Fermi Surfaces Of Metals Research Articles

Electronic structure in a transition metal dipnictide TaAs2

The family of transition-metal dipnictides has been of theoretical and experimental interest because this family hosts topological states and extremely large magnetoresistance (MR). Recently, , a member of this family, has been predicted to support a topological crystalline insulating state. Here, by using high-resolution angle-resolved photoemission spectroscopy (ARPES), we reveal both closed and open pockets in the metallic Fermi surface (FS) and linearly dispersive bands on the () surface, along with the presence of extreme MR observed from magneto-transport measurements. A comparison of the ARPES results with first-principles computations shows that the linearly dispersive bands on the measured surface of are trivial bulk bands. The absence of symmetry-protected surface state on the () surface indicates its topologically dark nature. The presence of open FS features suggests that the open-orbit fermiology could contribute to the extremely large MR of .

Colossal magnetoresistance in the multiple wave vector charge density wave regime of an antiferromagnetic Dirac semimetal.

Colossal negative magnetoresistance is a well-known phenomenon, notably observed in hole-doped ferromagnetic manganites. It remains a major research topic due to its potential in technological applications. In contrast, topological semimetals show large but positive magnetoresistance, originated from the high-mobility charge carriers. Here, we show that in the highly electron-doped region, the Dirac semimetal CeSbTe demonstrates similar properties as the manganites. CeSb0.11Te1.90 hosts multiple charge density wave modulation vectors and has a complex magnetic phase diagram. We confirm that this compound is an antiferromagnetic Dirac semimetal. Despite having a metallic Fermi surface, the electronic transport properties are semiconductor-like and deviate from known theoretical models. An external magnetic field induces a semiconductor metal-like transition, which results in a colossal negative magnetoresistance. Moreover, signatures of the coupling between the charge density wave and a spin modulation are observed in resistivity. This spin modulation also produces a giant anomalous Hall response.

Nontrivial Berry's phase and exotic quantum phenomena in the vicinity of the quantum limit in the three-dimensional Dirac system Bi0.97Sb0.03

Quantum oscillations are known as the manifestation of the Fermi surface of metals in the presence of a strong magnetic field and of associated geometrical phases such as Berry's phase, playing a critical role in topological quantum materials. This study reports quantum oscillations on the three-dimensional (3D) Dirac material Bi0.97Sb0.03 single crystals in pulsed magnetic fields up to 50 T with three possible orientations. Under a magnetic field, we observe very pronounced Shubnikov-de Haas (SdH) oscillations originating from the Landau quantization of T-point holes. The SdH oscillations were well indexed with a large Landé g factor. The anisotropic behaviors of the Landé g factor were also deduced based on a detailed analysis of SdH oscillations. The Landau fan diagram revealed a nontrivial Berry's phase. Beyond the quantum limit (QL) under an extremely strong magnetic field, unusual quantum oscillations (UQO) were found. The possible origin of this phenomenon may be electronic instability caused by interactions. These results offer insights into the unusual electronic properties under the extremely strong fields caused by the unique band topology in this case.

Quantum oscillations in the magnetization and density of states of insulators

The observation of [Formula: see text]-periodic behavior in Kondo insulators and semiconductor quantum wells challenges the conventional wisdom that quantum oscillations (QOs) necessarily arise from Fermi surfaces in metals. We revisit recently proposed theories for this phenomenon, focusing on a minimal model of an insulator with a hybridization gap between two opposite-parity light and heavy mass bands with an inverted band structure. We show that there are characteristic differences between the QO frequencies in the magnetization and the low-energy density of states (LE-DOS) of these insulators, in marked contrast to metals where all observables exhibit oscillations at the same frequency. The magnetization oscillations arising from occupied Landau levels occur at the same frequency that would exist in the unhybridized case. The LE-DOS oscillations in a disorder-free system are dominated by gap-edge states and exhibit a beat pattern between two distinct frequencies at low temperature. Disorder-induced in-gap states lead to an additional contribution to the DOS at the unhybridized frequency. The temperature dependence of the amplitude and phase of the magnetization and DOS oscillations are also qualitatively different and show marked deviations from the Lifshitz-Kosevich form well known in metals. We also compute transport to ensure that we are probing a regime with insulating upturns in the direct current (DC) resistivity.

Zeros of Green functions in topological insulators

This study demonstrates that the zeros of the diagonal components of Green functions are key quantities that can detect noninteracting topological insulators. We show that zeros of the Green functions traverse the band gap in the topological phases. The traverses induce the crosses of zeros, and the zeros' surface in the band gap, analogous to the Fermi surface of metals. By calculating the zeros of the microscopic models, we show the traverses of the zeros universally appear in all six classes of conventional noninteracting topological insulators. By utilizing the eigenvector-eigenvalue identity, which is a recently rediscovered relation in linear algebra, we prove that the traverses of the zeros in the bulk Green functions are guaranteed by the band inversions, which occur in the topological phases. The relevance of the zeros to detecting the exotic topological insulators such as the higher-order topological insulators is also discussed. For the Hamiltonians with the nearest-neighbor hoppings, we also show that the gapless edge state guarantees the zeros' surfaces in the band gap. The analysis demonstrates that the zeros can be used to detect a wide range of topological insulators and thus useful for searching new topological materials.

Resonant tunneling and bound states in the continuum

Many research topics these days involve the study of resonances in different structured media, especially in the field of optics. The seemingly different discovery of cyclotron resonance in metals by Azbel and Kaner in 1956 {Zh. Eksp. Teor. Fiz. 30, 811 (1956) [Sov. Phys. JETP 3, 772 (1956)]} that opened a new way to study the Fermi surface of metals has never been linked to optics. However, some later works of Azbel on resonant tunneling are closely related to the concepts currently developing in optics and photonics, and here I discuss some of those fundamental ideas underpinned by the physics of Fano resonances and bound states in the continuum with a few examples of their experimental demonstrations.

Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal.

The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition, and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wave vector nests the Fermi surface of metals. Here we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide. Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of the Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials.

Enhanced quantum oscillations in Kondo insulators

Quantum oscillations have long been regarded as the manifestation of the Fermi surface in metals. However, they were recently observed in Kondo insulators. We examine the Kondo screening due to Landau levels in Kondo insulators. It is shown that even for large Kondo insulating gaps, appreciable amplitudes of quantum oscillations that are consistent with experimental observations are present both in magnetization and resistivity. Specifically, we show that due to the periodic alignment between the Landau levels in the conduction and the f-orbit electrons, the Kondo screening itself undergoes oscillations so that the electronic structure oscillates with the magnetic field. Our results explain main features of quantum oscillations observed in experiments. They indicate that the non-rigidity of the electronic structure results in observable quantum oscillations Kondo insulators. This new effect provides a new way to probe the Fermi surface geometry of insulators.

Design of a 30-T Superconducting Magnet for Quantum Oscillation Application

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.

High-frequency size effect study of the Fermi surface of metals

This paper reviews experimental and theoretical work on the study of the Fermi surface of pure metals by the aid of high-frequency size effects. Particular attention is given to little-known new ballistic effects such as the time of flight effect, multichannel size effect, and nonlinear size effect.

Исследования поверхности Ферми металлов с помощью высокочастотных размерных эффектов

This paper reviews experimental and theoretical works on the study of the Fermi surface of pure metals with the aid of high-frequency size effects. Particular attention is given to little-known new ballistic effects such as the time of flight effect, multichannel size effect, and nonlinear size effect. Keywords: Fermi surface of metals, radio-frequency size effect, multichannel surface scattering, pure metals.

The Science Enjoyed by Professor Alec T. Stewart

Late Professor Stewart initiated and shaped the International Conference on Positron Annihilation (ICPA) series. As a first-generation experimental positron-annihilation scientist, he made full use of the angular correlation of annihilation radiation (ACAR) method. He applied this method to study Fermi surfaces of metals, positron wave-functions in crystals, positron-electron and -phonon many-body interactions, and the vacancy formation energy in solids. He also studied with this method positronium in liquids and solids (T. Hyodo, J. Phys. Conf. Series, 618 (2015) 012002). All these studies enjoyed by Professor Stewart will long be remembered by the positron study community.

Electrically tunable transverse magnetic focusing in graphene

Electrons in a periodic lattice can propagate without scattering for macroscopic distances despite the presence of the non-uniform Coulomb potential due to the nuclei. Such ballistic motion of electrons allows the use of a transverse magnetic field to focus electrons. This phenomenon, known as transverse magnetic focusing (TMF), has been used to study the Fermi surface of metals and semiconductor heterostructures, as well as to investigate Andreev reflection, spin-orbit interaction, and to detect composite fermions. Here we report on the experimental observation of transverse magnetic focusing in high mobility mono-, bi-, and tri-layer graphene devices. The ability to tune the graphene carrier density enables us for the first time to investigate TMF continuously from the hole to the electron regime and analyze the resulting focusing fan. Moreover, by applying a transverse electric field to tri-layer graphene, we use TMF as a ballistic electron spectroscopy method to investigate controlled changes in the electronic structure of a material. Finally, we demonstrate that TMF survives in graphene up to 300 K, by far the highest temperature reported for any system, opening the door to novel room temperature applications based on electron-optics.

Quantum oscillations and Berry's phase in topological insulator surface states with broken particle-hole symmetry

Quantum oscillations can be used to determine properties of the Fermi surface of metals by varying the magnitude and orientation of an external magnetic field. Topological insulator surface states are an unusual mix of normal and Dirac fermions. Unlike in graphene and simple metals, Berry's geometric phase in topological insulator surface states is not necessarily quantised. We show that reliably extracting this geometric phase from the phase offset associated with the quantum oscillations is subtle. This is especially so in the presence of a Dirac gap such as that associated with the Zeeman splitting or interlayer tunneling. We develop a semi-classical theory for general mixed normal-Dirac systems in the presence of a gap, and in doing so clarify the role of topology and broken particle-hole symmetry. We propose a systematic procedure of fitting Landau level index plots at large filling factors to reliably extract the phase offset associated with Berry's phase.

Thermal and electrical intralayer conductivities of underdoped cuprate superconductors from Fermi-pocket models

Several models of the electronic spectrum in the pseudogap state of underdoped cuprates have been proposed to explain photoemission and tunneling measurements, which reveal only truncated Fermi pockets instead of a full metallic Fermi surface. We consider the transport properties expected of four physically distinct models, and calculate the thermal and electrical conductivity of the electronic quasiparticles. By proposing transport currents that reflect the close correspondence between quasiparticles on the Fermi pockets in the pseudogap and those near nodes in the superconducting state, we show that measurable transport coefficients provide stringent tests of pseudogap models.

Spin polarization on Fermi surfaces of metals by the KKR method

With the implementation of a relativistic Korringa-Kohn-Rostoker Green's function and band-structure method, we analyze the spin-expectation value of the electron states on the Fermi surface of nonmagnetic as well as magnetic metals. It is shown that for relatively light elements such as Cu the spin states are well defined. A separation of all electron states to ``up'' and ``down'' spin-polarized states can be done even in the case of quite heavy but monovalent elements such as Au. In contrast, for heavy polyvalent metals such as Pt, the expectation value of the spin operator can be close to zero in large regions of the Fermi surface. In this case the nonrelativistic language of well-defined ``spin-up'' and ``spin-down'' states is not valid anymore. For magnetic materials, the relativistic Fermi surfaces change their topology with respect to the nonrelativistic majority and minority sheets because of spin-orbit driven avoided crossings of the bands. As a result, regions with vanishing spin polarization appear.

Apparent “three-dimensional” Fermi surface of transition-metal monolayers

One of the fundamental properties of an electronic system is its dimensionality. Novel photoemission apparatuses enable a direct determination of the Fermi surface of metals in all three dimensions but the data obtained from transition-metal monolayers appears to defy established experimental and theoretical insights. Our experimental approach resolves all of the existing conflicts surrounding Ni/Cu(100) and similar systems. We suggest the measurement of Fermi-surface projections to optimize spin-dependent transport properties.

Strongly coupled quantum criticality with a Fermi surface in two dimensions: Fractionalization of spin and charge collective modes

We describe two dimensional models with a metallic Fermi surface which display quantum phase transitions controlled by strongly interacting critical field theories below their upper critical dimension. The primary examples involve transitions with a topological order parameter associated with dislocations in collinear spin density wave ("stripe") correlations: the gapping of the order parameter fluctuations leads to a fractionalization of spin and charge collective modes, and this transition has been proposed as a candidate for the cuprates near optimal doping. The coupling between the order parameter and long-wavelength volume and shape deformations of the Fermi surface is analyzed by the renormalization group, and a runaway flow to a non-perturbative regime is found in most cases. A phenomenological scaling analysis of simple observable properties of possible second order quantum critical points is presented, with results quite similar to those near quantum spin glass transitions and to phenomenological forms proposed by Schroeder et al. (cond-mat/0011002).

Quantum states and Fermi surfaces in metals with an fcc lattice in an ultrastrong magnetic field

The expression for the electron wave function for a 3D crystal in a constant magnetic field is obtained in the strong coupling approximation. A 3D Harper-type equation describing the electron spectrum in magnetic 3D subbands is derived. The Fermi surfaces for monovalent noble metals are constructed for various orientations and magnitudes of magnetic fields corresponding to a rational number p/q of the magnetic flux quanta; radical changes in the topology of the Fermi surfaces in a strong magnetic field are observed. As a result, considerable changes in the physical properties of crystals in a strong magnetic field can be expected. In particular, a metal-semiconductor transition occurs for all even values of q, while metallic properties are preserved for odd values of q. The total energy of electrons as a function of the magnetic field is also calculated and shows a minimum for p/q=1/2. The type of thermodynamic oscillations in an ultrastrong magnetic field is discussed. The effects considered by the authors may be observed in fields with a strength of several tens of megagausses.

Using of slow positrons in various investigations – state of the art and perspectives

Slow positrons have become a unique probe in research of electronic structure of matter, in data acquisition on Fermi surface in metals and alloys, in studying of formation processes of structural and radiation defects in metals, semiconductors, polymers and nanocrystalline structures. First positron microscopes, PAES-spectrometers, positron ionization mass spectrometers and positron tomographs have been already created. In this paper results obtained by positron annihilation method in different research fields and possibilities of future progress in obtaining of high-intensity, high-brightness slow positron beams of variable energy are briefly discussed.