Fermi Vector Research Articles

Algebraic Hastatic Order in One-Dimensional Two-Channel Kondo Lattice.

The two-channel Kondo lattice likely hosts a rich array of phases, including hastatic order, a channel symmetry breaking heavy Fermi liquid. We revisit its one-dimensional phase diagram using density matrix renormalization group and, in contrast to previous work, find algebraic hastatic orders generically for stronger couplings. These are heavy Tomonaga-Luttinger liquids with nonanalyticities at Fermi vectors captured by hastatic density waves. We also find a predicted additional nonlocal order parameter due to interference between hastatic spinors, not present at large N, and residual repulsive interactions at strong coupling suggesting non-Fermi-liquid physics in higher dimensions.

Josephson current through a ferromagnetic bilayer: Beyond the quasiclassical approximation

Based on the Bogoliubov-de Gennes equations, we provide an exact numerical solution for the critical current of Josephson junctions with a composite ferromagnetic bilayer. We demonstrate that for the antiparallel orientation of the magnetic moments of the bilayer, the presence of a potential barrier at the bilayer interface results in large oscillations of the critical current as a function of ferromagnet thickness and/or exchange field. Because of this, and remarkably, in the range of small exchange field and thicknesses, the magnetism leads to the increase of the critical current. This effect is well pronounced at low temperature but disappears near $T_c$. If the potential barrier is replaced by a spin-active barrier at the bilayer interface the conventional 0-$\pi$ transition, similar to the case of an uniform ferromagnetic Josephson junction, is observed. Strikingly, for a parallel orientation of the magnetic moments of the bilayer, the presence of the spin-active barrier restores the anomalous behavior---potential barrier in the antiparallel case. These behaviors result from the resonant tunneling of Cooper pairs across the composite barrier---an effect related to the spin-dependent Fermi vector in the presence of the ferromagnets' exchange field.

Hybridization Effects Revealed by Angle-Resolved Photoemission Spectroscopy in Heavy-Fermion Ce2IrIn8**Supported by the National Key Research and Development Program of China under Grant Nos 2016YFA0401000, 2015CB921300, 2016YFA0300303, 2016YFA0401002 and 2017YFA0303103, the National Natural Science Foundation of China under Grant Nos 11674371, 11774401 and 11874330, the Strategic Priority Research Program (B) of the Chinese Academy of Sciences under Grant No XDB07000000, and

We utilize high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce2IrIn8. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature , which characterizes the electrical resistance maximum and indicates the onset temperature of hybridization. However, the Fermi vector and the Fermi surface volume have little change around Tcoh, which challenges the widely believed evolution from a high-temperature small Fermi surface to a low-temperature large Fermi surface. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory calculations.

Exploration of phase structure evolution induced by alloying elements in Ti alloys via a chemical-short-range-order cluster model

The prominent comprehensive properties of solid-solution- and intermetallic-based Ti alloys are derived from their diverse microstructures induced by multi-component alloying, which results in a chemical composition complexity. A cluster-plus-glue-atom model, characterizing the chemical short-range orders, was introduced to explore the relationships among the local atomic distributions of alloying elements in different phase structures of Ti alloys, including α-Ti, β-Ti, ω-Ti, α2-Ti3Al, γ-TiAl, O-Ti2AlNb, and B2-Ti(Al,Nb). Specific cluster structural units, i.e., cluster formulas, for these phases were determined with the guide of the Friedel oscillation theory for electron-structure stabilization. It is due to the change of cluster structural units that induces the phase transformation, which is attributed to the amounts of primary alloying elements of Al and Nb. The total atom number (Z) values in these cluster structural units, calculated by the Fermi vector kF, are all very close to the integer of Z = 16. Furthermore, the composition rules of industrial multi-component Ti alloys based on these phases were generalized in light of the cluster formula approach, which will open up a new route towards designing high-performance Ti alloys with complex compositions.

Plasmonic waves in two-dimensional electron gas

The electromagnetic waves applied on surfaces of materials with high density of free charges induce the generation of plasmonic waves both in volume and surface. Plasmonic waves in metals have been widely studied; also, recently plasmons have been investigated in intrinsic and doped semiconductor materials. In this work, plasmonic waves are simulated in a two-dimensional electron gas. Two-dimensional electron gas may be created at the interfaces between GaAs and AlxGa1-xAs. The plasmons propagation speed depends on Fermi velocity and Fermi vector of the two-dimensional electron gas. The 2D plasmons dispersion relations deviate from light dispersion at all frequencies and does not have an asymptotic character as in three-dimensional electron gas; this allows operating plasmonic devices with speeds much lower than the speed of light. At low frequencies the plasmon wave vector is much smaller than the Fermi vector and the geometric capacitance dominates the quantum capacitance. The propagation of plasmons in channels without gate is studied.

Bound-state dynamics in one-dimensional multispecies fermionic systems

In this work we provide for a description of the low-energy physics of interacting multi-species fermions in terms of the bound-states that are stabilized in these systems when a spin gap opens. We argue that, at energies much smaller than the spin gap, these systems are described by a Luttinger liquid of bound-states that depends, on top of the charge stiffness $\nu$ and the charge velocity $u$, on a "Fermi" momentum $P_F$ satisfying $qP_F = Nk_F$ where $q$ is the charge of the bound-state, $N$ the number of species and $k_F$ is the Fermi momentum in the non-interacting limit. We further argue that for generic interactions, generic bound-states are likely to be stabilized. They are associated with emergent, in general non-local, symmetries and are in the number of five. The first two consist of either a charge $q=N$ local $SU(N)$ singlet or a charge $q=N$ bound-state made of two local $SU(p)$ and $SU(N-p)$ singlets. In this case the Fermi momentum $P_F=k_F$ is preserved. The three others have an enhanced Fermi vector $P_F$. The latter are either charge $q=2$ bosonic p-wave and s-wave pairs with $SO(N)$ and $SP(N)$ symmetry and $P_F=Nk_F/2$ or a composite fermion of charge $q=1$ with $P_F=Nk_F$. The instabilities of these Luttinger liquid states towards incompressible phases and their possible topological nature are also discussed.

A model of the response of GMR of metallic multilayers to external magnetic field

AbstractIt has not been possible to transform resistivity models in terms of magnetic field in order to account for variation of giant magnetoresistance (GMR) with external magnetic field, which would have led to determination of material properties. This problem is approached mathematically via variation calculus to arrive at an exponential function that fits observed GMR values. Using this model in free electron approximation, the mean Fermi vector, susceptibility and total density of states of a number of metallic multilayers are determined from their reported GMR values. Susceptibility is found to depend on interface roughness and antiferromagnetic (AF) coupling; thus, it gives qualitative measure of interface quality and AF coupling. Comparison of susceptibilities and GMRs of electrodeposited and ion beam sputtered Co/Cu structures shows that a rough interface suppresses GMR in the former but enhances it in the latter.

Twofold Andreev reflections and coherent transport in ferromagnetic semiconductor/d-wave superconductor/ferromagnetic semiconductor double tunneling junctions

Coherent transport in a ferromagnetic semiconductor (FS)/d-wave superconductor (SC)/FS structure with {110} interfaces is studied by extending Bogoliubov-de Gennes equation into eight components, in which the interband coupling of heavy and light hole bands in the FS, the strengths of potential scattering at the interfaces, and the mismatches in the effective mass and Fermi vector between the FS and SC are taken into account. Twofold Andreev reflections exist due to the existence of two bands in the FS, in which the incident hole and the two Andreev-reflected electrons, belonging to the different spin subbands, form twofold spin-singlet pairing states near the FS/SC interface. It is shown that due to the interplay of the SC with unconventional d-wave pairing symmetry and FS, the differential conductance and tunneling magnetoresistance exhibit an abundant dependence on not only the interband coupling in the FS but also the strengths of potential scattering at the interfaces. More importantly, the properties are found to be quite different from those in the FS/s-wave SC/FS structure with conventional pairing symmetry for the SC.

Evidence of momentum-dependent hybridization inCe2Co0.8Si3.2

We studied the electronic structure of the Kondo lattice system Ce2Co0.8Si3.2 by angle-resolved photoemission spectroscopy (ARPES). The spectra obtained below the coherence temperature consist of a Kondo resonance, its spin-orbit partner and a number of dispersing bands. The quasiparticle weight related to the Kondo peak depends strongly on Fermi vectors associated with bulk bands. This indicates a highly anisotropic hybridization between conduction band and 4f electrons - V_{cf} in Ce2Co0.8Si3.2.

Suppression of 3D mobility of carrier and superconductivity by Y+3 substitution in Cu0.5Tl0.5Ba2(Ca2−xYx)Cu3O10−δ samples

Bulk Cu0.5Tl0.5Ba2(Ca2−xYx)Cu3O10−δ superconductor ceramic samples were synthesized by the conventional solid-state method and characterized by X-ray diffraction, dc-resistivity, ac-susceptibility and Fourier Transform Infrared spectroscopy. The main purpose of this study was to investigate the role of charge carriers and the effect of Y substitution at Ca sites in between the CuO2 planes on superconductivity. The superconducting properties are suppressed by Y substitution at Ca sites in between the CuO2 planes of Cu0.5Tl0.5Ba2(Ca2−xYx)Cu3O10−δ samples. It is most likely that Y3+ may create correlated domains in between the CuO2 planes and localizes the carriers, which lowers the diamagnetic screening and suppresses the superconductivity. Therefore, cationic substitution reduces the three dimensional (3D) mobility of carriers, resulting in the reduction of the Fermi vector and velocity of the carriers, which in turn suppresses the superconducting properties of the material.

Excess Conductivity Analysis of Tl1−x Y x Ba2Ca2Cu3O10−δ Superconductors

Excess conductivity analyses of resistivity data of Tl1−xYxBa2Ca2Cu3O10−δ (x=0, 0.04, 0.06, 0.08, 0.1) samples have been carried out by following Lawrence & Doniach (LD) and Maki-Thompson (MT) models. In the critical regime important superconductivity parameters have been elucidated by employing Ginzburg-Landau number NG of Ginzburg Landau theory. Our samples have shown a decrease in the Tc (R=0) and magnitude of diamagnetism with increased Y-doping. The cell parameters and volume of the unit cell increase with doping of Y+3 in Tl1−xYxBa2Ca2Cu3O10−δ (x=0, 0.04, 0.06, 0.08, 0.1) which shows a decrease in the density of charge carriers in the conducting CuO2 planes. Since the Fermi vectors of the carriers, KF=[3π2N/V]1/3=[3π2n]1/3, their coherence length along the c-axis, ξc=ħKF/2mΔ, and the Fermi velocity, VF=ħKF/m depend on density of mobile charge carriers, the doping of Y+3 suppresses it and hence the superconductivity parameters. We have confirmed these conjectures with the excess conductivity analyses (FIC) of our conductivity data. The FIC analysis of conductivity data has shown a decrease in the values of ξc, VF, Bc(0), Bc1(0) and Jc(0) with increase doping of Y (except for the samples with x=0.04). The width of two dimensional conductivity regimes is shrunken with increased Y-doping. From these studies it is concluded that presence of Y+3 in the unit cell of TlBa2Ca2Cu3O10−δ impedes the flow of the mobile charge carriers to the conducting CuO2 planes which induce suppression in the superconductivity parameters. The studies also stress the vital role of mobile charge carriers in the mechanism of high temperature superconductivity.

Inducing topological order in a honeycomb lattice

We explore the possibility of inducing a topological insulator phase in a honeycomb lattice lacking spin-orbit interaction using a metallic (or Fermi gas) environment. The lattice and the metallic environment interact through a density-density interaction without particle tunneling, and integrating out the metallic environment produces a honeycomb sheet with in-plane oscillating long-ranged interactions. We find the ground state of the interacting system in a variational mean-field method and show that the Fermi wave vector, kF, of the metal determines which phase occurs in the honeycomb lattice sheet. This is analogous to the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism in which the metal's kF determines the interaction profile as a function of the distance. Tuning kF and the interaction strength may lead to a variety of ordered phases, including a topological insulator and anomalous quantum-hall states with complex next-nearest-neighbor hopping, as in the Haldane and the Kane-Mele model. We estimate the required range of parameters needed for the topological state and find that the Fermi vector of the metallic gate should be of the order of 3Pi/8a (with a being the graphene lattice constant). The net coupling between the layers, which includes screening in the metal, should be of the order of the honeycomb lattice bandwidth. This configuration should be most easily realized in a cold-atoms setting with two interacting Fermionic species.

Axial Anomaly, Mismatched Fermi Surfaces and Vector Interaction in Dense Neutral Quark Matter

We report on effects of the q-$\bar{\rm q}$ vector interaction and/or the U(1)$_A$-anomaly-induced chiral-diquark coupling on the charge-neutral quark matter in $\beta$-equilibrium. We show that when the vector coupling is absent, there can appear a cross-over region sandwiched by two critical points in the intermediate temperature ($T$) region, while the phase transition in the low-$T$ region including zero temperature keeps being first order until the strength of the anomaly term is increased to have a critical value. On the other hand, when the vector coupling is also present, there appears a crossover region in the low-$T$ area including zero temperature with a new critical point, as was first demonstrated by Kitazawa et al and the present authors without and with the charge-neutral condition, respectively. We remark that the possible chromomagnetic instability is suppressed and can be even completely absent owing to the enhanced diquark coupling due to the anomaly term and/or by the vector interaction.

The e/ a values of ideal metallic glasses in relation to cluster formulae

The electrons per atom ratio e/ a is an important factor in discussing the stabilization mechanism of metallic glasses. However, the e/ a assignment is complicated by hybridization effects, particular in transition metal systems. We have previously proposed the “cluster-plus-glue-atom” model for metallic glasses and the relevant cluster formula [cluster](glue atom) x , x ∼ 1 or 3. In the present paper, this model is combined with Häussler’s global-resonance model into a so-called cluster-resonance model. This model allows the calculation of the Fermi vector and hence the effective e/ a of an ideal metallic glass. By assuming an ideal dense packing of the clusters, the atomic density can be obtained and e/ a becomes associated with the number of atoms in the unit cluster formula Z. Experiments are carried out to verify these correlations. Alloys like Cu 8Zr 5Al, Ni 4Zr 9Al 2 and [Ni 7Nb 5Zr]Ni 3, possessing locally the highest glass-forming abilities, are interpreted by cluster formulae, with their Zs being determined by their e/ a values. Such verifications not only validate the proposed e/ a calculation schemes and the models behind them, but also support the ideal metallic glass feature of bulk metallic glasses with high glass-forming abilities.

Exchange interaction between magnetic adatoms on surfaces of noble metals

We present first-principles calculations of the exchange interactions between magnetic impurities deposited on (001), (110), and (111) surfaces of Cu and Au and analyze them, in particular, in the asymptotic regime. For the (110) and the (111) surfaces, we demonstrate that the interaction shows an oscillatory behavior as a function of the distance, $R$, of the impurities and that the amplitude of the oscillations decays as 1/${R}^{2}$. Furthermore, the frequency of the oscillations is closely related to the length of the Fermi vector of the surface states existing on these surfaces. Due to the asymmetry of the surface-states dispersion, the frequency of the oscillations becomes also asymmetric on the (110) surfaces, while on the Au(111) surface two distinct frequencies are found in the oscillations as a consequence of the Bychkov-Rashba splitting of the surface states. Remarkably, no long-range oscillations of the exchange interaction are observed for the (001) surfaces where the surface states are unoccupied. When burying the impurities beneath the surface layer, oscillations mediated by the bulk states become visible.

Composition formulae of ideal metallic glasses and their relevant eutectics established by a cluster-resonance model

Composition formulae for ideal metallic glasses are explored by combining the cluster-plus-glue-atom model with the global resonance model, termed the cluster-resonance model for short. The former model gives the [cluster]1(glue atom) x cluster formulae, stressing the local cluster order of a glassy structure; the latter model extends the local cluster order to a medium-range one by introducing spherical periodicity that relates the cluster size with Fermi vector, k F. Such a correlation allows the calculation of Fermi energy, E F, and electrochemical potential of electrons of the system from any local clusters. The cluster-resonance model also implies the equilibrium of the electrochemical potentials of electrons between different clusters so that the number of glue atoms matching one cluster (x in the cluster formula) can be determined. Examples in the Cu–Zr–Al and B–Co–Si–Ta systems are given to show the effectiveness of the proposed model and the resulting cluster formulae in interpreting multicomponent metallic-glass compositions as well as their relevant binary eutectic points.

Electronic structure of U(Ru1−xRhx)2Si2 studied by laser angle-resolved photoemission spectroscopy

We have studied electronic structures of U(Ru1-xRhx)2Si2 employing ultrahigh-resolution laser angle-resolved photoemission spectroscopy to understand the effect of Rh substitution. A hole-like dispersive feature, which presumably has Rh d-band character, was observed in U(Ru1-xRhx)2Si2 for both x = 0 and x = 0.03. However, although a heavy quasiparticle band appears in the hidden-order state of URu2Si2 (x = 0), it was not observed for x = 0.03 over the temperature range studied. In addition, it was found that energy-distribution curves at the Fermi vector of the hole-like band for x = 0.03 behave similarly to the Fermi-Dirac function. We also present the mapping of Fermi surfaces formed by the hole-like band, which stay nearly unaffected by Rh substitution.

'Quantized' states of the charge-density wave in microcrystals of K0.3MoO3

Quantization of electrons in solids can typically be observed in microscopic samples if the mean free path of the electrons exceeds the dimensions of the sample. A special case is a quasi one-dimensional metal, in which electrons condense into a collective state. This state, a charge-density wave (CDW), is a periodic modulation of both the lattice and electron density. Here, we demonstrate that samples of K(0.3)MoO(3), a typical CDW conductor, show jumps in conduction, regular in temperature. The jumps correspond to transitions between discrete states of the CDW and reveal the quantization of the wave vector of electrons near the Fermi vector. The effect involves both quantum and classical features of the CDW: the quantum condensate demonstrates modes, resembling those of a classical wave in a resonator. The analysis of the steps allows extremely precise studies of the CDW wave-vector variations and reveals new prospects for structural studies of electronic crystals and fine effects in their electronic states and lattice motions.

Quantum state depressions in thin metal films with an indented surface

Modification of properties of metal films caused by indents on their surface are studied. It is shown that indents on a film surface lead to quantum state depression (QSD), i.e., a decrease in the density of quantum states of a free electron. The density of the wave vector in the k-space decreases throughout the Fermi sphere. At the same time, the total number of electrons is conserved, since the metal remains electrically neutral. According to the Pauli Exclusion Principle, some electrons will occupy states with higher wave numbers. The Fermi vector and the Fermi energy of the thin metal films increase and, therefore, the work function decreases. Experiments have demonstrated a decrease in the work function of the thin indented films of Au, Nb, Cr, and SiO2. Experimental results are qualitatively consistent with the theoretical predictions.

Hybridization effects inCeCoIn5observed by angle-resolved photoemission

We have investigated the low-energy electronic structure of the heavy fermion superconductor $\mathrm{Ce}\mathrm{Co}{\mathrm{In}}_{5}$ by angle-resolved photoemission. We focus on the dispersion and the peak width of the prominent quasi-two-dimensional Fermi surface sheet at the corner of the Brillouin zone as a function of temperature along certain $k$ directions with a photon energy of $h\ensuremath{\nu}=100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We find slight changes of the Fermi vector and an anomalous broadening of the peak width when the Fermi energy is approached. Additionally, we performed resonant angle-resolved photoemission spectroscopy experiments with $h\ensuremath{\nu}=121\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. A flat $f$-derived band is observed with a distinct temperature dependence and a $k$-dependent spectral weight. These results, including both off- and on-resonant measurements, fit qualitatively to a two-level mixing model derived from the periodic Anderson model.