Quantum States Of Matter Research Articles

Cloaking magnetic field and generating electric field with topological insulator and high permeability material

Topological insulator is a new state of quantum matter. When applied magnetic field is applied on a topological insulator, not only the magnetic field is induced, but also the electric field is induced, vice versa. We designed bi-layer magnetic cloak with topological insulator and high permeability material (HPM), derived the electric field and magnetic field inside and outside the bi-layer topological insulator and HPM. Calculation and simulation results show that the applied magnetic field is cloaked by the bi-layer topological insulator and HPM, and the uniform electric field is induced in the cloaked region.

Quantum Bose-Bose droplets at a dimensional crossover

We study a liquid quantum droplets in a mixture of two-component Bose-Einstein condensates under a variable confinement introduced along one or two spatial dimensions. Despite the atom-atom scattering has a three-dimensional character, discreetness of the available modes in the reduced dimension(s) strongly influences the zero-point energy -- the Lee-Huang-Yang term. In a weakly interaction limit, it is the leading correction to the mean-field energy at the crossover from three to two dimensions, or from three to one dimension. We analyze the properties of the droplets at the dimensional crossovers, and provide the demanding conditions for accessing quasi-low dimensions. We predict new kinds of droplets which are formed only due to the quantum fluctuations when the mean-field interaction vanishes. Our results pave the way for exploring new states of quantum matter, and are important for experiments with liquid quantum droplets in reduced dimensions.

Strong coupling and periodic potential at the Pb/Sb(111) interface

Proximity phenomena between superconductors and topological semimetals are at the heart of the search for novel quantum states of matter. Here we show that the epitaxial interface between one- to three-atomic-layer Pb and the Sb(111) substrate has extremely high electronic transparency. As a result, Pb loses its superconducting and even its metallic properties; its electronic spectrum mimics the semimetallic density of states of Sb. Additional states arise from a threefold symmetric potential which develops at the Pb/Sb interface. Our experimental results are supported by DFT simulations which demonstrate that this potential arises from the corrugated structure of the Pb/Sb interface.

Pseudogap, Fermi arc, and Peierls-insulating phase induced by 3D–2D crossover in monolayer VSe2

One of important challenges in condensed-matter physics is to realize new quantum states of matter by manipulating the dimensionality of materials, as represented by the discovery of high-temperature superconductivity in atomic-layer pnictides and room-temperature quantum Hall effect in graphene. Transition-metal dichalcogenides (TMDs) provide a fertile platform for exploring novel quantum phenomena accompanied by the dimensionality change, since they exhibit a variety of electronic/magnetic states owing to quantum confinement. Here we report an anomalous metal-insulator transition induced by three-dimensional (3D)–two-dimensional (2D) crossover in monolayer 1T-VSe2 grown on bilayer graphene. We observed a complete insulating state with a finite energy gap on the entire Fermi surface in monolayer 1T-VSe2 at low temperatures, in sharp contrast to metallic nature of bulk. More surprisingly, monolayer 1T-VSe2 exhibits a pseudogap with Fermi arc at temperatures above the charge-density-wave temperature, showing a close resemblance to high-temperature cuprates. This similarity suggests a common underlying physics between two apparently different systems, pointing to the importance of charge/spin fluctuations to create the novel electronic states, such as pseudogap and Fermi arc, in these materials.

Topolectrical-circuit realization of topological corner modes

Quantized electric quadrupole insulators have recently been proposed as novel quantum states of matter in two spatial dimensions. Gapped otherwise, they can feature zero-dimensional topological corner mid-gap states protected by the bulk spectral gap, reflection symmetries and a spectral symmetry. Here we introduce a topolectrical circuit design for realizing such corner modes experimentally and report measurements in which the modes appear as topological boundary resonances in the corner impedance profile of the circuit. Whereas the quantized bulk quadrupole moment of an electronic crystal does not have a direct analogue in the classical topolectrical-circuit framework, the corner modes inherit the identical form from the quantum case. Due to the flexibility and tunability of electrical circuits, they are an ideal platform for studying the reflection symmetry-protected character of corner modes in detail. Our work therefore establishes an instance where topolectrical circuitry is employed to bridge the gap between quantum theoretical modelling and the experimental realization of topological band structures.

Hydrostatic strain-induced topological phase of KNa2Sb

Topological insulators are novel state of quantum matter that have a bulk band gap like an ordinary insulator, but have protected conducting states by time reversal symmetry on their edge or surface. The spin-orbit coupling can play an important role in these materials, resulting in a band inversion at time reversal invariant momenta (TRIM) points. The topological phase and the effect of the hydrostatic pressure on the electronic structure and topological phase of the KNa2Sb compound are investigated by using both first-principles calculations and ab-initio based tight-binding computations. Under hydrostatic lattice strain until 5.6%, the KNa2Sb compound is semimetal with zero energy band gap and has an inverted band order. In this pressure, the Z2 invariants of this compound are calculated using the parity analysis at TRIM points and evolution of wannier charge centers at the six TRIM plane. The calculated surface states at (001) surface show a single Dirac cone exists on the X¯Γ¯W¯ line at the surface Brillouin zone. To investigate the stability of KNa2Sb compound the phonon dispersions and elastic tensors of this compound in the cubic structure are calculated.

Planar Hall effect in the type-II Weyl semimetal Td−MoTe2

Topological Weyl semimetals (WSMs), a new quantum state of matter, provide the realization of relativistic Weyl fermions in solid-state physics. The appearance of the Weyl nodes will demonstrate novel transport phenomena related to the chiral anomaly. Theoretical prediction hints that the giant planar Hall effect (PHE) can be induced by the chiral anomaly in the WSMs. In this work, we performed PHE measurements to probe chiral charge in type-II WSM ${T}_{d}\text{\ensuremath{-}}\mathrm{MoT}{\mathrm{e}}_{2}$. Robust transverse voltage can be acquired when the magnetic field and electric current is coplanar. Subsequently, the amplitude of intrinsic PHE increased sharply at $T=50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ with the decreasing temperature. The increasing trend in the temperature dependence is attributed to coupling effect between the chiral charge and the electric one, which drives the system into intermediate-coupling states at higher field. Our work confirms that the PHE measurement is a practical transport tool to probe chiral charge pumping effect in the WSMs systems.

Topological Type-II Dirac Fermions Approaching the Fermi Level in a Transition Metal Dichalcogenide NiTe2

Type-II Dirac/Weyl semimetals are characterized by strongly tilted Dirac cones such that the Dirac/Weyl node emerges at the boundary of electron and hole pockets as a new state of quantum matter, distinct from the standard Dirac/Weyl points with a point-like Fermi surface which are referred to as type-I nodes. The type-II Dirac fermions were recently predicted by theory and have since been confirmed in experiments in the PtSe2-class of transition metal dichal-cogenides. However, the Dirac nodes observed in PtSe2, PdTe2 and PtTe2 candidates are quite far away from the Fermi level, making the signature of topological fermions obscure as the physical properties are still dominated by the non-Dirac quasiparticles. Here we report the synthesis of a new type-II Dirac semimetal NiTe2 in which a pair of type-II Dirac nodes are located very close to the Fermi level. The quantum oscillations in this material reveal a nontrivial Berry's phase associated with these Dirac fermions. Our first principles calculations further unveil a topological Dirac cone in its surface states. Therefore, NiTe2 may not only represent an improved system to formulate the theoretical understanding of the exotic consequences of type-II Dirac fermions, it also facilitates possible applications based on these topological carriers.

Quantum oscillations and coherent interlayer transport in a new topological Dirac semimetal candidate YbMnSb2

Dirac semimetals (DSMs), which host Dirac fermions and represent new state of quantum matter, have been studied intensively in condensed matter physics. The exploration of new materials with topological states is im- portant in both physics and materials science. In this article, we report the synthesis and the transport properties of high quality single crystals of YbMnSb$_2$. YbMnSb$_2$ is a new compound with metallic behavior. Quantum oscillations, including Shubnikov-de Haas (SdH) oscillation and de Haas-van Alphen (dHvA) type oscillation, have been observed at low temperature and high magnetic field. Small effective masses and nontrivial Berry phase are extracted from the analyses of quantum oscillations, which provide the transport evidence for the possible existence of Dirac fermions in YbMnSb$_2$. The measurements of angular-dependent interlayer magnetoresistance (MR) indicate the interlayer transport is coherent. The Fermi surface (FS) of YbMnSb$_2$ possesses quasi-two dimensional (2D) characteristic as determined by the angular dependence of SdH oscillation frequency. These findings suggest that YbMnSb$_2$ is a new candidate of topological Dirac semimetal.

High-temperature quantum oscillations of the Hall resistance in bulk Bi2Se3

Helically spin-polarized Dirac fermions (HSDF) in protected topological surface states (TSS) are of high interest as a new state of quantum matter. In three-dimensional (3D) materials with TSS, electronic bulk states often mask the transport properties of HSDF. Recently, the high-field Hall resistance and low-field magnetoresistance indicate that the TSS may coexist with a layered two-dimensional electronic system (2DES). Here, we demonstrate quantum oscillations of the Hall resistance at temperatures up to 50 K in nominally undoped bulk Bi2Se3 with a high electron density n of about 2·1019 cm−3. From the angular and temperature dependence of the Hall resistance and the Shubnikov-de Haas oscillations we identify 3D and 2D contributions to transport. Angular resolved photoemission spectroscopy proves the existence of TSS. We present a model for Bi2Se3 and suggest that the coexistence of TSS and 2D layered transport stabilizes the quantum oscillations of the Hall resistance.

Diffusive-like effects and possible non trivial local topology on the half-Heusler YPdBi compound

The non-ambiguous experimental identification of topological states of matter is one of the main interesting problems regarding this new quantum state of matter. In particular, the half-Heusler family RMT (R = rare-earth, T = Pd, Pt or Au and T = Bi, Sb, Pb or Sn) could be a useful platform to explore these states due to their cubic symmetry and the topological properties tunable via their unit cell volume and/or the nuclear charges of the M and T atoms. In this work, we report electron spin resonance (ESR) and complementary macroscopic measurements in the Nd3 + -doped putative topologically trivial semimetal YPdBi. Following the Nd3 + ESR lineshape as a function of microwave power, size of the particle and temperature, we have been able to observe an evolution from a Dysonian lineshape to a diffusive-like lineshape. Furthermore, the Nd3 + ESR intensity saturation is concentration dependent, which could be due to a phonon-bottleneck process. Comparing these results with the Nd3 + -doped YPtBi, we discuss a possible scenario in which the Nd3 + ions could locally tune the topological properties of the system.

Anomalous Nernst effect in type-II Weyl semimetals

Topological Weyl semimetals (WSM), a new state of quantum matter with gapless nodal bulk spectrum and open Fermi arc surface states, have recently sparked enormous interest in condensed matter physics. Based on the symmetry and fermiology, it has been proposed that WSMs can be broadly classified into two types, type-I and type-II Weyl semimetals. While the undoped, conventional, type-I WSMs have point like Fermi surface and vanishing density of states (DOS) at the Fermi energy, the type-II Weyl semimetals break Lorentz symmetry explicitly and have tilted conical spectra with electron and hole pockets producing finite DOS at the Fermi level. The tilted conical spectrum and finite DOS at Fermi level in type-II WSMs have recently been shown to produce interesting effects such as a chiral anomaly induced longitudinal magnetoresistance that is strongly anisotropic in direction and a novel anomalous Hall effect. In this work, we consider the anomalous Nernst effect in type-II WSMs in the absence of an external magnetic field using the framework of semi-classical Boltzmann theory. Based on both a linearized model of time-reversal breaking WSM with a higher energy cut-off and a more realistic lattice model, we show that the anomalous Nernst response in these systems is strongly anisotropic in space, and can serve as a reliable signature of type-II Weyl semimetals in a host of magnetic systems with spontaneously broken time reversal symmetry.

Robust weak anti-localisation effect in strongly textured nanocrystalline [formula omitted] samples

Topological insulators are a quantum state of matter that has recently created a great interest among the scientific community, with Bi2Se3 being one of the most extensively studied materials. Here, we demonstrate that polycrystalline nanostructured samples of Bi2Se3 preserve the existence of topological surface states, where electrons cannot be localised. The nanosheet crystals were synthesised by a microwave-assisted method and their structure, composition and morphology thoroughly characterised. The transport properties of a textured polycrystalline sample with strong preferred orientation along the c-axis were measured, showing the presence of the weak anti-localisation effect and Shubnikov-de Haas oscillations. These features are robust against the presence of non-magnetic impurities and structural defects.

Quantum spin Hall effect and topological phase transition in InNxBiySb1−x−y/InSb quantum wells

Quantum spin Hall (QSH) effect, a fundamentally new quantum state of matter and topological phase transitions are characteristics of a kind of electronic material, popularly referred to as topological insulators (TIs). TIs are similar to ordinary insulator in terms of their bulk bandgap, but have gapless conducting edge-states that are topologically protected. These edge-states are facilitated by the time-reversal symmetry and they are robust against nonmagnetic impurity scattering. Recently, the quest for new materials exhibiting non-trivial topological state of matter has been of great research interest, as TIs find applications in new electronics and spintronics and quantum-computing devices. Here, we propose and demonstrate as a proof-of-concept that QSH effect and topological phase transitions can be realized in /InSb semiconductor quantum wells (QWs). The simultaneous incorporation of nitrogen and bismuth in InSb is instrumental in lowering the bandgap, while inducing opposite kinds of strain to attain a near-lattice-matching conducive for lattice growth. Phase diagram for bandgap shows that as we increase the QW thickness, at a critical thickness, the electronic bandstructure switches from a normal to an inverted type. We confirm that such transition are topological phase transitions between a traditional insulator and a TI exhibiting QSH effect—by demonstrating the topologically protected edge-states using the bandstructure, edge-localized distribution of the wavefunctions and edge-state spin-momentum locking phenomenon, presence of non-zero conductance in spite of the Fermi energy lying in the bandgap window, crossover points of Landau levels in the zero-mode indicating topological band inversion in the absence of any magnetic field and presence of large Rashba spin-splitting, which is essential for spin-manipulation in TIs.

Thickness-dependent carrier and phonon dynamics of topological insulator Bi2Te3 thin films.

As a new quantum state of matter, topological insulators offer a new platform for exploring new physics, giving rise to fascinating new phenomena and new devices. Lots of novel physical properties of topological insulators have been studied extensively and are attributed to the unique electron-phonon interactions at the surface. Although electron behavior in topological insulators has been studied in detail, electron-phonon interactions at the surface of topological insulators are less understood. In this work, using optical pump-optical probe technology, we performed transient absorbance measurement on Bi2Te3 thin films to study the dynamics of its hot carrier relaxation process and coherent phonon behavior. The excitation and dynamics of phonon modes are observed with a response dependent on the thickness of the samples. The thickness-dependent characteristic time, amplitude and frequency of the damped oscillating signals are acquired by fitting the signal profiles. The results clearly indicate that the electron-hole recombination process gradually become dominant with the increasing thickness which is consistent with our theoretical calculation. In addition, a frequency modulation phenomenon on the high-frequency oscillation signals induced by coherent optical phonons is observed.

Quantized chiral anomaly materials cloak

Chiral anomaly materials (CAM, e.g., axion insulator, topological insulator and some of Weyl semimetal) are new states of quantum matter. Anomalous Hall effect can occur in CAM, the anomalous Hall effect is closely related to the topological magneto-electric effect, i.e., when an electric field is applied to CAM, not only the electric field is induced, but also the magnetic field, vice versa. According to those properties, we design an electric cloak with quantized CAM and conductor, and a magnetic cloak with quantized CAM and superconductor. Simulation and calculation results show that the electric cloak can cloak applied electric field and induce magnetic field, and the magnetic cloak can cloak applied magnetic field and induce electric field. When applied electric field is generated by a point charge, the monopole can be obtained.

Nontrivial Berry phase in magnetic BaMnSb2 semimetal

The subject of topological materials has attracted immense attention in condensed-matter physics because they host new quantum states of matter containing Dirac, Majorana, or Weyl fermions. Although Majorana fermions can only exist on the surface of topological superconductors, Dirac and Weyl fermions can be realized in both 2D and 3D materials. The latter are semimetals with Dirac/Weyl cones either not tilted (type I) or tilted (type II). Although both Dirac and Weyl fermions have massless nature with the nontrivial Berry phase, the formation of Weyl fermions in 3D semimetals require either time-reversal or inversion symmetry breaking to lift degeneracy at Dirac points. Here we demonstrate experimentally that canted antiferromagnetic BaMnSb2 is a 3D Weyl semimetal with a 2D electronic structure. The Shubnikov-de Hass oscillations of the magnetoresistance give nearly zero effective mass with high mobility and the nontrivial Berry phase. The ordered magnetic arrangement (ferromagnetic ordering in the ab plane and antiferromagnetic ordering along the c axis below 286 K) breaks the time-reversal symmetry, thus offering us an ideal platform to study magnetic Weyl fermions in a centrosymmetric material.

Two-Dimensional Topological Insulators: Progress and Prospects

Two-dimensional topological insulators (2D TIs) are a remarkable class of atomically thin layered materials that exhibit unique symmetry-protected helical metallic edge states with an insulating interior. Recent years have seen a tremendous surge in research of this intriguing new state of quantum matter. In this Perspective, we summarize major milestones and the most significant progress in the latest developments of material discovery and property characterization in 2D TI research. We categorize the large number and rich variety of theoretically proposed 2D TIs based on the distinct mechanisms of topological phase transitions, and we systematically analyze and compare their structural, chemical, and physical characteristics. We assess the current status and challenges of experimental synthesis and potential device applications of 2D TIs and discuss prospects of exciting new opportunities for future research and development of this fascinating class of materials.

Intertwined superfluid and density wave order in two-dimensional 4He

A detailed analysis of low-temperature torsional oscillation measurements on two-dimensional 4He reveals evidence for intertwined superfluid and density wave order in this system. Superfluidity is a manifestation of the operation of the laws of quantum mechanics on a macroscopic scale. The conditions under which superfluidity becomes manifest have been extensively explored experimentally in both quantum liquids (liquid 4He being the canonical example) and ultracold atomic gases1,2, including as a function of dimensionality3,4. Of particular interest is the hitherto unresolved question of whether a solid can be superfluid5,6. Here we report the identification of a new state of quantum matter with intertwined superfluid and density wave order in a system of two-dimensional bosons subject to a triangular lattice potential. Using a torsional oscillator we have measured the superfluid response of the second atomic layer of 4He adsorbed on the surface of graphite, over a wide temperature range down to 2 mK. Superfluidity is observed over a narrow range of film densities, emerging suddenly and subsequently collapsing towards a quantum critical point. The unusual temperature dependence of the superfluid density in the limit of zero temperature and the absence of a clear superfluid onset temperature are explained, self-consistently, by an ansatz for the excitation spectrum, reflecting density wave order, and a quasi-condensate wavefunction breaking both gauge and translational symmetry.

Signature of type-II Weyl semimetal phase in MoTe2

Topological Weyl semimetal (TWS), a new state of quantum matter, has sparked enormous research interest recently. Possessing unique Weyl fermions in the bulk and Fermi arcs on the surface, TWSs offer a rare platform for realizing many exotic physical phenomena. TWSs can be classified into type-I that respect Lorentz symmetry and type-II that do not. Here, we directly visualize the electronic structure of MoTe2, a recently proposed type-II TWS. Using angle-resolved photoemission spectroscopy (ARPES), we unravel the unique surface Fermi arcs, in good agreement with our ab initio calculations that have nontrivial topological nature. Our work not only leads to new understandings of the unusual properties discovered in this family of compounds, but also allows for the further exploration of exotic properties and practical applications of type-II TWSs, as well as the interplay between superconductivity (MoTe2 was discovered to be superconducting recently) and their topological order.