Magnetic Quantum Oscillations Research Articles

Quantum oscillations evidence for topological bands in kagome metal ScV6Sn6

Metals with kagome lattice provide bulk materials to host both the flat-band and Dirac electronic dispersions. A new family of kagome metals is recently discovered in AV6Sn6. The Dirac electronic structures of this material needs more experimental evidence to confirm. In the manuscript, we investigate this problem by resolving the quantum oscillations in both electrical transport and magnetization in ScV6Sn6. The revealed orbits are consistent with the electronic band structure models. Furthermore, the Berry phase of a dominating orbit is revealed to be around π, providing direct evidence for the topological band structure, which is consistent with calculations. Our results demonstrate a rich physics and shed light on the correlated topological ground state of this kagome metal.

Quasi-2D Fermi surface in the anomalous superconductor UTe2

The heavy fermion paramagnet UTe2 exhibits numerous characteristics of spin-triplet superconductivity. Efforts to understand the microscopic details of this exotic superconductivity have been impeded by uncertainty regarding the underlying electronic structure. Here we directly probe the Fermi surface of UTe2 by measuring magnetic quantum oscillations in pristine quality crystals. We find an angular profile of quantum oscillatory frequency and amplitude that is characteristic of a quasi-2D Fermi surface, which we find is well described by two cylindrical Fermi sheets of electron- and hole-type respectively. Additionally, we find that both cylindrical Fermi sheets possess considerable undulation but negligible small-scale corrugation, which may allow for their near-nesting and therefore promote magnetic fluctuations that enhance the triplet pairing mechanism. Importantly, we find no evidence for the presence of any 3D Fermi surface sections. Our results place strong constraints on the possible symmetry of the superconducting order parameter in UTe2.

Magnetic breakdown and spin-zero effect in quantum oscillations in kagome metal CsV3Sb5

In the recently discovered kagome metal CsV3Sb5, an intriguing proposal invoking a doped Chern insulator state suggests the presence of small Chern Fermi pockets hosting spontaneous orbital-currents and large orbital magnetic moments. While the net thermodynamic magnetization is nearly insensitive to these moments, due to their antiferromagnetic alignment, their presence can be revealed by the Zeeman effect, which shifts electron energies in magnetic fields with a proportionality given by the effective g−factor. Here, we determine the g-factor using the spin-zero effect in magnetic quantum oscillations. A large g-factor enhancement is visible only in magnetic breakdown orbits between conventional and concentrated Berry curvature Fermi pockets that host large orbital moments. Such Berry-curvature-generated large orbital moments are almost always concealed by other effects. In this system, however, magnetic breakdown orbits due to the proximity to a conventional Fermi-surface section allow them to be visibly manifested in magnetic quantum oscillations. Our results provide a remarkable example of the interplay between electronic correlations and more conventional electronic bands in quantum materials.

Tunable non-Lifshitz–Kosevich temperature dependence of Shubnikov–de Haas oscillation amplitudes in SmSb

The Lifshitz–Kosevich (LK) theory is the pillar of magnetic quantum oscillations, which have been extensively applied to characterise a wide range of metallic states. In this study, we focus on the Shubnikov–de Haas (SdH) effect observed in SmSb, a rare-earth monopnictide. We observed a significant departure from the expected LK theory near TN = 2.4 K: both a peak-like anomaly and an enhancement in the temperature dependence of quantum oscillation amplitude are seen in SmSb. Moreover, we discovered a remarkable sensitivity of the SdH amplitudes to sample purity. By adjusting the sample purity, we were able to tune the temperature dependence of the α band’s SdH amplitudes from a peak-like anomalous behaviour to an enhancement. Therefore, SdH oscillations from the α band connect the two well-known non-LK behaviours, controllable through varying the sample purity, paving the way for developing further understanding of the mechanism leading to the anomalous quantum oscillations.

Inhomogeneity identification by measuring magnetic quantum oscillations

This study explores the identification of sample inhomogeneity via magnetic quantum oscillations analysis in semimetal NbSb2. By doping Bi and Cr, we obtained a homogenous Bi-doped sample and an inhomogeneous Cr-doped sample, whose homogeneity was confirmed by comparing the magnetic quantum oscillation before and after grinding the samples. The magnetic quantum oscillations in the inhomogeneous sample exhibited a distinct phase shift and unusual field-dependent amplitude, believed to result from a non-uniform Fermi energy. The analysis of the magnetic quantum oscillations demonstrated that the homogeneous Bi-doped sample can be interpreted by the symmetric and Lorentzian effective Fermi energy distribution, while the inhomogeneous Cr-doped sample exhibited an asymmetric distribution, illustrating an unconventional violation of the Lifshitz-Kosevich formula. This research provides a novel method for identifying material inhomogeneity and mitigating potential misinterpretations of magnetic quantum oscillations' unusual phase, commonly seen as a nontrivial Berry phase indicator in topological materials studies.

Coherent heavy charge carriers in an organic conductor near the bandwidth-controlled Mott transition

The physics of the Mott metal-insulator transition (MIT) has attracted huge interest in the last decades. However, despite broad efforts, some key theoretical predictions are still lacking experimental confirmation. In particular, it is not clear whether the large coherent Fermi surface survives in immediate proximity to the bandwidth-controlled first-order MIT. A quantitative experimental verification of the predicted behavior of the quasiparticle effective mass, renormalized by many-body interactions, is also missing. Here we address these issues by employing organic $\kappa$-type salts as exemplary quasi-two-dimensional bandwidth-controlled Mott insulators and gaining direct access to their charge carrier properties via magnetic quantum oscillations. We trace the evolution of the effective cyclotron mass as the conduction bandwidth is tuned very close to the MIT by means of precisely controlled external pressure. We find that the sensitivity of the mass renormalization to tiny changes of the bandwidth is significantly stronger than theoretically predicted and is even further enhanced upon entering the transition region where the metallic and insulating phases coexist. On the other hand, even at its very edge of stability the metallic ground state preserves a large coherent Fermi surface with no significant enhancement of scattering.

Magnetic oscillation spectrum of surface resistance in layered organic compounds

The magnetic quantum oscillations of surface resistance in quasi-two dimensional layered organic conductors have been obtained numerically for a magnetic field parallel to the surface. The resonance fields, which were theoretically calculated in a previous work on surface states, are found to coincide closely to the positions of the maxima in the ∂R/∂B curves. We find that, in quasi-two dimensional organic conductors, the transitions between the adjacent surface states are the most present transitions in the sum curve for surface resistance derivative oscillation spectrum. Our results, obtained from the calculations of the oscillation spectra of the individual series and the sum of six series, confirm that the theoretical description of surface states in the anisotropic organic conductors is essentially correct in its numerical aspects. These studies will be very helpful for analyzing and explaining the experimental curves and for their comparison with the theoretical results. Since there are yet no experimental data on surface impedance oscillations in quasi-two dimensional organic conductors with this work we would like to shed new light on this problem in order to motive new research in that direction mostly because of a need of such studies for a possible utilization of surface effects in the existing organic devices.

Low-temperature theory of inversion and quantum oscillations in Kondo insulators

The half-filled Kondo lattice model is studied at low temperatures on a simple cubic lattice using the self-consistent theory developed by Ram and Kumar [Phys. Rev. B 96, 075115 (2017)]. It is found to have three distinct insulating phases in the temperature-hopping plane, namely, the strong-coupling Kondo singlet (KS) phase, the inverted Kondo singlet (iKS) phase distinguished from the KS by inversion, and the antiferromagnetic (AFM) phase. The quasiparticle density of states across the inversion transition is noted to exhibit a dimensional reduction, which can differentiate between the KS and iKS insulators in experiments. Magnetic quantum oscillations obtained in the iKS and AFM phases are found to show a Lifshitz-Kosevich-like behavior with temperature as well as quantum fluctuations induced by the Kondo interaction.

Quantum oscillations of Kondo screening phases in strong magnetic fields

We generalize the iterative diagonalization procedure adopted in method of numerical renormalization group to analyze the Kondo effect in strong magnetic fields, where the density of states for itinerary electrons at the chemical potential varies discontinuously as the magnetic field changes. We first examine phases of many-body ground states in the presence of single impurity. By investigating change of $z$-component of total spin, $\Delta S_z$, and spin-spin correlation between the impurity and conduction electrons, we find that there are three states competing for the ground state when Zeeman splitting is present. One of the states is doublet in which the impurity spin is unscreened. The other two states are Kondo screening states with $\Delta S_z=1/2$ and $\Delta S_z=1$, in which the impurity spin is partially screened and completely screened respectively. For Kondo systems with two-impurities in strong magnetic fields, we find that the interplay between the Kondo screening effect, RKKY interaction, and quantum oscillations due to Landau levels determines the ground state of the system. Combination of these three factors results in different screening scenarios for different phases in which spins of two impurities can form spin-0 or spin-1 states, while impurity spins in these phases can be either screened, partially screened, or unscreened by conduction electrons. The emergence of the ground state from these competing states oscillates with the change of magnetic field, chemical potential or inter-impurity distance. This leads to quantum oscillations in magnetization and conductivity. In particular, we find extra peak structures in longitudinal conductivity that reflect changes of Kondo screening phases and are important features to be observed in experiments. Our results provide a complete characterization of phases for Kondo effect in strong magnetic fields.

Hall Anomaly, Quantum Oscillations and Possible Lifshitz Transitions in Kondo Insulator YbB12 : Evidence for Unconventional Charge Transport

The electronic behavior of the Kondo insulator YbB${}_{12}$ under strong magnetic fields reveals a possible origin of quantum oscillations in magnetization and resistivity that suggest the development of exotic electronic states.

Quantum oscillations in interaction-driven insulators

In recent years it has become understood that quantum oscillations of the magnetization as a function of magnetic field, long recognized as phenomena intrinsic to metals, can also manifest in insulating systems. Theory has shown that in certain simple band insulators, quantum oscillations can appear with a frequency set by the area traced by the minimum gap in momentum space, and are suppressed for weak fields by an intrinsic ``Dingle damping'' factor reflecting the size of the bandgap. Here we examine quantum oscillations of the magnetization in excitonic and Kondo insulators, for which interactions play a crucial role. In models of these systems, self-consistent parameters themselves oscillate with changing magnetic field, generating additional contributions to quantum oscillations. In the low-temperature, weak-field regime, we find that the lowest harmonic of quantum oscillations of the magnetization are unaffected, so that the zero-field bandgap can still be extracted by measuring the Dingle damping factor of this harmonic. However, these contributions dominate quantum oscillations of magnetization at all higher harmonics, thereby providing a route to measure this interaction effect.

The surface tension at the metal-insulator phase transition in the magnetic field

Any metal-insulator transition in a crystalline matter must be the transition from a situation in which the electronic bands overlap to that when they do not [1]. On the basis of the phenomenological theory [2], various singularities are considered in the magnetic field for the surface tension at the metal-insulator contact. The surface tension displays also the quantum magnetic oscillations at low temperatures [3]. The consideration is applied to the Mott insulator in the magnetic field.

Non-trivial impurity and field effects in topological Kondo insulator SmB6

Topological Kondo Insulator SmB6 is a strongly correlated material where a spin–orbit interaction between the localized odd-parity f-electron and even parity d-electron levels lead to a band inversion and opening of an insulating gap. The non-trivial topology (Z2 = -1) leads to the formation of a topologically protected conducting surface state due to the presence of strong correlation physics. Although the bulk material is known to form a topological Kondo gap, recent magnetic quantum oscillation experiments on SmB6 find an unconventional Fermi surface whose origin remains a matter of intense debate, and the possible role of topological surfacestate and impurity induced in-gap states have been proposed. By utilizing a realistic multi-orbital tight-binding Hamiltonian, this work aims to develop a microscopic model to study the effects of perturbations like external magnetic fields, and impurities on the low energy bulk and topological surface state properties of SmB6. We further examine the role of an external electric field in tuning the non-trivial topological surface state in SmB6 and provide experimentally observable signatures in the local density of state (LDOS) near non-magnetic and magnetic impurities. Additionally, we study the surface Ferromagnetism that arises in SmB6 as observed by magnetoresistance experiments.

Seebeck effect studies in the charge density wave state of organic conductor α-(BEDT–TTF)2KHg(SCN)4

Angular, magnetic field and temperature dependence of the interlayer Seebeck effect of the multiband organic conductor is experimentally studied at temperatures down to 0.55 K and fields up to 31 T in a wide range of angles. The background magnetic field and angular component of the Seebeck effect as well as the magnetic quantum oscillations that originate from the closed Fermi surface orbits are analyzed. The background interlayer Seebeck effect components show that above certain tilt angle of the magnetic field and above the kink field there is another CDW state in , between previously known CDW0 and CDW x states, in agreement with magnetoresistance and magnetization studies in this material. Our observations show that this state possesses some of the properties of the CDW0 state. The Fermi surface in the third CDW state is still reconstructed but less imperfectly nested as expected as this state develops above the kink field. The temperature dependence of the interlayer Seebeck effect reveals that this state is developed at temperatures below 3 K and at field orientations around the second AMRO maximum. In addition, for the first time, a detailed T − θ phase diagram of based purely on Seebeck effect measurements is presented. We find that other states and transitions, beside the CDW states, also exist in a given temperature and angular range that have not been previously reported. These observations change the whole picture about the transport processes in the organic conductor and allow to better understand the complex nature of the CDW order in this and similar compounds.

Quantum magnetic oscillations in Weyl semimetals with tilted nodes

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.

Large Magnetoresistance and Nontrivial Berry Phase in Nb3Sb Crystals with A15 StructureSupported by the National Key R&D Program of China (Grant No. 2016YFA0300402), the National Natural Science Foundation of China (Grant Nos. 12074335, and 11974095), and the Fundamental Research Funds for the Central Universities.

Compounds with the A15 structure have attracted extensive attention due to their superconductivity and nontrivial topological band structures. We have successfully grown Nb3Sb single crystals with the A15 structure and systematically measured the longitudinal resistivity, Hall resistivity and quantum oscillations in magnetization. Similar to other topological trivial/nontrivial semimetals, Nb3Sb exhibits large magnetoresistance (MR) at low temperatures (717%, 2 K and 9 T), unsaturating quadratic field dependence of MR and up-turn behavior in ρxx (T) curves under magnetic field, which is considered to result from a perfect hole-electron compensation, as evidenced by the Hall resistivity measurements. The nonzero Berry phase obtained from the de-Hass van Alphen (dHvA) oscillations demonstrates that Nb3Sb is topologically nontrivial. These results indicate that Nb3Sb superconductor is also a semimetal with large MR and nontrivial Berry phase. This indicates that Nb3Sb may be another platform to search for the Majorana zero-energy mode.

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.

Magnetic quantum oscillation in a monolayer insulator

Magnetic quantum oscillation in a monolayer insulator

Unusual high-field metal in a Kondo insulator

Within condensed-matter systems, strong electronic interactions often lead to exotic quantum phases. A recent manifestation of this is the unexpected observation of magnetic quantum oscillations and metallic thermal transport, both properties of systems with Fermi surfaces of itinerant quasiparticles, in the Kondo insulators SmB6 and YbB$_{12}$. To understand these phenomena, it is informative to study their evolution as the energy gap of the Kondo-Insulator state is closed by a large magnetic field. We show here that both the quantum-oscillation frequency and the cyclotron mass display a strong field dependence in the resulting high-field metallic state in $_{12}$. By tracking the Fermi-surface area, we conclude that the same quasiparticle band gives rise to the quantum oscillations in both insulating and metallic states. These data are understood most simply using a two-fluid picture where unusual quasiparticles, contributing little or nothing to charge transport, coexist with conventional fermions. In the metallic state this leads to a heavy-fermion bad metal with negligible magnetoresistance, relatively high resistivity and a very large Kadowaki-Woods ratio, underlining the exotic nature of the fermion ensemble inhabiting $_{12}$.

Fermi surface of the skutterudite CoSb3 : Quantum oscillations and band-structure calculations

The unfilled skutterudite ${\mathrm{CoSb}}_{3}$ is a small-gap semiconductor which was predicted to be close to a strain-induced transition to a topological-insulator phase passing through a topological quantum critical point. As a starting point to strain measurements, the aim of this work is to characterize the electronic structure of as-grown samples. Two types of samples, one grown in Sb flux and the other by an inclined rotary Bridgman technique, are compared based on structural properties, resistivity, Hall effect, and magnetization. All samples appear metallic with small growth-dependent hole doping. Measurements of quantum oscillations in magnetization and angle-dependent electronic transport confirm the calculated band structure at ambient pressure. The Fermi surface consists of a single spherical sheet at the $\mathrm{\ensuremath{\Gamma}}$ point, and the purest samples grown by the Bridgman technique have a Fermi energy lying 25 meV below the valence-band edge. Band-structure calculations provide an accurate description of the experimental results. Hence, this compound is suitable for an investigation of topological states under strain.