Small Fermi Surface Research Articles

Temperature-dependent change of the electronic structure in the Kondo lattice system YbRh2Si2

The heavy-fermion behavior in intermetallic compounds manifests itself in a quenching of local magnetic moments by developing Kondo spin-singlet many-body states combined with a drastic increase of the effective mass of conduction electrons, which occurs below the lattice Kondo temperature T K. This behavior is caused by interactions between the strongly localized 4f electrons and itinerant electrons. A controversially discussed question in this context is how the localized electronic states contribute to the Fermi surface upon changing the temperature. One expects that hybridization between the local moments and the itinerant electrons leads to a transition from a small Fermi surface in a non-coherent regime at high temperatures to a large Fermi surface once the coherent Kondo lattice regime is realized below T K. We demonstrate, using hard x-ray angle-resolved photoemission spectroscopy that the electronic structure of the prototypical heavy fermion compound YbRh2Si2 changes with temperature between 100 and 200 K, i.e. far above the Kondo temperature, T K = 25 K, of this system. Our results suggest a transition from a small to a large Fermi surface with decreasing temperature. This result is inconsistent with the prediction of the dynamical mean-field periodic Anderson model and supports the idea of an independent energy scale governing the change of band dispersion.

UTe2 : A nearly insulating half-filled j=52 5f3 heavy-fermion metal

Correlated band theory implemented as a combination of density functional theory with exact diagonalization [$\mathrm{DFT}+U$(ED)] of the Anderson impurity term with Coulomb repulsion $U$ in the open 14-orbital $5f$ shell is applied to ${\mathrm{UTe}}_{2}$. The small gap for $U=0$, evidence of the half-filled $j=\frac{5}{2}$ subshell of $5{f}^{3}$ uranium, is converted for $U=3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ to a flat-band semimetal with small heavy-carrier Fermi surfaces that will make properties sensitive to pressure, magnetic field, and off-stoichiometry, as observed experimentally. Two means of identification from the Green's function give a mass enhancement of the order of 12 for already heavy (flat) bands, consistent with the common heavy-fermion characterization of ${\mathrm{UTe}}_{2}$. The predicted Kondo temperature around 100 K matches the experimental values from resistivity. The electric field gradients for the two Te sites are calculated by $\mathrm{DFT}+U$(ED) to differ by a factor of 7, indicating a strong site distinction, while the anisotropy factor $\ensuremath{\eta}=0.18$ is similar for all three sites. The calculated uranium moment ${\ensuremath{\langle}{M}^{2}\ensuremath{\rangle}}^{1/2}$ of $3.5\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ is roughly consistent with the published experimental Curie-Weiss values of 2.8 and $3.3\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$ (which are field-direction dependent), and the calculated separate spin and orbital moments are remarkably similar to Hund's rule values for an ${f}^{3}$ ion. The $U=3\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ spectral density is compared with angle-integrated and angle-resolved photoemission spectra, with agreement that there is strong $5f$ character at, and for several hundred meV below, the Fermi energy. Our results support the picture that the underlying ground state of ${\mathrm{UTe}}_{2}$ is that of a half-filled $j=\frac{5}{2}$ subshell with two half-filled ${m}_{j}=\ifmmode\pm\else\textpm\fi{}\frac{1}{2}$ orbitals forming a narrow gap by hybridization and then driven to a conducting state by configuration mixing (spin-charge fluctuations). ${\mathrm{UTe}}_{2}$ displays similarities to ${\mathrm{UPt}}_{3}$ with its $5f$-dominated Fermi surfaces rather than a strongly localized Kondo lattice system.

Superconductivity and Normal-State Properties of Kagome Metal RbV3Sb5 Single CrystalsSupported by the National Natural Science Foundation of China (Grant Nos. 11822412 and 11774423), the National Key R&D Program of China (Grant Nos. 2018YFE0202600 and 2016YFA0300504), the Beijing Natural Science Foundation (Grant No. Z200005), the Fundamental Research Funds for the Central Universities, the Research Funds of Renmin University of China (RUC)

We report the discovery of superconductivity and detailed normal-state physical properties of RbV3Sb5 single crystals with V kagome lattice. RbV3Sb5 single crystals show a superconducting transition at T c ∼ 0.92 K. Meanwhile, resistivity, magnetization and heat capacity measurements indicate that it exhibits anomalies of properties at T * ∼ 102–103 K, possibly related to the formation of charge ordering state. When T is lower than T *, the Hall coefficient R H undergoes a drastic change and sign reversal from negative to positive, which can be partially explained by the enhanced mobility of hole-type carriers. In addition, the results of quantum oscillations show that there are some very small Fermi surfaces with low effective mass, consistent with the existence of multiple highly dispersive Dirac band near the Fermi energy level.

Anisotropic superconductivity and quantum oscillations in the layered dichalcogenide TaSnS2

${\mathrm{TaSnS}}_{2}$ single crystal and polycrystalline samples are investigated in detail by magnetization, electrical resistivity, and specific heat as well as Raman spectroscopy and nuclear magnetic resonance (NMR). Studies are focused on the temperature and magnetic field dependence of the superconducting state. We determine the critical fields for both directions $B\ensuremath{\parallel}c$ and $B\ensuremath{\perp}\phantom{\rule{0.16em}{0ex}}c$. Additionally, we investigate the dependence of the resistivity, the critical temperature, and the structure through Raman spectroscopy under high pressure up to 10 GPa. At a pressure of $\ensuremath{\approx}3\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ the superconductivity is suppressed below our minimum temperature. The Sn NMR powder spectrum shows a single line which is expected for the ${\mathrm{TaSnS}}_{2}$ phase and confirms the high sample quality. Pronounced de Haas-van Alphen oscillations in the ac susceptibility of polycrystalline sample reveal two pairs of frequencies indicating coexisting small and large Fermi surfaces. The effective mass of the smaller Fermi surface is $\ensuremath{\approx}0.5{m}_{\mathrm{e}}$. We compare these results with the band structures from DFT calculations. Our findings on ${\mathrm{TaSnS}}_{2}$ are discussed in terms of a quasi-two-dimensional BCS superconductivity.

Coexistence of Superconductivity and Charge Density Waves in Tantalum Disulfide: Experiment and Theory

The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H-TaS_{2}) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and magnetic transport properties. Around P_{c}, we observe a superconducting dome with a maximum superconducting transition temperature T_{c}=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field H_{c1} data, the temperature dependence of the H_{c1} can be well described by a single-gap anisotropic s-wave order parameter.

Deconfined criticality and ghost Fermi surfaces at the onset of antiferromagnetism in a metal

We propose a general theoretical framework, using two layers of ancilla qubits, for deconfined criticality between a Fermi liquid with a large Fermi surface, and a pseudogap metal with a small Fermi surface of electron-like quasiparticles. The pseudogap metal can be a magnetically ordered metal, or a fractionalized Fermi liquid (FL*) without magnetic order. A critical 'ghost' Fermi surface emerges (alongside the large electron Fermi surface) at the transition, with the ghost fermions carrying neither spin nor charge, but minimally coupled to $(U(1) \times U(1))/Z_2$ or $(SU(2) \times U(1))/Z_2$ gauge fields. The $(U(1) \times U(1))/Z_2$ case describes simultaneous Kondo breakdown and onset of magnetic order: the two gauge fields induce nearly equal attractive and repulsive interactions between ghost Fermi surface excitations, and this competition controls the quantum criticality. Away from the transition on the pseudogap side, the ghost Fermi surface absorbs part of the large electron Fermi surface, and leads to a jump in the Hall co-efficient. We also find an example of an "unnecessary quantum critical point" between a metal with spin density wave order, and a metal with local moment magnetic order. The ghost fermions contribute an enhanced specific heat near the transition, and could also be detected in other thermal probes. We relate our results to the phases of correlated electron compounds.

Large magnetoresistance and quantum oscillations of a ternary boride MoAlB single crystal

We report transport properties in a ternary compound MoAlB single crystal. It shows a metallic behavior with a residual resistivity ratio of $\ensuremath{\rho}$(300 K)/$\ensuremath{\rho}$(2 K)=40. After applied magnetic fields, the resistivity shows a quick upturn and saturates to a plateau as the temperature decreasing. At 2 K, the magnetoresistance (MR) increases quadratically with the magnetic field and reaches up to $1650%$ ($B=14$ T) without saturation. The two-band fitting of the Hall conductivity suggests that the unsaturated large quadratic MR in MoAlB was originated from the carriers' compensation with high mobility (${\ensuremath{\mu}}_{h}=7104\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$ and ${\ensuremath{\mu}}_{e}=1432\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$). The angle dependence of the ${\ensuremath{\rho}}_{xx}$ shows significant anisotropy. Furthermore, pronounced Shubnikov-de Haas (SdH) oscillations are observed in both magnetoresistivity and Hall resistivity. We analyzed these quantum oscillations carefully and revealed several small Fermi surfaces with the light cyclotron masses in MoAlB. The angle dependence of SdH oscillations reveals quasi-two-dimensional Fermi surface structure in MoAlB.

T-square resistivity without Umklapp scattering in dilute metallic Bi2O2Se

Fermi liquids (FLs) display a quadratic temperature (T) dependent resistivity. This can be caused by electron-electron (e-e) scattering in presence of inter-band or Umklapp scattering. However, dilute metallic SrTiO3 was found to display T2 resistivity in absence of either of the two mechanisms. The presence of soft phonons as possible scattering centers raised the suspicion that T2 resistivity is not due to e-e scattering. Here, we present the case of Bi2O2Se, a layered semiconductor with hard phonons, which becomes a dilute metal with a small single-component Fermi surface upon doping. It displays T2 resistivity well below the degeneracy temperature in absence of Umklapp and inter-band scattering. We observe a universal scaling between the T2 resistivity prefactor (A) and the Fermi energy (EF), an extension of the Kadowaki-Woods plot to dilute metals. Our results imply the absence of a satisfactory understanding of the ubiquity of e-e T2 resistivity in FLs.

Effect of uniaxial stress on the electronic band structure of NbP

The Weyl semimetal NbP exhibits a very small Fermi surface consisting of two electron and two hole pockets, whose fourfold degeneracy in $k$ space is tied to the rotational symmetry of the underlying tetragonal crystal lattice. By applying uniaxial stress, the crystal symmetry can be reduced, which successively leads to a degeneracy lifting of the Fermi-surface pockets. This is reflected by a splitting of the Shubnikov-de Haas frequencies when the magnetic field is aligned along the $c$ axis of the tetragonal lattice. In this study, we present the measurement of Shubnikov-de Haas oscillations of single-crystalline NbP samples under uniaxial tension, combined with state-of-the-art calculations of the electronic band structure. Our results show qualitative agreement between calculated and experimentally determined Shubnikov-de Haas frequencies, demonstrating the robustness of the band-structure calculations upon introducing strain. Furthermore, we predict a significant shift of the Weyl points with increasing uniaxial tension, allowing for an effective tuning to the Fermi level at only 0.8% of strain along the $a$ axis.

Hard antinodal gap revealed by quantum oscillations in the pseudogap regime of underdoped high-Tc superconductors

An understanding of the missing antinodal electronic excitations in the pseudogap state is essential for uncovering the physics of the underdoped cuprate high temperature superconductors. The majority of high temperature experiments performed thus far, however, have been unable to discern whether the antinodal states are rendered unobservable due to their damping, or whether they vanish due to their gapping. Here we distinguish between these two scenarios by using quantum oscillations to examine whether the small Fermi surface pocket, found to occupy only 2% of the Brillouin zone in the underdoped cuprates, exists in isolation against a majority of completely gapped density of states spanning the antinodes, or whether it is thermodynamically coupled to a background of ungapped antinodal states. We find that quantum oscillations associated with the small Fermi surface pocket exhibit a signature sawtooth waveform characteristic of an isolated two-dimensional Fermi surface pocket. This finding reveals that the antinodal states are destroyed by a hard gap that extends over the majority of the Brillouin zone, placing strong constraints on a drastic underlying origin of quasiparticle disappearance over almost the entire Brillouin zone in the pseudogap regime.

Linear nonsaturating magnetoresistance in the Nowotny chimney ladder compound Ru2Sn3

We present magnetoresistivity measurements in high-quality single crystals of the Nowotny chimney ladder compound Ru$_2$Sn$_3$. We find a linear and nonsaturating magnetoresistance up to 20 T. The magnetoresistance changes with the magnetic field orientation at small magnetic fields, from a positive to a negative curvature. Above 5 T, the magnetoresistance shows no sign of saturation up to 20 T for any measured angle. The shape of the anisotropy in the magnetoresistance remains when increasing temperature and Kohler's rule is obeyed. We associate the linear and nonsaturating magnetoresistance to a small Fermi surface with hot spots, possibly formed as a consequence of the structural transition. We discuss the relevance of electron-electron interactions under magnetic fields and aspects of the topologically nontrivial properties expected in Ru$_2$Sn$_3$.

From the pseudogap metal to the Fermi liquid using ancilla qubits

We propose a new parton theory of the hole-doped cuprates, describing the evolution from the pseudogap metal with small Fermi surfaces to the conventional Fermi liquid with a large Fermi surface. We introduce 2 ancilla qubits per square lattice site, and employ them to obtain a variational wavefunction of a fractionalized Fermi liquid for the pseudogap metal state. We propose a multi-layer Hamiltonion for the cuprates, with the electrons residing in the 'physical' layer, and the ancilla qubits in two 'hidden' layers: the hidden layers can be decoupled from the physical layer by a canonical transformation which leaves the hidden layers in a trivial gapped state. This Hamiltonian yields an emergent gauge theory which describes not only the fractionalized Fermi liquid, but also the conventional Fermi liquid, and possible exotic intermediate phases and critical points. The fractionalized Fermi liquid has hole pockets with quasiparticle weight which is large only on "Fermi arcs", and fermionic spinon excitations which carry charges of the emergent gauge fields.

Extent of Fermi-surface reconstruction in the high-temperature superconductor HgBa2CuO4+δ.

High magnetic fields have revealed a surprisingly small Fermi surface in underdoped cuprates, possibly resulting from Fermi-surface reconstruction due to an order parameter that breaks translational symmetry of the crystal lattice. A crucial issue concerns the doping extent of such a state and its relationship to the principal pseudogap and superconducting phases. We employ pulsed magnetic-field measurements on the cuprate [Formula: see text]Cu[Formula: see text] to identify signatures of Fermi-surface reconstruction from a sign change of the Hall effect and a peak in the temperature-dependent planar resistivity. We trace the termination of Fermi-surface reconstruction to two hole concentrations where the superconducting upper critical fields are found to be enhanced. One of these points is associated with the pseudogap endpoint near optimal doping. These results connect the Fermi-surface reconstruction to both superconductivity and the pseudogap phenomena.

Large- N approach to the two-channel Kondo lattice

This paper studies the two-channel Kondo lattice in the large-N limit at half-filling. In this model, the continuous channel-symmetry is spontaneously broken, forming a channel ferromagnet in which one conduction channel forms a Kondo insulator, while the other remains conducting. The paper discusses how this ground-state can be understood using the concept of "order fractionalization", in which the channel magnetization breaks up into an emergent spinor order parameter. By integrating out the fermions we derive an effective action that describes this symmetry breaking and its emergent collective modes. A remarkable observation is that topological defects in the order parameter carry a U(1) flux, manifested in the Aharonov-Bohm phase picked by electrons that orbit the defect. By studying the effective action, we argue that the phase diagram contains a non-magnetic transition between a large and a small Fermi surface.

Spin liquids and pseudogap metals in the SU(4) Hubbard model in a moiré superlattice

Motivated by the realization of spin-valley Hubbard on triangular moir\'e superlattice in ABC trilayer graphene aligned with hexagon boron nitride (hBN) and possibly also in twisted transition metal dichalcogenide homobilayers, we study possible Mott insulating phases and pseudogap metals based on symmetry constraint and parton mean field theories. First we show that Luttinger constraint allows two distinct symmetric and featureless Fermi liquids when there is an inter-valley Hund's term breaking $SU(4)$ spin rotation. Especially, there exists a symmetric and featureless "pseudogap metal" with small Fermi surfaces. Then we suggest to search for such an unconventional metallic state by doping the Mott insulator at $\nu_T=2$. For this purpose, we study the $\nu_T=2$ Mott insulator using $SO(6)$ Schwinger boson or Schwinger fermion parton. At the $SU(4)$ symmetric point, we find two symmetric $Z_2$ spin liquids. With a large anti inter-valley Hund's term, a featureless Mott insulator is natural. Next we show that doping the featureless Mott insulator or a $Z_2$ spin liquid can lead to featureless or orthogonal "pseudogap metal" with small Fermi surfaces proportional to the doping. Besides, we also provide one scenario for the evolution from "pseudogap metal" to the conventional Fermi liquid through an intermediate exotic "deconfined metal" phase. Last, we give brief comments on the possibility of $U(1)$ spinon fermi surface state or $Z_4$ spin liquid at $\nu_T=1$.

Interplay of structure and charge order revealed by quantum oscillations in thin films of Pr2CuO4±δ

For both electron- and hole-doped cuprates, superconductivity appears in the vicinity of suppressed broken symmetry order, suggesting that quantum criticality plays a vital role in the physics of these systems. A confounding factor in identifying the role of quantum criticality in the electron-doped systems is the competing influence of chemical doping and oxygen stoichiometry. Using high quality thin films of Pr$_{2}$CuO$_{4\pm\delta}$, we tune superconductivity and uncover the influence of quantum criticality without Ce substitution. We observe magnetic quantum oscillations that are consistent with the presence of small hole-like Fermi surface pockets, and a large mass enhancement near the suppression of superconductivity. Tuning these materials using only oxygen stoichiometry allows the observation of quantum oscillations and provides a new axis with which to explore the physics underlying the electron-doped side of the cuprate phase diagram.

Quenched nematic criticality and two superconducting domes in an iron-based superconductor

The nematic electronic state and its associated critical fluctuations have emerged as a potential candidate for the superconducting pairing in various unconventional superconductors. However, in most materials their coexistence with magnetically ordered phases poses a significant challenge in determining their importance. Here, by combining chemical and hydrostatic physical pressure in FeSe0.89S0.11, we access a nematic quantum phase transition isolated from any other competing magnetic phases. From quantum oscillations in high magnetic fields, we trace the evolution of the Fermi surface and electronic correlations as a function of applied pressure and detect a Lifshitz transition that separates two distinct superconducting regions. One emerges from the nematic phase with a small Fermi surface and strong electronic correlations, while the other one has a large Fermi surface and weak correlations that promotes nesting and stabilization of a magnetically ordered phase at high pressures. The absence of mass divergence at the nematic quantum phase transition suggests that the nematic fluctuations could be quenched by the strong coupling to the lattice or local strain effects. A direct consequence is the weakening of superconductivity at the nematic quantum phase transition in the absence of magnetically driven fluctuations. Using sulfur doping and pressure, the authors separate the nematic and magnetic phase transitions in FeSe. They find that a Lifshitz transition sits between two independent superconducting domes where nematic critical fluctuations remain finite.

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.

Characterization of quantum spin liquids and their spinon band structures via functional renormalization

We combine the pseudofermion functional renormalization group (PFFRG) method with a self-consistent Fock-like mean-field scheme to calculate low-energy effective theories for emergent spinon excitations in spin-1/2 quantum spin liquids. Using effective spin interactions from PFFRG as an input for the Fock equation and allowing for the most general types of free spinon ans\"atze as classified by the projective symmetry group (PSG) method, we are able to systematically determine spinon band structures for spin-liquid candidate systems beyond mean-field theory. We apply this approach to the antiferromagnetic $J_1$-$J_2$ Heisenberg model on the square lattice and to the antiferromagnetic nearest-neighbor Heisenberg model on the kagome lattice. For the $J_1$-$J_2$ model, we find that in the regime of maximal frustration a SU(2) $\pi$-flux state with Dirac spinons yields the largest mean-field amplitudes. For the kagome model, we identify a gapless $\mathbb{Z}_2$ spin liquid with a small circular spinon Fermi surface and approximate Dirac-cones at low but finite energies.

Angular dependence of the upper critical field in the high-pressure 1T′ phase of MoTe2

Superconductivity in the type-II Weyl semimetal candidate MoTe$_2$ has attracted much attention due to the possible realization of topological superconductivity. Under applied pressure, the superconducting transition temperature is significantly enhanced, while the structural transition from the high-temperature 1$T'$ phase to the low-temperature $T_d$ phase is suppressed. Hence, applying pressure allows us to investigate the dimensionality of superconductivity in 1$T'$-MoTe$_2$. We have performed a detailed study of the magnetotransport properties and upper critical field $H_{c2}$ of MoTe$_2$ under pressure. The magnetoresistance (MR) and Hall coefficient of MoTe$_2$ are found to be decreasing with increasing pressure. In addition, the Kohler's scalings for the MR data above $\sim$11 kbar show a change of exponent whereas the data at lower pressure can be well scaled with a single exponent. These results are suggestive of a Fermi surface reconstruction when the structure changes from the $T_d$ to 1$T'$ phase. The $H_{c2}$-temperature phase diagram constructed at 15 kbar, with $H\parallel ab$ and $H\perp ab$, can be satisfactorily described by the Werthamer-Helfand-Hohenberg model with the Maki parameters $\alpha \sim$ 0.77 and 0.45, respectively. The relatively large $\alpha$ may stem from a small Fermi surface and a large effective mass of semimetallic MoTe$_2$. The angular dependence of $H_{c2}$ at 15 kbar can be well fitted by the Tinkham model, suggesting the two-dimensional nature of superconductivity in the high-pressure 1$T'$ phase.