Orbit topology analyzed from π phase shift of magnetic quantum oscillations in three-dimensional Dirac semimetal.

With the emergence of Dirac fermion physics in the field of condensed matter, magnetic quantum oscillations (MQOs) have been used to discern the topology of orbits in Dirac materials. However, many previous researchers have relied on the single-orbit Lifshitz-Kosevich (LK) formula, which overlooks the significant effect of degenerate orbits on MQOs. Since the single-orbit LK formula is valid for massless Dirac semimetals with small cyclotron masses, it is imperative to generalize the method applicable to a wide range of Dirac semimetals, whether massless or massive. This report demonstrates how spin-degenerate orbits affect the phases in MQOs of three-dimensional massive Dirac semimetal, NbSb2 With varying the direction of the magnetic field, an abrupt π phase shift is observed due to the interference between the spin-degenerate orbits. We investigate the effect of cyclotron mass on the π phase shift and verify its close relation to the phase from the Zeeman coupling. We find that the π phase shift occurs when the cyclotron mass is half of the electron mass, indicating the effective spin gyromagnetic ratio as g s = 2. Our approach is not only useful for analyzing MQOs of massless Dirac semimetals with a small cyclotron mass but also can be used for MQOs in massive Dirac materials with degenerate orbits, especially in topological materials with a sufficiently large cyclotron mass. Furthermore, this method provides a useful way to estimate the precise g s value of the material.

Large anomalous Hall current induced by topological nodal lines in a ferromagnetic van der Waals semimetal.

Topological semimetals host electronic structures with several band-contact points or lines and are generally expected to exhibit strong topological responses. Up to now, most work has been limited to non-magnetic materials and the interplay between topology and magnetism in this class of quantum materials has been largely unexplored. Here we utilize theoretical calculations, magnetotransport and angle-resolved photoemission spectroscopy to propose Fe3GeTe2, a van der Waals material, as a candidate ferromagnetic (FM) nodal line semimetal. We find that the spin degree of freedom is fully quenched by the large FM polarization, but the line degeneracy is protected by crystalline symmetries that connect two orbitals in adjacent layers. This orbital-driven nodal line is tunable by spin orientation due to spin-orbit coupling and produces a large Berry curvature, which leads to a large anomalous Hall current, angle and factor. These results demonstrate that FM topological semimetals hold significant potential for spin- and orbital-dependent electronic functionalities.

Colossal Seebeck Coefficient of Hopping Electrons in (TMTSF)_{2} PF_{6}.

We report on a study of the Seebeck coefficient and resistivity in the quasi-one-dimensional conductor (TMTSF)_{2} PF_{6} extended deep into the spin-density-wave state. The metal-insulator transition at T_{SDW}=12 K leads to a reduction in carrier concentration by 7 orders of magnitude. Below 1 K, charge transport displays the behavior known as variable range hopping. Until now, the Seebeck response of electrons in this regime has barely been explored and is even less understood. We find that, in this system, residual carriers, hopping from one trap to another, generate a Seebeck coefficient as large as 400 k_{B}/e. The results provide the first solid evidence for a long-standing prediction according to which hopping electrons in the presence of the Coulomb interaction can generate a sizable Seebeck coefficient in the zero-temperature limit.

Origin of the Large Anisotropic g Factor of Holes in Bismuth.

The ratio of the Zeeman splitting to the cyclotron energy (M=ΔE_{Z}/ℏω_{c}) for holelike carriers in bismuth has been quantified with great precision by many experiments performed during the past five decades. It exceeds 2 when the magnetic field is along the trigonal axis and vanishes in the perpendicular configuration. Theoretically, however, M is expected to be isotropic and equal to unity in a two-band Dirac model. We argue that a solution to this half-a-century-old puzzle can be found by extending the k·p theory to multiple bands. Our model not only gives a quantitative account of the magnitude and anisotropy of M for holelike carriers in bismuth, but also explains its contrasting evolution with antimony doping and pressure, both probed by new experiments reported here. The present results have important implications for the magnitude and anisotropy of M in other systems with strong spin-orbit coupling.

Cernox thermometer under hydrostatic pressure for enhanced temperature accuracy.

Sensors that can be used in a pressurized environment are few. Thus, it is generally considered that the accuracy of temperature measurements decreases in experiments carried out under pressure. Based on a commercially available cryogenic temperature sensor, Cernox from Lake Shore Cryotronics, Inc., we developed and tested an enclosure that enables Cernox to be directly used inside a cylindrical pressure cell, next to the specimen under investigation. To isolate the Cernox thermometer from the pressurized medium inside the pressure cell, we combined the principles of an encapsulated Pt sensor and a cylindrical clamped piston cell. The encapsulated Cernox allows for precise measurement and control of the specimen's temperature. It is also beneficial for accurately determining small changes in physical properties, such as temperature, or measuring the amount of hysteresis in the first-order phase transition. It would also be useful for accurately controlling the sample's temperature when the sample approaches the transition temperature from one side.

Electrochemical fabrication of (TMTSF)2X (X = PF6, BF4, CIO4) nanowires.

TMTSF-based (TMTSF = tetramethyltetraselenafulvalene = C10H12Se4) charge-transfer salt nanowires were fabricated using the galvanostatic deposition technique that was assisted by an anodic aluminum oxide (AAO) template. By applying a low current density of 1-2 microA/cm2 for more than one month, nanowire arrays with diameters of approximately 150 nm and lengths of approximately 6 microm were obtained. The length of nanowires can be controlled by the duration of the constant current application. Energy-dispersive X-ray spectroscopic (EDX) analysis confirmed that selenium is one of the main components of the nanowires. The micro-Raman (v3C == C) and FT-IR spectra (v3PF6-, v3BF4-, v3CIO4-) indicated that the nanowire arrays had the (TMTSF)2X (X = PF6, BF4, CIO4) phase. The TEM images and the selected area electron diffraction (SAED) patterns indicate that the nanowires were not single crystals, but the current-voltage characteristic that was measured with the four-terminal method showed the conductivity of the (TMTSF)2PF6 single crystals (sigmaRT = 1.6 S/cm) at room temperature.

Boundary conductance in macroscopic bismuth crystals.

The interface between a solid and vacuum can become electronically distinct from the bulk. This feature, encountered in the case of quantum Hall effect, has a manifestation in insulators with topologically protected metallic surface states. Non-trivial Berry curvature of the Bloch waves or periodically driven perturbation are known to generate it. Here, by studying the angle-dependent magnetoresistance in prismatic bismuth crystals of different shapes, we detect a robust surface contribution to electric conductivity when the magnetic field is aligned parallel to a two-dimensional boundary between the three-dimensional crystal and vacuum. The effect is absent in antimony, which has an identical crystal symmetry, a similar Fermi surface structure and equally ballistic carriers, but an inverted band symmetry and a topological invariant of opposite sign. Our observation confirms that the boundary interrupting the cyclotron orbits remains metallic in bismuth, which is in agreement with what was predicted by Azbel decades ago. However, the absence of the effect in antimony indicates an intimate link between band symmetry and this boundary conductance.

Low-field magnetic anisotropy of Sr2IrO4.

Magnetic anisotropy in strontium iridate (Sr2IrO4) is essential because of its strong spin-orbit coupling and crystal field effect. In this paper, we present a detailed mapping of the out-of-plane (OOP) magnetic anisotropy in Sr2IrO4for different sample orientations using torque magnetometry measurements in the low-magnetic-field region before the isospins are completely ordered. Dominant in-plane anisotropy was identified at low fields, confirming thebaxis as an easy magnetization axis. Based on the fitting analysis of the strong uniaxial magnetic anisotropy, we observed that the main anisotropic effect arises from a spin-orbit-coupled magnetic exchange interaction affecting the OOP interaction. The effect of interlayer exchange interaction results in additional anisotropic terms owing to the tilting of the isospins. The results are relevant for understanding OOP magnetic anisotropy and provide a new way to analyze the effects of spin-orbit-coupling and interlayer magnetic exchange interactions. This study provides insight into the understanding of bulk magnetic, magnetotransport, and spintronic behavior on Sr2IrO4for future studies.

Quantum transport evidence of isolated topological nodal-line fermions.

Anomalous transport responses, dictated by the nontrivial band topology, are the key for application of topological materials to advanced electronics and spintronics. One promising platform is topological nodal-line semimetals due to their rich topology and exotic physical properties. However, their transport signatures have often been masked by the complexity in band crossings or the coexisting topologically trivial states. Here we show that, in slightly hole-doped SrAs3, the single-loop nodal-line states are well-isolated from the trivial states and entirely determine the transport responses. The characteristic torus-shaped Fermi surface and the associated encircling Berry flux of nodal-line fermions are clearly manifested by quantum oscillations of the magnetotransport properties and the quantum interference effect resulting in the two-dimensional behaviors of weak antilocalization. These unique quantum transport signatures make the isolated nodal-line fermions in SrAs3 desirable for novel devices based on their topological charge and spin transport.

Antiferromagnet-Based Spintronic Functionality by Controlling Isospin Domains in a Layered Perovskite Iridate.

The novel electronic state of the canted antiferromagnetic (AFM) insulator strontium iridate (Sr2 IrO4 ) is well described by the spin-orbit-entangled isospin Jeff = 1/2, but the role of isospin in transport phenomena remains poorly understood. In this study, antiferromagnet-based spintronic functionality is demonstrated by combining the unique characteristics of the isospin state in Sr2 IrO4 . Based on magnetic and transport measurements, a large and highly anisotropic magnetoresistance (AMR) is obtained by manipulating the AFM isospin domains. First-principles calculations suggest that electrons whose isospin directions are strongly coupled to the in-plane net magnetic moment encounter an isospin mismatch when moving across the AFM domain boundaries, which generates a high resistance state. By rotating a magnetic field that aligns in-plane net moments and removes domain boundaries, the macroscopically ordered isospins govern dynamic transport through the system, which leads to the extremely angle-sensitive AMR. As this work establishes a link between isospins and magnetotransport in strongly spin-orbit-coupled AFM Sr2 IrO4 , the peculiar AMR effect provides a beneficial foundation for fundamental and applied research on AFM spintronics.

New 1:3 type nickel-bis(dithiolene) salt (FcCHCHPymCH3)[Ni(dmit)2]3 (dmit: 2-thioxo-1,3-dithiole-4,5-dithiolate): its electrocrystallization, crystal structure, electrical properties, and electronic band structure analysis.

Black single crystals of Ni(dmit)(2) complex (dmit: 2-thioxo-1,3-dithiole-4,5-dithiolate) with trans-4-[2-(1-ferrocenyl)vinyl]-1-methylpyridinium chromophore as a countercation, (FcCHCHPymCH(3))[Ni(dmit)(2)](3), were prepared by the electrocrystallization technique. In the triclinic structure of the complex (P, a = 11.430(5) A, b = 13.349(2) A, c = 19.355(6) A, alpha = 75.15(2) degrees , beta = 79.19(3) degrees , gamma = 82.12(2) degrees , Z = 2), Ni(dmit)(2) anion layers are separated by the cations with a relatively rare 1:3 cation-to-anion ratio. Detailed crystal and electronic structure analysis revealed that the anions are stacked in the layers to form alternating dimers and monomers rather than trimers. The measured electrical conductivity indicates a semiconducting property of the compound with an estimated energy gap of 0.06 eV. The calculated LUMO bands are very narrow, and the semiconducting behavior is more likely due to the electron localization mainly on the dimers, consistent with the observed longer Ni-S bond distances in the dimers.