Showing papers on "Fermi energy published in 2017"
TL;DR: In this article, the lattice relaxation in the twisted bilayer graphene (TBG) and its effect on the electronic band structure was theoretically studied and an effective continuum theory was developed to obtain the optimized structure to minimize the total energy.
Abstract: We theoretically study the lattice relaxation in the twisted bilayer graphene (TBG) and its effect on the electronic band structure. We develop an effective continuum theory to describe the lattice relaxation in general TBGs and obtain the optimized structure to minimize the total energy. At small rotation angles $l{2}^{\ensuremath{\circ}}$, in particular, we find that the relaxed lattice drastically reduces the area of the AA stacking region and forms a triangular domain structure with alternating AB and BA stacking regions. We then investigate the effect of the domain formation on the electronic band structure. The most notable change from the nonrelaxed model is that an energy gap of up to 20 meV opens at the superlattice subband edges on the electron and hole sides. We also find that the lattice relaxation significantly enhances the Fermi velocity, which was strongly suppressed in the nonrelaxed model.
363 citations
TL;DR: In this paper, the authors used a scanning tunneling microscope on the newly discovered superconducting Dirac surface state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducted transition temperature Tc = 14.5 K), and clearly observed a sharp and non-split zero bias peak inside a vortex core.
Abstract: The search for Majorana bound state (MBS) has recently emerged as one of the most active research areas in condensed matter physics, fueled by the prospect of using its non-Abelian statistics for robust quantum computation. A highly sought-after platform for MBS is two-dimensional topological superconductors, where MBS is predicted to exist as a zero-energy mode in the core of a vortex. A clear observation of MBS, however, is often hindered by the presence of additional low-lying bound states inside the vortex core. By using scanning tunneling microscope on the newly discovered superconducting Dirac surface state of iron-based superconductor FeTe1-xSex (x = 0.45, superconducting transition temperature Tc = 14.5 K), we clearly observe a sharp and non-split zero-bias peak inside a vortex core. Systematic studies of its evolution under different magnetic fields, temperatures, and tunneling barriers strongly suggest that this is the case of tunneling to a nearly pure MBS, separated from non-topological bound states which is moved away from the zero energy due to the high ratio between the superconducting gap and the Fermi energy in this material. This observation offers a new, robust platform for realizing and manipulating MBSs at a relatively high temperature.
347 citations
TL;DR: Graphene plasmons are used, propagating at extremely slow velocities close to the electron Fermi velocity, to probe the nonlocal response of the graphene electron liquid, and a parameter-free match with the full quantum description of the massless Dirac electron gas is revealed.
Abstract: The response of electron systems to electrodynamic fields that change rapidly in space is endowed by unique features, including an exquisite spatial nonlocality This can reveal much about the materials’ electronic structure that is invisible in standard probes that use gradually varying fields Here, we use graphene plasmons, propagating at extremely slow velocities close to the electron Fermi velocity, to probe the nonlocal response of the graphene electron liquid The near-field imaging experiments reveal a parameter-free match with the full quantum description of the massless Dirac electron gas, which involves three types of nonlocal quantum effects: single-particle velocity matching, interaction-enhanced Fermi velocity, and interaction-reduced compressibility Our experimental approach can determine the full spatiotemporal response of an electron system
289 citations
TL;DR: In this paper, the authors reported the large intrinsic anomalous Hall effect (AHE) in the half-metallic ferromagnet Co3Sn2S2 single crystal.
Abstract: The origin of anomalous Hall effect (AHE) in magnetic materials is one of the most intriguing aspect in condensed matter physics and has been controversial for a long time. Recent studies indicate that the intrinsic AHE is closely related to the Berry curvature of occupied electronic states. In a magnetic Weyl semimetal with broken time-reversal symmetry, there are significant contributions on Berry curvature around Weyl nodes, which would lead to a large intrinsic AHE. Here, we report the large intrinsic AHE in the half-metallic ferromagnet Co3Sn2S2 single crystal. By systematically mapping out the electronic structure of Co3Sn2S2 theoretically and experimentally, the large intrinsic AHE should originate from the Weyl fermions near the Fermi energy. Furthermore, the intrinsic anomalous Hall conductivity depends linearly on the magnetization and this can be attributed to the sharp decrease of magnetization and the change of topological characteristics.
289 citations
TL;DR: Focusing on the regime of strong repulsive interactions, the many-body system is characterized by extracting the key properties of repulsive Fermi polarons: the energy E_{+}, the effective mass m^{*}, the residue Z, and the decay rate Γ.
Abstract: We employ radio-frequency spectroscopy to investigate a polarized spin mixture of ultracold ^{6}Li atoms close to a broad Feshbach scattering resonance. Focusing on the regime of strong repulsive interactions, we observe well-defined coherent quasiparticles even for unitarity-limited interactions. We characterize the many-body system by extracting the key properties of repulsive Fermi polarons: the energy E_{+}, the effective mass m^{*}, the residue Z, and the decay rate Γ. Above a critical interaction, E_{+} is found to exceed the Fermi energy of the bath, while m^{*} diverges and even turns negative, thereby indicating that the repulsive Fermi liquid state becomes energetically and thermodynamically unstable.
240 citations
TL;DR: 2D Fe2Si nanosheet, one counterpart of Hapkeite mineral discovered in meteorite with novel magnetism is proposed on the basis of first-principles calculations and has a high thermodynamic stability and its 2D lattice can be retained at the temperature up to 1200 K.
Abstract: Searching experimental feasible two-dimensional (2D) ferromagnetic crystals with large spin-polarization ratio, high Curie temperature and large magnetic anisotropic energy is one key to develop next-generation spintronic nanodevices. Here, 2D Fe2Si nanosheet, one counterpart of Hapkeite mineral discovered in meteorite with novel magnetism is proposed on the basis of first-principles calculations. The 2D Fe2Si crystal has a slightly buckled triangular lattice with planar hexacoordinated Si and Fe atoms. The spin-polarized calculations with hybrid HSE06 function method indicate that 2D Fe2Si is a ferromagnetic half metal at its ground state with 100% spin-polarization ratio at Fermi energy level. The phonon spectrum calculation and ab initio molecular dynamic simulation shows that 2D Fe2Si crystal has a high thermodynamic stability and its 2D lattice can be retained at the temperature up to 1200 K. Monte Carlo simulations based on the Ising model predict a Curie temperature over 780 K in 2D Fe2Si crystal, ...
189 citations
TL;DR: In this paper, a theory of N species of fermions at nonzero density coupled to a U ( 1 ) gauge field in two spatial dimensions is examined and the Lyapunov rate and butterfly velocity in an extended random-phase approximation.
Abstract: We compute parameters characterizing many-body quantum chaos for a critical Fermi surface without quasiparticle excitations. We examine a theory of N species of fermions at nonzero density coupled to a U ( 1 ) gauge field in two spatial dimensions and determine the Lyapunov rate and the butterfly velocity in an extended random-phase approximation. The thermal diffusivity is found to be universally related to these chaos parameters; i.e., the relationship is independent of N , the gauge-coupling constant, the Fermi velocity, the Fermi surface curvature, and high-energy details.
158 citations
TL;DR: In this paper, a low-valent, quasi-two-dimensional trilayer metallic nickelate Pr4Ni3O8 is presented, revealing this system to be a close analogue of cuprate systems, and offering tantalizing hope that it may superconduct if appropriate electron doping can be achieved.
Abstract: High-temperature cuprate superconductivity remains a defining problem in condensed-matter physics. Among myriad approaches to addressing this problem has been the study of alternative transition metal oxides with similar structures and 3d electron count that are suggested as proxies for cuprate physics. None of these analogues has been superconducting, and few are even metallic. Here, we report that the low-valent, quasi-two-dimensional trilayer compound Pr4Ni3O8 avoids a charge-stripe-ordered phase previously reported for La4Ni3O8, leading to a metallic ground state. X-ray absorption spectroscopy shows that metallic Pr4Ni3O8 exhibits a low-spin configuration with significant orbital polarization and pronounced character in the unoccupied states above the Fermi energy, a hallmark of the cuprate superconductors. Density functional theory calculations corroborate this finding, and reveal that the orbital dominates the near-Ef occupied states as well. Belonging to a regime of 3d electron count found for hole-doped cuprates, Pr4Ni3O8 thus represents one of the closest analogues to cuprates yet reported and a singularly promising candidate for high-Tc superconductivity if electron doping could be achieved. A careful study of the low-valent, quasi-two-dimensional trilayer metallic nickelate Pr4Ni3O8 is presented, revealing this system to be a close analogue of cuprate systems, and offering tantalizing hope that it may superconduct if appropriate electron doping can be achieved.
152 citations
TL;DR: The development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 thin films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness is reported.
Abstract: A well known semiconductor Cd3As2 has reentered the spotlight due to its unique electronic structure and quantum transport phenomena as a topological Dirac semimetal. For elucidating and controlling its topological quantum state, high-quality Cd3As2 thin films have been highly desired. Here we report the development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness. With decreasing the film thickness to 10 nm, the quantum Hall states exhibit variations such as a change in the spin degeneracy reflecting the Dirac dispersion with a large Fermi velocity. Details of the electronic structure including subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness, suggesting the presence of a two-dimensional topological insulating phase. The demonstration of quantum Hall states in our high-quality Cd3As2 films paves a road to study quantum transport and device application in topological Dirac semimetal and its derivative phases. Despite many achievements in the topological semimetal Cd3As2, the high-quality Cd3As2 films are still rare. Here, Uchida et al. grow high-crystallinity and high-mobility Cd3As2 thin films and observe quantum Hall states dependent on the confinement thickness.
144 citations
TL;DR: It is shown that the Fermi arcs can give rise to a distinctive 3D quantum Hall effect in topological semimetals, and the edge states of the Fermani arcs show a unique 3D distribution, giving an example of (d-2)-dimensional boundary states.
Abstract: The quantum Hall effect is usually observed in 2D systems We show that the Fermi arcs can give rise to a distinctive 3D quantum Hall effect in topological semimetals Because of the topological constraint, the Fermi arc at a single surface has an open Fermi surface, which cannot host the quantum Hall effect Via a ``wormhole'' tunneling assisted by the Weyl nodes, the Fermi arcs at opposite surfaces can form a complete Fermi loop and support the quantum Hall effect The edge states of the Fermi arcs show a unique 3D distribution, giving an example of ($d\ensuremath{-}2$)-dimensional boundary states This is distinctly different from the surface-state quantum Hall effect from a single surface of topological insulator As the Fermi energy sweeps through the Weyl nodes, the sheet Hall conductivity evolves from the $1/B$ dependence to quantized plateaus at the Weyl nodes This behavior can be realized by tuning gate voltages in a slab of topological semimetal, such as the TaAs family, ${\mathrm{Cd}}_{3}{\mathrm{As}}_{2}$, or ${\mathrm{Na}}_{3}\mathrm{Bi}$ This work will be instructive not only for searching transport signatures of the Fermi arcs but also for exploring novel electron gases in other topological phases of matter
136 citations
TL;DR: In this paper, Boltzmann transport theory and first-principles calculations as a function of Fermi energy and crystal orientation were used to investigate thermoelectric properties of monolayer indium selenide (InSe).
Abstract: Thermoelectric properties of monolayer indium selenide (InSe) are investigated by using Boltzmann transport theory and first-principles calculations as a function of Fermi energy and crystal orientation. We find that the maximum power factor of p-type (n-type) monolayer InSe can be as large as 0.049 (0.043) W/K2m at 300 K in the armchair direction. The excellent thermoelectric performance of monolayer InSe is attributed to both its Seebeck coefficient and electrical conductivity. The large Seebeck coefficient originates from the moderate (about 2 eV) bandgap of monolayer InSe as an indirect gap semiconductor, while its large electrical conductivity is due to its unique two-dimensional density of states (DOS), which consists of an almost constant DOS near the conduction band bottom and a sharp peak near the valence band top.
TL;DR: A direct comparison of bulk and surface spectral functions reveals a time-reversal-invariant surface state in a local bandgap in the (110) bulk band structure, which connects hole and electron pockets that would otherwise be separated by an indirect local band gap.
Abstract: Time-of-flight momentum microscopy is developed. It enables direct three-dimensional mapping of the topology of the Fermi surface, identification of electron and hole pockets, and quantification of Fermi velocity as a function of wavevector.
TL;DR: In this paper, the stable YN2 monolayer with octahedral coordination is a novel p-state Dirac half metal, which not only has a fully spin-polarized Dirac state, but also has the highest Fermi velocity (3.74 × 105 m/s) of the DHMs reported to date.
Abstract: In spintronics, it is highly desirable to find new materials that can simultaneously possess complete spin-polarization, high-speed conduction electrons, large Curie temperature, and robust ferromagnetic ground states. Using first-principles calculations, we demonstrate that the stable YN2 monolayer with octahedral coordination is a novel p-state Dirac half metal (DHM), which not only has a fully spin-polarized Dirac state, but also the highest Fermi velocity (3.74 × 105 m/s) of the DHMs reported to date. In addition, its half-metallic gap of 1.53 eV is large enough to prevent the spin-flip transition. Because of the strong nonlocal p orbitals of N atoms (N-p) direct exchange interaction, the Curie temperature reaches over 332 K. Moreover, its ferromagnetic ground state can be well preserved under carrier doping or external strain. Therefore, the YN2 monolayer is a promising DHM for high-speed spintronic devices and would lead to new opportunities in designing other p-state DHMs.
TL;DR: This work realizes the BCS-BEC crossover in a nearly compensated semimetal, Fe1+ySexTe1−x, by tuning the Fermi energy εF via chemical doping, which permits us to systematically change Δ/εF from 0.16 to 0.50.
Abstract: The crossover from Bardeen-Cooper-Schrieffer (BCS) superconductivity to Bose-Einstein condensation (BEC) is difficult to realize in quantum materials because, unlike in ultracold atoms, one cannot tune the pairing interaction. We realize the BCS-BEC crossover in a nearly compensated semimetal, Fe1+y Se x Te1-x , by tuning the Fermi energy eF via chemical doping, which permits us to systematically change Δ/eF from 0.16 to 0.50, where Δ is the superconducting (SC) gap. We use angle-resolved photoemission spectroscopy to measure the Fermi energy, the SC gap, and characteristic changes in the SC state electronic dispersion as the system evolves from a BCS to a BEC regime. Our results raise important questions about the crossover in multiband superconductors, which go beyond those addressed in the context of cold atoms.
TL;DR: Structural and electronic characterizations as well as theoretical calculations unequivocally demonstrate for the first time the presence of a nearly linear energy dispersion in the vicinity of the Fermi energy, as expected of the Dirac signature for theoretical freestanding germanene.
Abstract: Bernal-stacked bilayer germanene with a stable buckled honeycomb structure has been successfully synthesized on Cu(111). Structural and electronic characterizations as well as theoretical calculations unequivocally demonstrate for the first time the presence of a nearly linear energy dispersion in the vicinity of the Fermi energy, as expected of the Dirac signature for theoretical freestanding germanene.
TL;DR: In this paper, the authors used a full van der Waals heterostructure to perform tunnelling spectroscopy measurements of the proximity effect in superconductor-graphene-superconductor junctions.
Abstract: Van der Waals heterostructures provide a tunable platform for probing the Andreev bound states responsible for proximity-induced superconductivity, helping to establish a connection between Andreev physics at finite energy and the Josephson effect. A normal conductor placed in good contact with a superconductor can inherit its remarkable electronic properties1,2. This proximity effect microscopically originates from the formation in the conductor of entangled electron–hole states, called Andreev states3,4,5,6,7,8. Spectroscopic studies of Andreev states have been performed in just a handful of systems9,10,11,12,13. The unique geometry, electronic structure and high mobility of graphene14,15 make it a novel platform for studying Andreev physics in two dimensions. Here we use a full van der Waals heterostructure to perform tunnelling spectroscopy measurements of the proximity effect in superconductor–graphene–superconductor junctions. The measured energy spectra, which depend on the phase difference between the superconductors, reveal the presence of a continuum of Andreev bound states. Moreover, our device heterostructure geometry and materials enable us to measure the Andreev spectrum as a function of the graphene Fermi energy, showing a transition between different mesoscopic regimes. Furthermore, by experimentally introducing a novel concept, the supercurrent spectral density, we determine the supercurrent–phase relation in a tunnelling experiment, thus establishing the connection between Andreev physics at finite energy and the Josephson effect. This work opens up new avenues for probing exotic topological phases of matter in hybrid superconducting Dirac materials16,17,18.
TL;DR: It is found that many half-Heusler compounds exhibit multiple triple points along four independent C_{3} axes, through which the doubly degenerate conduction bands and the nondegenerate valence band cross each other linearly nearby the Fermi energy.
Abstract: We predict the existence of triple point fermions in the band structure of several half-Heusler topological insulators by ab initio calculations and the Kane model We find that many half-Heusler compounds exhibit multiple triple points along four independent C_{3} axes, through which the doubly degenerate conduction bands and the nondegenerate valence band cross each other linearly nearby the Fermi energy When projected from the bulk to the (111) surface, most of these triple points are located far away from the surface Γ[over ¯] point, as distinct from previously reported triple point fermion candidates These isolated triple points give rise to Fermi arcs on the surface, that can be readily detected by photoemission spectroscopy or scanning tunneling spectroscopy
TL;DR: In this article, a degenerate four-wave mixing experiment on a silicon nitride (SiN) waveguide covered with graphene which was gated using a polymer electrolyte was performed.
Abstract: Third order optical nonlinearities in graphene have been demonstrated to be large and have been predicted to be highly dependent on the Fermi energy of the graphene. This prediction suggests that graphene can be used to make systems with large and electrically tunable optical nonlinearities. In this work, we present what is to our knowledge the first experimental observation of this Fermi energy dependence of the optical nonlinearity. We have performed a degenerate four-wave mixing experiment on a silicon nitride (SiN) waveguide covered with graphene which was gated using a polymer electrolyte. We observe strong dependencies of the four-wave mixing conversion efficiency on the signal-pump detuning and Fermi energy, that is, the optical nonlinearity is indeed demonstrated to be electrically tunable. In the vicinity of the interband absorption edge (2|EF| ≈ ℏω), a peak value of the waveguide nonlinear parameter of ≈6400 m–1W−1, corresponding to a graphene nonlinear sheet conductivity |σs(3)| ≈ 4.3 × 10–19 A...
TL;DR: In this article, a spin-polarized multiple Dirac ring feature is reported for the first time in an experimentally realized material, and the 3D band structure further reveals that MnF3 possesses rings of Dirac nodes.
Abstract: Spin-polarized materials with Dirac features have sparked great scientific interest due to their potential applications in spintronics. But such a type of structure is very rare and none has been fabricated. Here, we investigate the already experimentally synthesized manganese fluoride (MnF3) as a novel spin-polarized Dirac material by using first-principles calculations. MnF3 exhibits multiple Dirac cones in one spin orientation, while it behaves like a large gap semiconductor in the other spin channel. The estimated Fermi velocity for each cone is of the same order of magnitude as that in graphene. The 3D band structure further reveals that MnF3 possesses rings of Dirac nodes in the Brillouin zone. Such a spin-polarized multiple Dirac ring feature is reported for the first time in an experimentally realized material. Moreover, similar band dispersions can be also found in other transition metal fluorides (e.g., CoF3, CrF3, and FeF3). Our results highlight a new interesting single-spin Dirac material with promising applications in spintronics and information technologies.
TL;DR: The analysis indicates that the electron−electron interaction is not the sole driving force of these transitions and that lattice degrees of freedom play an important role, and the evolution of elastic constant anomalies with temperature and magnetic field allows for a detailed phase diagram.
Abstract: When a magnetic field confines the carriers of a Fermi sea to their lowest Landau level, electron-electron interactions are expected to play a significant role in determining the electronic ground state. Graphite is known to host a sequence of magnetic field-induced states driven by such interactions. Three decades after their discovery, thermodynamic signatures of these instabilities are still elusive. Here, we report the first detection of these transitions with sound velocity. We find that the phase transition occurs in the vicinity of a magnetic field at which at least one of the Landau levels cross the Fermi energy. The evolution of elastic constant anomalies with temperature and magnetic field draws a detailed phase diagram which shows that the ground state evolves in a sequence of thermodynamic phase transitions.Our analysis indicates that electron-electron interaction is not the sole driving force of these transitions and that lattice degrees of freedom play an important role.
TL;DR: Using the global particle-swarm optimization method and density functional theory, a new stable graphene-like 2D Dirac material is predicted: a Be3C2 monolayer with a hexagonal honeycomb structure that exhibits a high Fermi velocity in the same order of graphene.
Abstract: Two-dimensional (2D) materials with Dirac cones exhibit rich physics and many intriguing properties, but the search for new 2D Dirac materials is still a current hotspot. Using the global particle-swarm optimization method and density functional theory, we predict a new stable graphene-like 2D Dirac material: a Be3C2 monolayer with a hexagonal honeycomb structure. The Dirac point occurs exactly at the Fermi level and arises from the merging of the hybridized pz bands of Be and C atoms. Most interestingly, this monolayer exhibits a high Fermi velocity in the same order of graphene. Moreover, the Dirac cone is very robust and retains even included spin–orbit coupling or external strain. These outstanding properties render the Be3C2 monolayer a promising 2D material for special electronics applications.
TL;DR: In this article, van-der-Waals (vdW) tunnel barriers, fabricated by stacking layered semiconductors on top of the transition metal dichalcogenide (TMD) superconductor NbSe$_2), were used to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) devices at 70 mK.
Abstract: Tunnel junctions, a well-established platform for high-resolution spectroscopy of superconductors, require defect-free insulating barriers with clean engagement to metals on both sides. Extending the range of materials accessible to tunnel junction fabrication, beyond the limited selection which allows high-quality oxide formation, requires the development of alternative fabrication techniques. Here we show that van-der-Waals (vdW) tunnel barriers, fabricated by stacking layered semiconductors on top of the transition metal dichalcogenide (TMD) superconductor NbSe$_2$, sustain a stable, low noise tunneling current, and exhibit strong suppression of sub-gap tunneling. We utilize the technique to measure the spectra of bulk (20 nm) and ultrathin (3- and 4-layer) devices at 70 mK. The spectra exhibit two distinct energy gaps, the larger of which decreases monotonously with thickness and $T_C$, in agreement with BCS theory. The spectra are analyzed using a two-band model modified to account for depairing. We show that in the bulk, the smaller gap exhibits strong depairing in an in-plane magnetic field, consistent with a high Fermi velocity. In the few-layer devices, depairing of the large gap is negligible, consistent with out-of-plane spin-locking due to Ising spin-orbit coupling. Our results demonstrate the utility of vdW tunnel junctions in mapping the intricate spectral evolution of TMD superconductors over a range of magnetic fields.
TL;DR: In this article, the effects of the adsorption of gas molecules (CO, NO, NO2, H2S, N2, O2, NH3, and H2) on the electronic properties of atomically thin indium selenium (InSe) were investigated.
Abstract: First-principles calculations are performed to investigate the effects of the adsorption of gas molecules (CO, NO, NO2, H2S, N2, H2O, O2, NH3, and H2) on the electronic properties of atomically thin indium selenium (InSe). Our study shows that the lone-pair states of Se are located at the top of the valence band of InSe and close to the Fermi energy level, implying its high sensitivity to external adsorbates. Among these gas molecules, H2 and H2S are strong donors; NO, NO2, H2O, and NH3 are effective acceptors; while CO and N2 exhibit negligible charge transfer. The O2 molecule has very limited oxidizing ability and a relatively weak interaction with InSe which is comparable to the N2 adsorption. A clear band gap narrowing is found for the H2S, NO2, and NH3 adsorbed systems, whereas a Fermi level shifting to the conduction band is observed upon a moderate uptake of H2 molecules. Our analysis suggests several interesting applications of InSe: (1) due to the different interaction behaviors with these extern...
TL;DR: A Hartree-Fock theory for electrons on a honeycomb lattice aiming to solve a long-standing problem of the Fermi velocity renormalization in graphene is developed and an explicit expression for the optical conductivity is derived via precise quantum Monte Carlo calculations which compares well to the mean-field approach.
Abstract: We have developed a Hartree-Fock theory for electrons on a honeycomb lattice aiming to solve a long-standing problem of the Fermi velocity renormalization in graphene. Our model employs no fitting parameters (like an unknown band cutoff) but relies on a topological invariant (crystal structure function) that makes the Hartree-Fock sublattice spinor independent of the electron-electron interaction. Agreement with the experimental data is obtained assuming static self-screening including local field effects. As an application of the model, we derive an explicit expression for the optical conductivity and discuss the renormalization of the Drude weight. The optical conductivity is also obtained via precise quantum Monte Carlo calculations which compares well to our mean-field approach.
TL;DR: It is shown that including this coupling leads to entirely different conclusions because the critical fluctuations are mostly cut off by the noncritical lattice shear modes, and these predictions are relevant for the iron-based superconductors.
Abstract: Theoretically, it is commonly held that in metals near a nematic quantum critical point the electronic excitations become incoherent on the entire ``hot'' Fermi surface, triggering non-Fermi-liquid behavior. However, such conclusions are based on electron-only theories, ignoring a symmetry-allowed coupling between the electronic nematic variable and a suitable crystalline lattice strain. Here, we show that including this coupling leads to entirely different conclusions because the critical fluctuations are mostly cut off by the noncritical lattice shear modes. At sufficiently low temperatures the thermodynamics remain Fermi-liquid type, while, depending on the Fermi surface geometry, either the entire Fermi surface stays cold, or at most there are hot spots. In particular, our predictions are relevant for the iron-based superconductors.
TL;DR: In this paper, a co-precipitation method was used to synthesize Co3O4, NiO, and mixed Ni-Co oxide nanoparticles (NPs) of ∼20nm.
TL;DR: In this paper, a double layer graphene metasurface with various geometric dimensions was used to achieve near-unity absorption of the incident electromagnetic waves in the terahertz regime.
TL;DR: It is reported that ZrO is a topological material with the coexistence of three pairs of type-II triply degenerate nodal points (TNPs) and three nodal rings (NRs), when spin-orbit coupling (SOC) is ignored.
Abstract: Using first-principles calculations, we report that ZrO is a topological material with the coexistence of three pairs of type-II triply degenerate nodal points (TNPs) and three nodal rings (NRs), when spin–orbit coupling (SOC) is ignored. Noticeably, the TNPs reside around the Fermi energy with a large linear energy range along the tilt direction (>1 eV), and the NRs are formed by three strongly entangled bands. Under symmetry-preserving strain, each NR would evolve into four droplet-shaped NRs before fading away, producing distinct evolution compared with that in usual two-band NR. When SOC is included, TNPs would transform into type-II Dirac points while all of the NRs are gapped. Remarkably, the type-II Dirac points inherit the advantages of TNPs: residing around the Fermi energy and exhibiting a large linear energy range. Both features facilitate the observation of interesting phenomena induced by type-II dispersion. The symmetry protections and low-energy Hamiltonian for the nontrivial band crossings...
TL;DR: In this article, various minimal lattice models for type II Weyl semimetals were considered, with a pair of Weyl nodes sharing a single electron pocket and a single hole pocket (hydrogen model).
Abstract: Topological Weyl semimetals (TWS) can be classified as type I TWS, in which the density of states vanishes at the Weyl nodes, and type II TWS, in which an electron pocket and a hole pocket meet at a singular point of momentum space, allowing for distinct topological properties. We consider various minimal lattice models for type II TWS. The simplest time-reversal-breaking band structure, with a pair of Weyl nodes sharing a single electron pocket and a single hole pocket (hydrogen model), exhibits relics of surface Fermi arc states only away from the Fermi energy, with no topological protection. Topologically protected Fermi arcs can be restored by an additional term (hydrogen model) that produces a bulk structure where the electron and hole pockets of each Weyl point are disjoint. In time-reversal-symmetric but inversion-breaking models, we identify nontopological surface track states that arise out of the topological Fermi arc states at the transition from type I to type II and persist in the type II TWS. The distinctions among these minimal models can aid in distinguishing between generic and model-dependent behavior in studies of superconductivity, magnetism, and quantum oscillations of type II Weyl semimetals.
TL;DR: In this article, Ga-doping is used to relocate the Fermi energy in NbP sufficiently close to the W2 Weyl points, for which the different surfaces are verified by resultant quantum oscillations.
Abstract: NbP is a recently realized Weyl semimetal (WSM), hosting Weyl points through which conduction and valence bands cross linearly in the bulk and exotic Fermi arcs appear. The most intriguing transport phenomenon of a WSM is the chiral anomaly-induced negative magnetoresistance (NMR) in parallel electric and magnetic fields. In intrinsic NbP the Weyl points lie far from the Fermi energy, making chiral magneto-transport elusive. Here, we use Ga-doping to relocate the Fermi energy in NbP sufficiently close to the W2 Weyl points, for which the different Fermi surfaces are verified by resultant quantum oscillations. Consequently, we observe a NMR for parallel electric and magnetic fields, which is considered as a signature of the chiral anomaly in condensed-matter physics. The NMR survives up to room temperature, making NbP a versatile material platform for the development of Weyltronic applications.