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Fermi surface volume of interacting systems
Three Fermion sumrules for interacting systems are derived at T=0, involving the number expectation N-bar (μ), canonical chemical potentials μ(m), a logarithmic time derivative of the Greens function γ{sub k-vector σ} and the static Greens function. In essence we establish at zero temperature the sumrules linking: N-bar (μ)↔∑{sub m}Θ(μ−μ(m))↔∑{sub k-vector ,σ}Θ(γ{sub k-vector σ})↔∑{sub k-vector ,σ}Θ(G{sub σ}( k-vector ,0)). Connecting them across leads to the Luttinger and Ward sumrule, originally proved perturbatively for Fermi liquids. Our sumrules are nonperturbative in character and valid in a considerably broader setting that additionally includes non-canonical Fermions and Tomonaga–Luttinger models. Generalizations are given for singlet-paired superconductors, where one of the sumrules requires a testable assumption of particle–hole symmetry at all couplings. The sumrules are found by requiring a continuous evolution from the Fermi gas, and by assuming a monotonic increase of μ(m) with particle number m. At finite T a pseudo-Fermi surface, accessible to angle resolved photoemission, is defined using the zero crossings of the first frequency moment of a weighted spectral function.
Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB 6
The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator–metal transition, where theory predicts a Fermi surface associated with neutral low-energy excitations. In this paper, we reveal another route to experimentally realize a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB 6 positioned close to the insulator–metal transition. We present experimental signatures down to low temperatures (<<1 K) associated with a Fermi surface in the bulk, including a sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Finally, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB 6 , a Fermi surface arises from novel itinerant low-energy excitations that couple to magnetic fields, but not weak DC electric fields.
Spinon Fermi Surface in a Cluster Mott Insulator Model on a Triangular Lattice and Possible Application to 1 T –TaS 2
1T-TaS 2 is a cluster Mott insulator on the triangular lattice with 13 Ta atoms forming a star of David cluster as the unit cell. We derive a two-dimensional XXZ spin-1/2 model with a four-spin ring exchange term to describe the effective low energy physics of a monolayer 1T-TaS 2 , where the effective spin-1/2 degrees of freedom arises from the Kramers degenerate spin-orbital states on each star of David. Here, a large scale density matrix renormalization group simulation is further performed on this effective model and we find a gapless spin liquid phase with a spinon Fermi surface at a moderate to large strength region of the four-spin ring exchange term. All peaks in the static spin structure factor are found to be located on the “2k F ” surface of a half-filled spinon on the triangular lattice. Experiments to detect the spinon Fermi surface phase in 1T-TaS 2 are discussed.
Competition of spinon Fermi surface and heavy Fermi liquid states from the periodic Anderson to the Hubbard model
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Bulk Fermi Surfaces of the Dirac Type-II Semimetallic Candidates M Al 3 (Where M = V , Nb, and Ta)
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Discovery of segmented Fermi surface induced by Cooper pair momentum
Imaging a peculiar Fermi surface Running a current through a superconductor can cause the superconducting gap to close on a section of the Fermi surface. However, observing this segmented Fermi surface directly is tricky. To do so, Zhu et al . worked with a thin film of the topological insulator bismuth telluride placed on top of superconducting niobium diselenide. A small applied magnetic field caused a screening current, which in turn led to a segmented Fermi surface in the topological insulator layer. Using a scanning tunneling microscope, the researchers were able to map out this Fermi surface. —JS
Number of holes contained within the Fermi surface volume in underdoped high-temperature superconductors
Here, we provide a potential solution to the longstanding problem relating Fermi surface reconstruction to the number of holes contained within the Fermi surface volume in underdoped high T c superconductors. On considering uniaxial and biaxial charge-density wave order, we show that there exists a relationship between the ordering wave vector, the hole doping, and the cross-sectional area of the reconstructed Fermi surface whose precise form depends on the volume of the starting Fermi surface. We consider a “large” starting Fermi surface comprising 1+p hole carriers, as predicted by band structure calculations, and a “small” starting Fermi surface comprising p hole carriers, as proposed in models in which the Coulomb repulsion remains the dominant energy. Using the reconstructed Fermi surface cross-sectional area obtained in quantum oscillation experiments in YBa 2 Cu 3 O 6+x and HgBa 2 CuO 4+x and the established methods for estimating the chemical hole doping, we find the ordering vectors obtained from x-ray scattering measurements to show a close correspondence with those expected for the small starting Fermi surface. We therefore show the quantum oscillation frequency and charge-density wave vectors provide accurate estimates for the number of holes contributing to the Fermi surface volume in the pseudogap regime.
Electronic density of states of a U ( 1 ) quantum spin liquid with spinon Fermi surface. I. Orbital magnetic field effects
A quantum spin liquid with a spinon Fermi surface is an exotic insulator that hosts neutral Fermi surfaces inside the insulating gap. In an external magnetic field, it has been pointed out that the neutral Fermi surfaces are Landau quantized to form Landau levels due to the coupling to the induced emergent gauge magnetic field. In this paper, we calculate the electronic density of states (as observed in tunneling experiments) of the quantum spin liquid in an orbital magnetic field. Here, we find that the Landau levels from the neutral Fermi surfaces give rise to a set of steps emerging at the upper and lower Hubbard band edges. Each of the Hubbard band edge steps further develop into a band edge resonance peak when a weak gauge binding arising from the gauge field fluctuations is taken into account. Importantly, each Hubbard band edge step and its resulting resonance peak in the weak gauge binding are found to have a corresponding Landau level from the neutral Fermi surfaces, so the Hubbard band edge steps and the band edge resonance peaks provide signatures to the unique feature that characterizes the Landau quantization of the in-gap neutral Fermi surfaces in the spin liquid. We further consider the strong gauge-binding regime where the band edge resonance peaks move into the Mott gap and develop into true in-gap bound states. In the strong gauge binding regime, we solve the Landau-level spectrum of the in-gap bound states in an orbital magnetic field. For the in-gap bound state with a Mexican-hat-like band dispersion, we find that the envelop energy to have a state excited from the bound state Landau levels decreases quadratically with the magnetic field. The quadratic decrease behavior of the envelop energy is consistent with the intuition that applying magnetic field localizes the states and energetically promotes the in-gap bound states formation. Finally, we discuss the connection of our results to the electronic density of states spectra measured in the layered 1T-TaS 2 . We point out that a quantum spin liquid with a quasibound state in the upper Hubbard band can give the density of states spectra similar to the one measured in the experiment.
Surface Fermi contours and phonon anomalies at the surface of the random alloy
Abstract The impact of compositional disorder on physical properties is a subject of prime focus in many materials systems. In this paper, we report photoemission measurements of Mo 1− x Re x (110)-(1 × 1)H alloy surfaces to probe the relationship between the surface Fermi contours and giant Kohn anomalies observed in the surface phonon dispersion relations. Our measured nesting vectors precisely predict the composition dependence of the anomalous wave vectors.
Effective field theories for superconducting systems with multiple Fermi surfaces
In this work we investigate the description of superconducting systems with multiple Fermi surfaces. For the case of one Fermi surface we re-obtain the result that the superconductor is more precisely described as a topological state of matter. Studying the case of more than one Fermi surface, we obtain the effective theory describing a time reversal symmetric topological superconductor. These results are obtained by employing a general procedure to construct effective low energy actions describing states of electromagnetic systems interacting with charges and defects. The procedure consists in taking into account the proliferation or dilution of these charges and defects and its consequences for the low energy description of the electromagnetic response of the system. We find that the main ingredient entering the low energy characterization of the system with more than one Fermi surface is a non-conservation of the canonical supercurrent triggered by particular vortex configurations.
Single monkey-saddle singularity of a Fermi surface and its instabilities
Fermi surfaces can undergo sharp transitions under smooth changes of parameters. Such transitions can have a topological character, as is the case when a higher-order singularity, one that requires cubic or higher-order terms to describe the electronic dispersion near the singularity, develops at the transition. When time-reversal and inversion symmetries are present, odd singularities can only appear in pairs within the Brillouin zone. In this case, the combination of the enhanced density of states that accompanies these singularities and the nesting between the pairs of singularities leads to interaction-driven instabilities. Here, we present examples of single n = 3 (monkey-saddle) singularities when time-reversal and inversion symmetries are broken. We then turn to the question of what instabilities are possible when the singularities are isolated. For spinful electrons, we find that the inclusion of repulsive interactions destroys any isolated monkey-saddle singularity present in the noninteracting spectrum by developing Stoner or Lifshitz instabilities. In contrast, for spinless electrons and at the mean-field level, we show that an isolated monkey-saddle singularity can be stabilized in the presence of short-range repulsive interactions.
Predicting the galvanomagnetic coefficients of tungsten from Fermi-surface data
Tungsten crystal Fermi surface galvanomagnetic coefficients in scattering process by semiempirical scheme
Nontrivial Fermi surface topology of the kagome superconductor CsV 3 Sb 5 probed by de Haas–van Alphen oscillations
Here, we have investigated the normal state Fermi-surface properties of the kagome superconductor CsV 3 Sb 5 using torque magnetometry with applied fields (H) up to 35 T. The torque signal shows clear de Haas-van Alphen (dHvA) oscillations above 15 T. The oscillations are smooth and consist of seven distinct frequencies with values from similar to ~18 T to 2135 T. The presence of higher frequencies in CsV 3 Sb 5 is further confirmed by carrying out additional measurements using the tunnel diode oscillator technique. All frequencies measured at different tilt angles (θ) of the field direction with respect to the c axis show a 1/cosθ dependence, implying that the Fermi surfaces corresponding to these frequencies are two dimensional (2D). The absence of dHvA oscillations at θ = 90° further supports the presence of 2D Fermi surfaces. The Berry phase (φ) determined from the Landau level fan diagram for all frequencies is similar to ~ 0.4. This value is close to the theoretical value of φ = 0.5 for a nontrivial system, which strongly supports the nontrivial topology of the Fermi surfaces of these frequencies. Several quantities characterizing the Fermi surface are calculated employing the Lifshitz-Kosevich theory. These findings are crucial for exploring the interplay between nontrivial band topology, charge-density wave, and unconventional superconductivity of CsV 3 Sb 5 .
Super-geometric electron focusing on the hexagonal Fermi surface of PdCoO 2
Geometric electron optics may be implemented in solids when electron transport is ballistic on the length scale of a device. Currently, this is realized mainly in 2D materials characterized by circular Fermi surfaces. Here we demonstrate that the nearly perfectly hexagonal Fermi surface of PdCoO 2 gives rise to highly directional ballistic transport. We probe this directional ballistic regime in a single crystal of PdCoO 2 by use of focused ion beam (FIB) micro-machining, defining crystalline ballistic circuits with features as small as 250 nm. The peculiar hexagonal Fermi surface naturally leads to enhanced electron self-focusing effects in a magnetic field compared to circular Fermi surfaces. This super-geometric focusing can be quantitatively predicted for arbitrary device geometry, based on the hexagonal cyclotron orbits appearing in this material. These results suggest a novel class of ballistic electronic devices exploiting the unique transport characteristics of strongly faceted Fermi surfaces.
Distinct Superconducting Gap on Two Bilayer-Split Fermi Surface Sheets in Bi 2 Sr 2 CaCu 2 O 8+ δ Superconductor
High resolution laser-based angle-resolved photoemission measurements are carried out on an overdoped superconductor Bi 2 Sr 2 CaCu 2 O 8+ δ with a T c of 75 K. Two Fermi surface sheets caused by bilayer splitting are clearly identified with rather different doping levels: the bonding sheet corresponds to a doping level of 0.14, which is slightly underdoped while the antibonding sheet has a doping of 0.27 that is heavily overdoped, giving an overall doping level of 0.20 for the sample. Different superconducting gap sizes on the two Fermi surface sheets are revealed. The superconducting gap on the antibonding Fermi surface sheet follows a standard d-wave form while it deviates from the standard d-wave form for the bonding Fermi surface sheet. The maximum gap difference between the two Fermi surface sheets near the antinodal region is ~2 meV. Furthermore, these observations provide important information for studying the relationship between the Fermi surface topology and superconductivity, and the layer-dependent superconductivity in high temperature cuprate superconductors.