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A new class of kagome compounds exhibits several intriguing properties, including a novel charge-density-wave state. Quantum oscillation experiments reveal the origin of this state.

We report a comprehensive magnetization and magnetotransport study on a metallic-type of ${\mathrm{EuMnSb}}_{2}$ single crystals. A large Hall signal up to room temperature is found in our metallic ${\mathrm{EuMnSb}}_{2}$, which is dominated by hole carriers with very low carrier density and high mobility. Clear Shubnikov--de Haas oscillations are observed at low temperatures (T $l$ 30 K), from which a dominant tiny hole pocket with small effective mass is deduced. The interplay of carrier density and Mn and Eu moments results in complex magnetic ground states with two successive antiferromagnetic (AFM) transitions appearing at ${T}_{N1}=24 \mathrm{K}$ and ${T}_{N2}=9 \mathrm{K}$. In line with the rich magnetic phases, the transport properties, including magnetoresistance, quantum oscillation, and Berry phase, experience abrupt disturbances when crossing the magnetic phase boundaries. The Dirac state is likely quenched once the AFM orders set in, indicating a strong coupling between the AFM magnetic order and the band topology. Our results suggest that metallic-type ${\mathrm{EuMnSb}}_{2}$ is an ideal platform for the study of the interplay between magnetism, charge transport, and band topology.

Calculation of conductivity in the Hubbard model is a challenging task. Recent years have seen much progress in this respect and numerically exact solutions are now possible in certain regimes. In this paper we discuss the calculation of conductivity for the square lattice Hubbard model in the presence of a perpendicular magnetic field, focusing on orbital effects. We present the relevant formalism in all detail and in full generality, and then discuss the simplifications that arise at the level of the dynamical mean field theory (DMFT). We prove that the Kubo bubble preserves gauge and translational invariance, and that in the DMFT the vertex corrections cancel regardless of the magnetic field. We present the DMFT results for the spectral function and both the longitudinal and Hall conductivity in several regimes of parameters. We analyze thoroughly the quantum oscillations of the longitudinal conductivity and identify a high-frequency oscillation component, arising as a combined effect of scattering and temperature, in line with recent experimental observations in moir\'e systems., Comment: 32 pages, 21 figures

At high temperatures, quantum oscillations are predicted to emerge in materials containing correlated electrons, with the oscillations behaving differently from those seen at low temperatures.

The identification of a non-trivial band topology usually relies on directly probing the protected surface/edge states. But, it is difficult to achieve electronically in narrow-gap topological materials due to the small (meV) energy scales. Here, we demonstrate that band inversion, a crucial ingredient of the non-trivial band topology, can serve as an alternative, experimentally accessible indicator. We show that an inverted band can lead to a four-fold splitting of the non-zero Landau levels, contrasting the two-fold splitting (spin splitting only) in the normal band. We confirm our predictions in magneto-transport experiments on a narrow-gap strong topological insulator, zirconium pentatelluride (ZrTe5), with the observation of additional splittings in the quantum oscillations and also an anomalous peak in the extreme quantum limit. Our work establishes an effective strategy for identifying the band inversion as well as the associated topological phases for future topological materials research., It remains difficult to identify non-trivial band topology through electronic transport in narrow-gap topological materials. Here, the authors demonstrate that an inverted band leads to a four-fold splitting of non-zero Landau levels and an anomalous zero Landau level peak in the density of states of ZrTe5, hence can serve as an indicator to explore non-trivial topology.

Metals composed of weakly-coupled, stacked layers possess a Fermi surface that slightly varies in size along the stacking direction. This appears in de Haas-van Alphen (dHvA) oscillations of the magnetisation with magnetic field as two close frequencies, corresponding to the two extremal Fermi surface cross-sectional areas. We show that, for layered materials of sufficiently high mobility, Coulomb interactions can have a dramatic effect on the form of the dHvA oscillations: there is also generically an oscillation at the small difference of the two large frequencies. We determine the size and form of this effect, and show that it probes the short-range part of the Coulomb interactions within the layered material. We argue that this interaction effect may explain recent experimental observations of anomalous low-frequency dHvA oscillations in the ultrapure delafossites., 5 + 5 pages, 1 figure

Quantum spin liquids are exotic states of matter that may be realized in frustrated quantum magnets and feature fractionalized excitations and emergent gauge fields. Here, we consider a gapless U(1) spin liquid with spinon Fermi pockets in two spatial dimensions. Such a state appears to be the most promising candidate to describe the exotic field-induced behavior observed in numerical simulations of the antiferromagnetic Kitaev honeycomb model. A similar such state may also be responsible for the recently-reported quantum oscillations of the thermal conductivity in the field-induced quantum paramagnetic phase of $\alpha$-RuCl$_3$. We consider the regime close to a Lifshitz transition, at which the spinon Fermi pockets shrink to small circles around high-symmetry points in the Brillouin zone. By employing renormalization group and mean-field arguments, we demonstrate that interactions lead to a gap opening in the spinon spectrum at low temperatures, which can be understood as a nesting instability of the spinon Fermi surface. This leads to proliferation of monopole operators of the emergent U(1) gauge field and confinement of spinons. While signatures of fractionalization may be observable at finite temperatures, the gapless U(1) spin liquid state with nested spinon Fermi pockets is ultimately unstable at low temperatures towards a conventional long-range-ordered ground state, such as a valence bond solid. Implications for Kitaev materials in external magnetic fields are discussed., Comment: 19 pages, 7 figures, 1 table; v2: added several minor improvements and references

Novel compounds with two-dimensional square lattices formed by group IV or V elements (Si, Sn, Ge, Bi, and Sb) have been attracting a great deal of attention in recent times, mainly because of the possible emergence of various topological phases therein. Here, we successfully grow the single crystals of Sn-square-net based material $\mathrm{Sc}{\mathrm{Sn}}_{2}$, and systematically perform their magnetization and magnetotransport measurements. Clear quantum oscillations emerge in the magnetization isotherms along different field orientations, from which nonzero Berry phases are extracted, implying that $\mathrm{Sc}{\mathrm{Sn}}_{2}$ harbors three-dimensional Fermi surfaces and nontrivial electronic states. Similar to many other topological semimetals with extremely large magnetoresistance, $\mathrm{Sc}{\mathrm{Sn}}_{2}$ shows field-induced resistivity enhancement as well, which has been proven to be not of a gap opening origin. Besides, at low temperature, large magnetoresistance with a quasilinear field dependence is observed. Our analysis of the magnetotransport data finds that the quasilinear magnetoresistance in $\mathrm{Sc}{\mathrm{Sn}}_{2}$ cannot be understood by several familiar mechanisms proposed in the literature. These findings are expected to have far reaching implications for both the fundamental understanding and magnetoresistance device application of topological semimetal materials.

A Weyl semimetal (WSM)\ is a three-dimensional topological phase of matter where pairs of nondegenerate bands cross at isolated points in the Brillouin zone called Weyl nodes. Near these points, the electronic dispersion is gapless and linear. A magnetic field $B$ changes this dispersion into a set of Landau levels which are dispersive along the direction of the magnetic field only. The $n=0$ Landau level is special since its dispersion$\ $is linear and unidirectional. The presence of this chiral level distinguishes Weyl from Schr\"{o}dinger fermions. In this paper, we study the quantum oscillations of the orbital magnetization and magnetic susceptibility in Weyl semimetals. We generalise earlier works% \cite{Mikitik2019} on these De Haas-Van Alphen oscillations by considering the effect of a tilt of the Weyl nodes. We study how the fundamental period of the oscillations in the small $B$ limit and the strength of the magnetic field $B_{1}$ required to reach the quantum limit are modified by the magnitude and orientation of the tilt vector $\mathbf{t}$. We show that the magnetization from a single node is finite in the $B\rightarrow 0$ limit. Its sign depends on the product of the chirality and sign of the tilt component along the magnetic field direction. We also study the magnetic oscillations from a pair of Weyl nodes with opposite chirality and with opposite or identical tilt. Our calculation shows that these two cases lead to a very different behavior of the magnetization in the small and large $B$ limits. We finally consider the effect of an energy shift $\pm \Delta _{0}$ of a pair of Weyl nodes on the magnetic oscillations. We assume a constant density of carriers so that both nodes share a common Fermi level. Our calculation can easily be extended to a WSM with an arbitrary number of pairs of Weyl nodes., Comment: 15 pages with 11 figures

The topological effect in thermoelectric transport is an important research aspect in semimetals, but whether an anomalous Nernst effect exists in nonmagnetic topological semimetals in a finite external field is still under debate. To demonstrate how to discern the topological effect in nonmagnetic topological semimetals, we present a comprehensive study on the magnetothermoelectric properties for four Weyl semimetals: TaAs, TaP, NbAs, and NbP. We observe large magneto-Seebeck and Nernst signals at intermediate temperatures, which are attributed to a multiband ordinary contribution and an inelastic, phonon-drag effect, while the latter is ignored in previous studies. This phonon-drag effect also induces an unusual, prominent temperature and field dependence of quantum oscillations in thermoelectric transport signals. On the other hand, only signatures of a relatively small anomalous thermoelectric effect are found in TaAs compared with the ordinary effect.

${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ is a correlated and spin-orbit coupled system with an extraordinary anisotropy. It is both interesting and unique largely because this material exhibits conflicting phenomena that are often utterly inconsistent with traditional precedents, particularly, the quantum oscillations in the nonmetallic state and colossal magnetoresistivity achieved by avoiding a fully spin-polarized state. This work focuses on the relationship between the lattice and transport properties along each crystalline axis and reveals that application of magnetic field, $H$, along different crystalline axes readily $\mathit{stretches}$ or $\mathit{shrinks}$ the lattice in a uniaxial manner, resulting in distinct electronic states. Furthermore, application of modest pressure drastically amplifies the $\mathit{anisotropic}$ magnetoelastic effect, leading to either an occurrence of a robust metallic state at $H||\mathrm{hard}$ axis or a reentrance of the nonmetallic state at $H||\mathrm{easy}$ axis. ${\mathrm{Ca}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ presents a rare lattice-dependent magnetotransport mechanism, in which the extraordinary lattice flexibility enables an exquisite control of the electronic state via magnetically stretching or shrinking the crystalline axes, and the spin polarization plays an unconventional role unfavorable for maximizing conductivity. At the heart of the intriguing physics is the anisotropic magnetostriction that leads to exotic states.

Superconductivity in topological quantum materials is much sought after, as it represents the key avenue to searching for topological superconductors, which host a full pairing gap in the bulk but Majorana bound states at the surface. To date, however, the simultaneous realization of nontrivial band topology and superconductivity in the same material under ambient conditions remains rare. In this paper, we study both superconducting and topological properties of the binary compound ${\mathrm{PtPb}}_{4}$ $({T}_{c}\ensuremath{\sim}2.7 \mathrm{K})$ that was recently reported to exhibit large Rashba splitting, inherent to the heavy $5d$ Pt and $6p$ Pb. We show that in ${\mathrm{PtPb}}_{4}$, the specific heat jump at ${T}_{c}$ reaches $\mathrm{\ensuremath{\Delta}}C/\ensuremath{\gamma}{T}_{c}\ensuremath{\sim}1.70\ifmmode\pm\else\textpm\fi{}0.04$, larger than the 1.43 expected for weak-coupling BCS superconductors. Moreover, the measurement of quantum oscillation suggests the possibility of a topological band structure, which is further studied by density functional theory calculations. Our study may stimulate future experimental and theoretical investigations in this intriguing material.

The operation of transposing particles (or holes) on one side of a Fermi surface into holes (or particles) on the other side is shown to be a symmetry operation of the Bardeen, Cooper, and Schrieffer reduced Hamiltonian, and an approximate symmetry operation of the full two-particle scalar interaction Hamiltonian, insofar as particle-conserving collective excitations are concerned. Calling this operation $f$ inversion, and the conserved quantity associated with it $f$ parity, the zero-rest-energy collective excitations in a Fermi gas with weak attractive interactions prove to have $f\mathrm{parity}=\ensuremath{-}1$.

Investigating Shubnikov-de Haas (SdH) oscillations in high magnetic fields, we experimentally infer the electronic band structure of the quasi-two-dimensional electron gas (2DEG) at the ionic-liquid gated amorphous ($a$)-${\mathrm{LaAlO}}_{3}/{\mathrm{KTaO}}_{3}$ interface. The angular dependence of SdH oscillations indicates a 2D confinement of a majority of electrons at the interface. However, additional SdH oscillations with an angle-independent frequency observed at high tilt angles indicate the coexistence of 3D charge carriers extending deep into the ${\mathrm{KTaO}}_{3}$. The SdH oscillations measured in magnetic fields perpendicular to the interface show four frequencies corresponding to four 2D subbands with different effective masses ($0.20\phantom{\rule{0.16em}{0ex}}{m}_{e}\ensuremath{-}0.55\phantom{\rule{0.16em}{0ex}}{m}_{e}$). The single-frequency oscillations originating from 3D electrons yields a larger effective mass of $\ensuremath{\sim}0.70\phantom{\rule{0.16em}{0ex}}{m}_{e}$. Overall, the inferred subbands are in good agreement with the theoretical calculations and angle-resolved photoemission spectroscopy studies on 2DEG at ${\mathrm{KTaO}}_{3}$ surface.

Unlocking the exotic properties promised to occur in topologically non-trivial semi-metals currently requires significant fine-tuning. Crystalline symmetry restricts the location of topological defects to isolated points (0D) or lines (1D), as formalized by the Wigner-Von Neumann theorem. The scarcity of materials in which these anomalies occur at the chemical potential is a major obstacle towards their applications. Here we show how non-crystalline quasi-symmetries stabilize near-degeneracies of bands over extended regions in energy and in the Brillouin zone. Specifically, a quasi-symmetry is an exact symmetry of a k∙p Hamiltonian to lower-order that is broken by higher-order terms. Hence quasi-symmetric points are gapped, yet the gap is parametrically small and therefore does not influence the physical properties of the system. We demonstrate that in the eV-bandwidth semi-metal CoSi an internal quasi-symmetry stabilizes gaps in the 1-2 meV range over a large near-degenerate plane (2D). This quasi-symmetry is key to explaining the surprising simplicity of the experimentally observed quantum oscillations of four interpenetrating Fermi surfaces around the R-point. Untethered from the limitations of crystalline symmetry, quasi-symmetries eliminate the need for fine-tuning as they enforce sources of large Berry curvature to occur at the chemical potential, and thereby lead to new Wigner-Von Neumann classifications of solids. Quasi-symmetries arise from a comparable splitting of degenerate states by spin-orbit coupling and by orbital dispersion - suggesting a hidden classification framework for symmetry groups and materials in which quasi-symmetries are critical to understand the low-energy physics.

The NV center in a diamond is a quantum sensor with exceptional quality for highly sensitive nanoscale analysis of NMR spectra and thermometry. In this study, we investigate nanoscale phase change detection of lipid bilayers utilizing ensemble-averaged nuclear spin detection from small volume ~ (6 nm)$^{3}$, which was determined by the depth of the NV center. Analysis of nanoscale NMR signal confirm thickness of lipid bilayer to be 6.2 nm $\pm$ 3.4 nm with proton density of 65 proton/nm$^{3}$ verifying formation of lipid bilayer on top of diamond sample. Correlation spectroscopy from nanoscale volume reveals quantum oscillation at 3.06 MHz corresponding to the Larmor frequency of proton at an applied magnetic field of 71.8 mT. The result of the correlation spectroscopy was compared with the 2D molecular diffusion model constructed by Monte Carlo simulation combined with results from molecular dynamics simulation. There is a change in diffusion constant from 1.5 $\pm$ 0.25 nm$^{2}$/${\mu}$s to 3.0 $\pm$ 0.5 nm$^{2}$/${\mu}$s when the temperature changes from 26.5 $^\circ$C to 36.0 $^\circ$C. Our results demonstrate that simultaneous observation of changes in translational diffusion and temperature is possible in label-free measurements using nanoscale diamond magnetometry. Our method paves the way for label-free imaging of cell membranes for understanding its phase composition and dynamics.

From measurements of the oscillatory magnetoresistance in weak perpendicular magnetic fields, we found that the quantum oscillations in two-dimensional electron systems in Si-MOS structures are observed down to the critical carrier density nc of the transition to the strongly localized state. For such low densities, the oscillations exhibit an anticipated period, phase, and amplitude, even though the conductivity becomes less than e2/h, and, hence, the mean free path becomes less than the Fermi wavelength λF. We believe that this apparent contradiction with the Ioffe-Regel criterion for diffusive transport is caused by emergence of an inhomogeneous state of the 2D system, in which the regions of diffusive and hopping conduction are spatially separated.

SmB6 has drawn much attention in recent times due to the discovery of anomalies in its ground state properties as well as prediction of topologically protected gapless surface states. Varied theories have been proposed to capture the ground state anomalies. Here, we studied the electronic structure of SmB6 employing density functional theory using different exchange correlation potentials, spin-orbit coupling and electron correlation strength. We discover that a suitable choice of interaction parameters such as spin-orbit coupling, electron correlation strength and exchange interaction within the generalized gradient approximation provides a good description of the spectral functions observed in the angle-resolved photoemission spectroscopy (ARPES) studies. The Fermi surface plots exhibit electron pockets around X-point and hole pockets around ΓX line having dominant Sm 4f character. These observations corroborate well with the recent experimental results involving quantum oscillation measurements, ARPES, etc. In addition to primarily Sm 4f contributions observed at the Fermi level, the results exhibit significantly large contribution from B 2p states compared to weak Sm 5d contributions. This suggests important role of B 2p - Sm 4f hybridization in the exotic physics of this system.

Intrinsic functionality of 2D materials is crucial for both fundamental studies and practical applications. In particular, 2D magnetism has recently emerged as a major research field bringing in new concepts, physical effects, and device designs. However, the list of stoichiometric 2D magnets is still rather short and would benefit from extension by magnetic graphene derivatives. Here, the graphite intercalation compound EuC6 is scaled down to an Eu/graphene bilayer, an approach inspired by studies of magnetic Eu/silicene (EuSi2) and Eu/germanene (EuGe2). The material, synthesized by Eu deposition on graphene, exhibits 2D ferromagnetism evidenced by low-magnetic-field control over the effective transition temperature. The ferromagnetic state is further manifested by the emergence of negative magnetoresistance and an anomalous Hall effect. Remarkably, induced magnetism does not destroy the parental electronic structure – in the Eu/graphene bilayer, quantum oscillations reveal low-mass carriers, a property inherited from graphene. The material can make an important ingredient of van der Waals heterostructures to be applied in ultracompact spintronics and quantum computing.

We investigate the quantum dynamics of neutral atoms subject to a uniform spin-dependent gauge field. In particular, we analyze a simple experimental scheme to generate the Landau quantization in a two dimensional atomic gas with internal three-level $\Lambda$-type configuration. We show how energy eigenfunctions and eigenvalues are obtained and discuss the experimental conditions for which a variety of physical quantities of the atomic gas can exhibit quantum oscillations., Comment: 11 pages, two figures

As personal electronic devices increasingly rely on cloud computing for energy-intensive calculations, the power consumption associated with the information revolution is rapidly becoming an important environmental issue. Several approaches have been proposed to construct electronic devices with low-energy consumption. Among these, the low-dissipation surface states of topological insulators (TIs) are widely employed. To develop TI-based devices, a key factor is the maximum temperature at which the Dirac surface states dominate the transport behavior. Here, we employ Shubnikov-de Haas oscillations (SdH) as a means to study the surface state survival temperature in a high-quality vanadium doped Bi1.08Sn0.02Sb0.9Te2S single crystal system. The temperature and angle dependence of the SdH show that: (1) crystals with different vanadium (V) doping levels are insulating in the 3–300 K region; (2) the SdH oscillations show two-dimensional behavior, indicating that the oscillations arise from the pure surface states; and (3) at 50 K, the V0.04 single crystals (Vx:Bi1.08-xSn0.02Sb0.9Te2S, where x = 0.04) still show clear sign of SdH oscillations, which demonstrate that the surface dominant transport behavior can survive above 50 K. The robust surface states in our V doped single crystal systems provide an ideal platform to study the Dirac fermions and their interaction with other materials above 50 K.

We present the systematic de Haas–van Alphen (dHvA) quantum oscillations studies on the recently discovered topological Dirac semimetal pyrite PtBi2 single crystals. Remarkable dHvA oscillations are emerged at a low field about 1.5 T. From the analyses of the dHvA oscillations, we extract the high quantum mobilities, light effective masses and phase shift factors for the Dirac fermions in pyrite PtBi2. From the angular dependence of the dHvA oscillations, we map out the topology of the Fermi surface. Furthermore, we identify two additional oscillation frequencies that are not probed by the SdH oscillations, which provides us with opportunities to further understand its Fermi surface topology.

The discovery of two-dimensional gapless Dirac fermions in graphene and topological insulators (TI) has sparked extensive ongoing research toward applications of their unique electronic properties. The gapless surface states in three-dimensional insulators indicate a distinct topological phase of matter with a non-trivial Z2 invariant that can be verified by angle-resolved photoemission spectroscopy or magnetoresistance quantum oscillation. In TI nanowires, the gapless surface states exhibit Aharonov-Bohm (AB) oscillations in conductance, with this quantum interference effect accompanying a change in the number of transverse one-dimensional modes in transport. Thus, while the density of states (DOS) of such nanowires is expected to show such AB oscillation, this effect has yet to be observed. Here, we adopt nanomechanical measurements that reveal AB oscillations in the DOS of a topological insulator. The TI nanowire under study is an electromechanical resonator embedded in an electrical circuit, and quantum capacitance effects from DOS oscillation modulate the circuit capacitance thereby altering the spring constant to generate mechanical resonant frequency shifts. Detection of the quantum capacitance effects from surface-state DOS is facilitated by the small effective capacitances and high quality factors of nanomechanical resonators, and as such the present technique could be extended to study diverse quantum materials at nanoscale., 15+16 pages, 4+11 figures

High field superconducting magnets, as important experimental facilities in condensed matter physics, provide high magnetic field for mapping the Fermi surface of metals, which is a basic approach to study many material properties like resistance, Hall resistance, and magnetic susceptibility. Up to now, the highest magnetic field ever achieved with superconducting magnets is 32 T, obtained at national high magnetic field laboratory (NHMFL) on Dec. 8, 2017, with a 17-T REBCO insert in a 15-T LTS background magnet. Institute of Electrical Engineering (IEE, CAS), has developed high field superconducting magnet for many years, and we successfully tested a 27.2-T superconducting magnet at IEE on Nov. 28, 2017. In the next step, we want to utilize our magnet techniques to fabricate a 30-T superconducting magnet for quantum oscillation application. The magnet will consist of a 15-T/160-mm LTS magnet and a 15-T/35-mm REBCO insert. Electromagnetic design to obtain a higher operation margin and detailed stress analysis will be introduced. Other design considerations will also be given in details and engineering issues will also be discussed.

Surface Fermi arcs (SFAs), the unique open Fermi-surfaces (FSs) discovered recently in topological Weyl semimetals (TWSs), are unlike closed FSs in conventional materials and can give rise to many exotic phenomena, such as anomalous SFA-mediated quantum oscillations, chiral magnetic effects, three-dimensional quantum Hall effect, non-local voltage generation and anomalous electromagnetic wave transmission. Here, by using in-situ surface decoration, we demonstrate successful manipulation of the shape, size and even the connections of SFAs in a model TWS, NbAs, and observe their evolution that leads to an unusual topological Lifshitz transition not caused by the change of the carrier concentration. The phase transition teleports the SFAs between different parts of the surface Brillouin zone. Despite the dramatic surface evolution, the existence of SFAs is robust and each SFA remains tied to a pair of Weyl points of opposite chirality, as dictated by the bulk topology., Surface Fermi arcs (SFAs) are characteristic features of a topological Weyl semimetal but there is no easy way to manipulate them so far. Here, the authors report manipulation of the shape, size and connections of SFAs in a Weyl semimetal NbAs, leading to an unusual topological Lifshitz transition.

We have studied the response of a layered conductor with a quasi-two-dimensional electron spectrum and a multisheet Fermi surface (FS) formed by two weakly corrugated cylinders and two planar sheets adjoining these cylinders to nonuniform heating. As a result of action of pressure on the conductor or its doping with impurity atoms, distance Δp between the FS cylinder and planar sheets can be reduced to such an extent that conduction electrons begin roaming over these sheets, tunneling from one FS sheet (cavity) to the other. If a conduction electron can visit several times all FS sheets during its mean free time, its motion in the plane orthogonal to the magnetic field becomes finite. In this case, Shubnikov–de Haas oscillations are excited with a period determined by the closed area circumscribed by the electron during its motion in the magnetic-breakdown trajectory in the momentum space. We have calculated the dependence of the thermoelectric field on the magnitude and orientation of a quite strong quantizing magnetic field. In a magnetic field normal to the layers, the cross sections of the cylindrical part of the FS are equidistant from both FS planar sheets. However, this equidistance is violated even for a small deviation of the field from the normal to the layers by angle ϑ, and, at a certain value ϑk of this angle, the probability of magnetic breakdown to one of the FS planar sheets can be so low that the electron cannot close the magnetic-breakdown trajectory, and its motion over the other planar sheet with a visit to the FS cylindrical part becomes infinite. In this case, the magnetic-breakdown quantum oscillations of magnetization and all kinetic characteristics of the conductor disappear, and their disappearance is repeated periodically with a change in the slope of the magnetic field to the layers as a function of tanϑ.

Three-dimensional (3D) topological semimetals represent a new class of topological matters. The study of this family of materials has been at the frontiers of condensed matter physics, and many breakthroughs have been made. Several topological semimetal phases, including Dirac semimetals (DSMs), Weyl semimetals (WSMs), nodal-line semimetals (NLSMs), and triple-point semimetals, have been theoretically predicted and experimentally demonstrated. The low-energy excitation around the Dirac/Weyl nodal points, nodal line, or triply degenerated nodal point can be viewed as emergent relativistic fermions. Experimental studies have shown that relativistic fermions can result in a rich variety of exotic transport properties, e.g., extremely large magnetoresistance, the chiral anomaly, and the intrinsic anomalous Hall effect. In this review, we first briefly introduce band structural characteristics of each topological semimetal phase, then review the current studies on quantum oscillations and exotic transport properties of various topological semimetals, and finally provide a perspective of this area.

We systematically study the bulk and finite size effects on the band structure of prototypical line node materials using density functional theory based computations. For the bulk system, we analyze quantum oscillations with changes in the Fermi surface topology. We show that ultra thin slabs are gapped, with the first signatures of the line node appearing beyond a critical thickness. Further, motivated by possibility of tuning the bulk line node, we find that the line node radius is rather sensitive to bulk volume change. Based on this observation, we propose that application of pressure or suitable substrate engineering can be used as an effective means to control and tune the line node and the accompanying surface drumhead states., 14 pages, 8 figures. Contribution to special issue of J. Phys. Chem. Solids on spin-orbit materials

Electron transport in single GaAs quantum wells of widths from 22 to 46 nm with two populated quantum-confinement subbands ES and EAS is investigated at a temperature of T = 4.2 K in tilted magnetic fields B 72° and α > 85°, respectively. The origin of this unexpected behavior of magnetointersubband oscillations in tilted magnetic fields is discussed.

Weyl nodes and Fermi arcs in type-II Weyl semimetals (WSMs) have led to lots of exotic transport phenomena. Recently, ${\mathrm{Mo}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{Te}}_{2}$ has been established as a type-II WSM with Weyl points located near Fermi level, which offers an opportunity to study its intriguing band structure by electrical transport measurements. Here, by selecting a special sample with the thickness gradient across two- (2D) and three-dimensional (3D) regimes, we show strong evidence that ${\mathrm{Mo}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{Te}}_{2}$ is a type-II Weyl semimetal by observing the following two dimensionality-dependent transport features: (1) a chiral-anomaly-induced anisotropic magnetoconductivity enhancement, proportional to the square of in-plane magnetic field (${B}_{\mathrm{in}}^{2}$); and (2) an additional quantum oscillation with thickness-dependent phase shift. Our theoretical calculations show that the observed quantum oscillation originates from a Weyl-orbit-like scenario due to the unique band structure of ${\mathrm{Mo}}_{0.25}{\mathrm{W}}_{0.75}{\mathrm{Te}}_{2}$. The in situ dimensionality-tuned transport experiment offers an alternative strategy to search for type-II WSMs.

We measure the Shubnikov--de Haas effect in thin-film ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ grown on an $({\mathrm{LaAlO}}_{3}{)}_{0.29}\text{\ensuremath{-}}({\mathrm{SrAl}}_{1/2}{\mathrm{Ta}}_{1/2}{\mathrm{O}}_{3}{)}_{0.71}$ substrate. We detect all three known Fermi surfaces and extract the Fermi surface volumes, cyclotron effective masses, and quantum lifetimes. We show that the electronic structure is nearly identical to that of single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, and that the quasiparticle lifetime is consistent with the ${T}_{\mathrm{c}}$ of comparably clean, single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$. Unlike single-crystal ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$, where the quantum and transport lifetimes are roughly equal, we find that the transport lifetime is $1.3\ifmmode\pm\else\textpm\fi{}0.1$ times longer than the quantum lifetime. This may suggest that extended (rather than point) defects could be the dominant source of quasiparticle scattering in these films. To test this hypothesis, we perform cross-sectional scanning transmission electron microscopy and find that out-of-phase boundaries extending the entire thickness of the film occur with a density that is consistent with the quantum mean free path. The long quasiparticle lifetimes make these films ideal for studying the unconventional superconducting state in ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ through the fabrication of devices---such as planar tunnel junctions and superconducting quantum interference devices.

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 Lifshiftz-Kosevich 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, NbSb$_2$. With varying the direction of the magnetic field, an abrupt $\pi$ phase shift is observed due to the interference between the spin-degenerate orbits. We investigate the effect of cyclotron mass on the $\pi$ phase shift and verify its close relation to the phase from the Zeeman coupling. We find that the $\pi$ phase shift occurs when the cyclotron mass is 1/2 of the electron mass, indicating the effective spin gyromagnetic ratio is $g_s$ = 2. Our approach is not only useful for analysing 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., Comment: 17 pages, 4 figures

Electronic transport properties of GeSn as a quantum material have been studied. Superconductivity, large magnetoresistance, and quantum oscillations were observed. The combination of such transport properties in one material is rare, implying the great potential of GeSn in fundamental physics research and device applications.

Many conductors, including recently studied Dirac materials, show saturation of coherence length on decreasing temperature. This surprising phenomenon is assigned to external noise, residual magnetic impurities, or two-level systems specific to noncrystalline solids. Here, by considering the SnTe-class of compounds as an example, we show theoretically that breaking of mirror symmetry deteriorates Berry's phase quantization, leading to additional dephasing in weak-antilocalization magnetoresistance (WAL-MR). Our experimental studies of WAL-MR corroborate these theoretical expectations in (111) Pb1-xSnxSe thin film with Sn contents x corresponding to both topological crystalline insulator and topologically trivial phases. In particular, we find the shortening of the phase coherence length in samples with intentionally broken mirror symmetry. Our results indicate that the classification of quantum transport phenomena into universality classes should encompass, in addition to time-reversal and spin-rotation invariances, spatial symmetries in specific systems.

Exploration of the topological quantum materials with electron correlation is at the frontier of physics, as the strong interaction may give rise to new topological phases and transitions. Here we report that a family of kagome magnets RMn_{6}Sn_{6} manifest the quantum transport properties analogical to those in the quantum-limit Chern magnet TbMn_{6}Sn_{6}. The topological transport in the family, including quantum oscillations with nontrivial Berry phase and large anomalous Hall effect arising from Berry curvature field, points to the existence of Chern gapped Dirac fermions. Our observation demonstrates a close relationship between rare-earth magnetism and topological electron structure, indicating the rare-earth elements can effectively engineer the Chern quantum phase in kagome magnets.