comprehensive investigation of the electronic structure using angle-resolved photoemission and q-dependent electron energy loss spectroscopy as experimental tools we develop, in a first step, an adequate theoretical understanding using density functional theory (DFT). In a second step the DFT is used as input for a dedicated multiplet theory by which we achieve excellent agreement with available multiplet spectroscopy. Our Letter connects the understanding of the electronic structure and of optical processes in NiPS_{3} and related materials as a prerequisite for further progress of the field.

Symmetry breaking in topological matter has become in recent years a key concept in condensed matter physics to unveil novel electronic states. In this work, we predict that broken inversion symmetry and strong spin-orbit coupling in trigonal PtBi 2 lead to a type-I Weyl semimetal band structure. Transport measurements show an unusually robust low dimensional superconductivity in thin exfoliated flakes up to 126 nm in thickness (with T c ∼ 275-400 mK), which constitutes the first report and study of unambiguous superconductivity in a type-I Weyl semimetal. Remarkably, a Berezinskii-Kosterlitz-Thouless transition with T BKT ∼ 310 mK is revealed in up to 60 nm thick flakes, which is nearly an order of magnitude thicker than the rare examples of two-dimensional superconductors exhibiting such a transition. This makes PtBi 2 an ideal platform to study low dimensional and unconventional superconductivity in topological semimetals.

Fermi surfaces are essential for predicting, characterizing and controlling the properties of crystalline metals and semiconductors. Angle-resolved photoemission spectroscopy (ARPES) is the only technique directly probing the Fermi surface by measuring the Fermi momenta (k F ) from energy- and angular distribution of photoelectrons dislodged by monochromatic light. Existing apparatus is able to determine a number of k F -vectors simultaneously, but direct high-resolution 3D Fermi surface mapping remains problematic. As a result, no such datasets exist, strongly limiting our knowledge about the Fermi surfaces. Here we show that using a simpler instrumentation it is possible to perform 3D-mapping within a very short time interval and with very high resolution. We present the first detailed experimental 3D Fermi surface as well as other experimental results featuring advantages of our technique. In combination with various light sources our methodology and instrumentation offer new opportunities for high-resolution ARPES in the physical and life sciences., (© 2022. The Author(s).)

Layered van der Waals materials of the family TaTMTe 4 (TM = Ir, Rh, Ru) are showing interesting electronic properties. We report the growth and characterization of TaIrTe 4 , TaRhTe 4 , TaIr 1- x Rh x Te 4 ( x = 0.06, 0.14, 0.78, 0.92), Ta 1+ x Ru 1- x Te 4 single crystals. X-ray powder diffraction confirms that TaRhTe 4 is isostructural to TaIrTe 4 . All these compounds are metallic with diamagnetic behavior. Below T ≈ 4 K we observed signatures of the superconductivity in the TaIr 1- x Rh x Te 4 compounds for x = 0.92. All samples show weak quadratic-in-field magnetoresistance (MR). However, for TaIr 1- x Rh x Te 4 with x ≈ 0.78, the MR has a linear term dominating in low fields that indicates the presence of Dirac cones in the vicinity of the Fermi energy. For TaRhTe 4 series the MR is almost isotropic. Electronic structure calculations for TaIrTe 4 and TaRhTe 4 reveal appearance of the Rh band close to the Fermi level.

We present a systematic study of the evolution of structural parameters and electronic correlations as a function of 3d band filling in a single crystal series of BaT 2 As 2 (T = Cr-Cu). The structure trends are discussed in relation to the orbital occupation of the corresponding d elements supported by calculations of the charge density and electron localization function. Analysis of our specific heat data yields the mass enhancement ( m */ m b ) throughout the series. By combining the structural data with the mass enhancement values, we find that the decrease in m */ m b for n > 5 follows an increase of the crystal field splitting, determined by the progressive distortion of the As-T-As angle from the ideal tetrahedral environment. This study finds a strong interplay between crystal structure, bonding behavior, band filling, and electronic properties.

The nematic phase in iron based superconductors (IBSs) has attracted attention with a notion that it may provide important clue to the superconductivity. A series of angle-resolved photoemission spectroscopy (ARPES) studies were performed to understand the origin of the nematic phase. However, there is lack of ARPES study on LaFeAsO nematic phase. Here, we report the results of ARPES studies of the nematic phase in LaFeAsO. Degeneracy breaking between the [Formula: see text] and [Formula: see text] hole bands near the [Formula: see text] and M point is observed in the nematic phase. Different temperature dependent band splitting behaviors are observed at the [Formula: see text] and M points. The energy of the band splitting near the M point decreases as the temperature decreases while it has little temperature dependence near the [Formula: see text] point. The nematic nature of the band shift near the M point is confirmed through a detwin experiment using a piezo device. Since a momentum dependent splitting behavior has been observed in other iron based superconductors, our observation confirms that the behavior is a universal one among iron based superconductors.

We report a systematic elastoresistivity study on LaFe_{1-x}Co_{x}AsO single crystals, which have well separated structural and magnetic transition lines. All crystals show a Curie-Weiss-like nematic susceptibility in the tetragonal phase. The extracted nematic temperature is monotonically suppressed upon cobalt doping, and changes sign around the optimal doping level, indicating a possible nematic quantum critical point beneath the superconducting dome. The amplitude of the nematic susceptibility shows a peculiar double-peak feature. This could be explained by a combined effect of different contributions to the nematic susceptibility, which are amplified at separated doping levels of LaFe_{1-x}Co_{x}AsO.

The ternary iron arsenide compound BaFe 2 As 2 exhibits a structural phase transition from tetragonal to orthorhombic at a temperature of about 140 K. The twin lamellae arising below this transition temperature were studied in undoped single crystalline bulk and epitaxial thin film samples using electron backscatter diffraction in a scanning electron microscope equipped with a helium cryostat. Applying this technique on bulk single crystals a characteristic twin lamella size in the range of 0.1 μm up to a few μm was observed. In contrast, in epitaxially strained thin films the phase transition is not observed at temperatures above 19 K., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Novel functionalities may be achieved in oxide electronics by appropriate stacking of planar oxide layers of different metallic species, MO p and M'O q . The simplest mechanism allowing the tailoring of the electronic states and physical properties of such heterostructures is of electrostatic nature-charge imbalance between the M and M' cations. Here we clarify the effect of interlayer electrostatics on the anisotropic Kitaev exchange in H 3 LiIr 2 O 6 , a recently proposed realization of the Kitaev spin liquid. By quantum chemical calculations, we show that the precise position of H + cations between magnetically active [LiIr 2 O 6 ] 3- honeycomb-like layers has a strong impact on the magnitude of Kitaev interactions. In particular, it is found that stacking with straight interlayer O-H-O links is detrimental to in-plane Kitaev exchange since coordination by a single H-ion of the O ligand implies an axial Coulomb potential at the O site and unfavorable polarization of the O 2p orbitals mediating the Ir-Ir interactions. Our results therefore provide valuable guidelines for the rational design of Kitaev quantum magnets, indicating unprecedented Kitaev interactions of ≈40 meV if the linear interlayer linkage is removed.

The Hall effect is a powerful tool for investigating carrier type and density. For single-band materials, the Hall coefficient is traditionally expressed simply by , where e is the charge of the carrier, and n is the concentration. However, it is well known that in the critical region near a quantum phase transition, as it was demonstrated for cuprates and heavy fermions, the Hall coefficient exhibits strong temperature and doping dependencies, which can not be described by such a simple expression, and the interpretation of the Hall coefficient for Fe-based superconductors is also problematic. Here, we investigate thin films of Ba(Fe1-xCox)2As2 with compressive and tensile in-plane strain in a wide range of Co doping. Such in-plane strain changes the band structure of the compounds, resulting in various shifts of the whole phase diagram as a function of Co doping. We show that the resultant phase diagrams for different strain states can be mapped onto a single phase diagram with the Hall number. This universal plot is attributed to the critical fluctuations in multiband systems near the antiferromagnetic transition, which may suggest a direct link between magnetic and superconducting properties in the BaFe2As2 system.

Measurements of magneto-resistivity and magnetic susceptibility were performed on single crystals of superconducting Ba(Fe0.9Co0.1)2As2 close to the conditions of optimal doping. The high quality of the investigated samples allows us to reveal dynamic scaling behaviour associated with a vortex-glass phase transition in the limit of a weak degree of quenched disorder. Accordingly, the dissipative component of the ac susceptibility is reproduced well within the framework of Havriliak-Negami relaxation, assuming a critical power-law divergence for the characteristic correlation time τ of the vortex dynamics. Remarkably, the random disorder introduced by the Fe1-xCox chemical substitution is found to act on the vortices as a much weaker quenched disorder than previously reported for cuprate superconductors such as Y1-xPrxBa2Cu3O7-δ.

Superconducting (SC) and non-superconducting (non-SC)Rb(x)Fe(2-y)Se2 crystals were grown using the "self-flux" technique in order to assign the microstructural changes to the onset of superconductivity in complex iron selenides. The crystals were thoroughly characterized by magnetic susceptibility and transport measurements as well as powder X-ray diffraction. Special attention was paid to the comparison of the microstructure of the crystals with and without the superconducting transition by means of transmission electron microscopy (TEM). It is shown that the alternation of ordered and disordered regions on the sample surface and along the c-axis is characteristic for both SC and non-SC materials and therefore does not necessarily represent a trigger of superconductivity. Three types of electron diffraction patterns were found for the superconducting Rb(x)Fe(2-y)Se2 sample, of which one is observed for the first time and originates from alkali metal ordering. Moreover, for the superconducting Rb(x)Fe(2-y)Se2 material a monoclinic distortion with β ∼ 87° was observed, leading to the space group I2/m. This monoclinic distortion seems to be an attribute of the superconducting material only, whereas in the non-superconducting sample the orthogonality of the crystallographic axes is preserved.

In this work we present the results of the bulk magnetization measurements in a superconducting state of single crystals of Ba0.65Na0.35Fe2As2. The isothermal magnetic field (H ∥ c axis) dependent magnetization (M) loops exhibit a second peak (SP) or 'fishtail effect', as well as remarkable flux jumps at low temperatures. The critical current density Jc obtained from the M(H) loops is rather high, of the order of 10(6) A cm(-2). The analysis of the temperature- and field-dependent Jc implies that high Jc is mainly due to collective (weak) pinning of vortices by dense microscopic point defects with some contribution from a strong pinning mechanism. Pronounced magnetic instabilities in terms of flux jumps depend strongly on temperature as well as on the field sweep rate. The field for the first flux jump as calculated from an adiabatic model, however, is much lower than the experimentally observed values, and this enhanced stability is attributed to a flux creep phenomenon. The analysis of field-dependent magnetic relaxation data additionally supports a collective pinning model. The data further suggest that SP in M(H) is likely related to the crossover in creep dynamics from an elastic to a plastic mechanism. We have constructed the vortex phase diagram on the field-temperature plane.