Small Fermi Pockets Research Articles

Anomalous normal-state gap in an electron-doped cuprate.

In the underdoped n-type cuprate Nd2-xCexCuO4, long-range antiferromagnetic order reconstructs the Fermi surface, resulting in a putative antiferromagnetic metal with small Fermi pockets. Using angle-resolved photoemission spectroscopy, we observe an anomalous energy gap, an order of magnitude smaller than the antiferromagnetic gap, in a wide portion of the underdoped regime and smoothly connecting to the superconducting gap at optimal doping. After considering all the known ordering tendencies in tandem with the phase diagram, we hypothesize that the normal-state gap in the underdoped n-type cuprates originates from Cooper pairing. The high temperature scale of the normal-state gap raises the prospect of engineering higher transition temperatures in the n-type cuprates comparable to those of the p-type cuprates.

Small Fermi Pockets Intertwined with Charge Stripes and Pair Density Wave Order in a Kagome Superconductor

Small Fermi Pockets Intertwined with Charge Stripes and Pair Density Wave Order in a Kagome Superconductor

Electric transport properties of Cu-doped NbTe2 single crystals

NbTe2 is a monoclinic quasi-two-dimensional layered semimetal. This paper reports the change of physical properties of NbTe2 single crystals under the action of copper substitution. The zero-field ab-plane resistivity ρxx shows that the samples havegood metallicity. Magnetoresistance changes of up to 23% were observed at a magnetic field of 13 T without any tendency to saturate. We have observed linear magnetoresistance under high magnetic field, which will be attributed to the linear band dispersion of small Fermi pockets. And we use a formula to describe the linear relationship: ρ=NiBπn2ec∝B. The curve of the Hall resistance changing with the magnetic field at the same temperature has a linear relationship with the magnetic field, and the slope is positive, which indicates that the carrier type is mainly hole-type carriers.

The thickness dependence of quantum oscillations in ferromagnetic Weyl metal SrRuO3

In a thin Weyl semimetal, a thickness dependent Weyl-orbit quantum oscillation was proposed to exist, originating from a nonlocal cyclotron orbit via electron tunnelings between top and bottom Fermi-arc surface states. Here, magneto-transport measurements were carried out on untwinned Weyl metal SrRuO3 thin films. In particular, quantum oscillations with a frequency Fs1 ≈ 30 T were identified, corresponding to a small Fermi pocket with a light effective mass. Its oscillation amplitude appears to be at maximum for thicknesses in a range of 10 to 20 nm, and the phase of oscillation exhibits a systematic change with film thickness. The constructed Landau fan diagram shows an unusual concave downward curvature in the 1/μ0Hn-n curve, where n is the Landau level index. From thickness and field-orientation dependence, the Fs1 oscillation is attributed to be of surface origin. Those findings can be understood within the framework of the Weyl-orbit quantum oscillation effect with non-adiabatic corrections.

Searching for a promising topological Dirac nodal-line semimetal by angle resolved photoemission spectroscopy

Topological semimetals, in which conduction and valence bands cross each other at either discrete points or along a closed loop with symmetry protected in the momentum space, exhibited great potential in applications of optical devices as well as heterogeneous catalysts or antiferromagnetic spintronics, especially when the crossing points/lines matches Fermi level (E F). It is intriguing to find the ‘ideal’ topological semimetal material, in which has a band structure with Dirac band-crossing located at E F without intersected by other extraneous bands. Here, by using angle resolved photoemission spectroscopy, we investigate the band structure of the so-called ‘square-net’ topological material ZrGeS. The Brillouin zone (BZ) mapping shows the Fermi surface of ZrGeS is composed by a diamond-shaped nodal line loop at the center of BZ and small electron-like Fermi pockets around X point. The Dirac nodal line band-crossing located right at E F, and shows clearly the linear Dirac band dispersions within a large energy range >1.5 eV below E F, without intersected with other bands. The obtained Fermi velocities and effective masses along Γ–X, Γ–M and M–X high symmetry directions were 4.5–5.9 eV Å and 0–0.50 m e, revealing an anisotropic electronic property. Our results suggest that ZrGeS, as a promising topological nodal line semimetal, could provide a promising platform to investigate the Dirac-fermions related physics and the applications of topological devising.

Superconducting FeSe monolayer with millielectronvolt Fermi energy

Iron selenide (FeSe) is an iron-based superconductor which shows unique properties, including strongly anisotropic superconducting gap, paramagnetism in undoped compound and extremely small Fermi pocket size. In this work, we demonstrate that the sizes of electron and hole pockets in FeSe monolayer become much smaller than those in bulk. The Fermi energy is in the order of a few meV and can be fine-tuned by the thickness of graphene layers underneath. Despite the low carrier density, the FeSe monolayers grown on trilayer or multi-layer graphene are superconducting. The superconducting gap size is sensitive to the Fermi energy of the hole band. Remarkably, the FeSe monolayer provides the opportunity to study the physics in the crossover regime where the Fermi energy and superconducting gap are comparable to each other.

Fractional Fermi liquid in a generalized t−J model

Inspired by the recent discovery of superconductivity in the nickelate Nd$_{1-x}$Sr$_x$NiO$_2$, we study a generalized $t-J$ model to investigate the correlated phases induced by doping spin-one Ni$^{2+}$ into a spin $1/2$ Mott insulator formed by Ni$^{1+}$. Based on a three-fermion parton mean field analysis, we identify a robust fractional Fermi liquid (FL*) phase for almost every doping level. The FL* state is characterized by a small Fermi pocket on top of a spin liquid, which violates the Luttinger theorem of a conventional Fermi liquid and is an example of a symmetric pseudogap metal. Furthermore, we verify our theory in one dimension through density matrix renormalization group (DMRG) simulations on both the generalized $t-J$ model and a two-orbital Hubbard model. The fractional Fermi liquid reduces to fractional Luttinger liquid (LL*) in one dimension, which is connected to the conventional Luttinger liquid through a continuous quantum phase transition by tuning interaction strength. Our findings offer new insights into correlated electron phenomena in nickelate superconductors and other multi-orbital transition metal oxide with spin-triplet $d^8$ state.

Nonsaturating magnetoresistance, anomalous Hall effect, and magnetic quantum oscillations in the ferromagnetic semimetal PrAlSi

We report a comprehensive investigation of the structural, magnetic, transport and thermodynamic properties of a single crystal PrAlSi, in comparison to its nonmagnetic analogue LaAlSi. PrAlSi exhibits a ferromagnetic transition at $T_C$ = 17.8 K which, however, is followed by two weak phase transitions at lower temperatures. Based on the combined dc and ac magnetic susceptibility measurements, we propose the two reentrant magnetic phases below $T_C$ to be spin glasses or ferromagnetic cluster glasses. When the magnetic glassy states are suppressed by small field, several remarkable features appear. These include a linear, nonsaturating magnetoresistance as a function of field that is reminiscent of a topological or charge-compensated semimetal, and a large anomalous Hall conductivity amounting to $\sim$2000 $\Omega ^{-1}$cm$^{-1}$. Specific-heat measurements indicate a non-Kramers doublet ground state and a relatively low crystal electric field splitting of the Pr$^{3+}$ multiplets of less than 100 K. Shubnikov-de Hass oscillations are absent in LaAlSi, whereas they are clearly observed below about 25 K in PrAlSi, with an unusual temperature dependence of the dominating oscillation frequency $F$. It increases from $F$ = 18 T at 25 K to $F$ = 33 T at 2 K, hinting at an emerging Fermi pocket upon cooling into the ordered phase. These results suggest that PrAlSi is a new system where a small Fermi pocket of likely relativistic fermions is strongly coupled to magnetism. Whether hybridization between $f$ and conduction band is also involved remains an intriguing open problem.

Observation of small Fermi pockets protected by clean CuO2 sheets of a high-T c superconductor.

In cuprate superconductors with high critical transition temperature (T c), light hole-doping to the parent compound, which is an antiferromagnetic Mott insulator, has been predicted to lead to the formation of small Fermi pockets. These pockets, however, have not been observed. Here, we investigate the electronic structure of the five-layered Ba2Ca4Cu5O10(F,O)2, which has inner copper oxide (CuO2) planes with extremely low disorder, and find small Fermi pockets centered at (π/2, π/2) of the Brillouin zone by angle-resolved photoemission spectroscopy and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between superconductivity and antiferromagnetic ordering in the same CuO2 sheet. These data further indicate that superconductivity can occur without contribution from the antinodal region around (π, 0), which is shared by other competing excitations.

Electronic and magnetic structure of infinite-layer NdNiO2: trace of antiferromagnetic metal

The recent discovery of Sr-doped infinite-layer nickelate NdNiO2 offers a new platform for investigating unconventional superconductivity in nickelate-based compounds. Most intriguingly, the resistivity minimum and Hall coefficient drop were identified simultaneously in the experiment, reflecting a novel electronic structure and transport property of NdNiO2. Driven by this pioneering work, we present a first-principles calculation for the electronic and magnetic structure of undoped parent NdNiO2. By taking into account experimentally relevant interaction strength, we found that (π, π, π) antiferromagnetic NdNiO2 is a compensated bad metal with small Fermi pockets. However, due to the small exchange coupling between 3d-electrons of Ni and strong hybridization with 5d-electrons of Nd, the discovered antiferromagnetic ordering is very weak. Crucially, with the decreasing of temperature, there exists a phase transition between good paramagnetic metal and bad AFM metal. The estimated transition temperature is ~70–90 K, which is consistent with that for observing the resistivity minimum and Hall coefficient drop. In this regarding, our results provide a plausible physical interpretation for these significant experimental observations.

A New Three-Dimensional Subsulfide Ir2In8S with Dirac Semimetal Behavior

Dirac and Weyl semimetals host exotic quasiparticles with unconventional transport properties, such as high magnetoresistance and carrier mobility. Recent years have witnessed a huge number of newly predicted topological semimetals from existing databases; however, experimental verification often lags behind such predictions. Common reasons are synthetic difficulties or the stability of predicted phases. Here, we report the synthesis of the type-II Dirac semimetal Ir2In8S, an air-stable compound with a new structure type. This material has two Dirac crossings in its electronic structure along the Γ-Z direction of the Brillouin zone. We further show that Ir2In8S has a high electron carrier mobility of ∼10 000 cm2/(V s) at 1.8 K and a large, nonsaturating transverse magnetoresistance of ∼6000% at 3.34 K in a 14 T applied field. Shubnikov de-Haas oscillations reveal several small Fermi pockets and the possibility of a nontrivial Berry phase. With its facile crystal growth, novel structure type, and striking electronic structure, Ir2In8S introduces a new material system to study topological semimetals and enable advances in the field of topological materials.

Comprehensive search for topological materials using symmetry indicators.

Over the past decade, topological materials-in which the topology of electron bands in the bulk material leads to robust, unconventional surface states and electromagnetism-have attracted much attention. Although several theoretically proposed topological materials have been experimentally confirmed, extensive experimental exploration of topological properties, as well as applications in realistic devices, has been restricted by the lack of topological materials in which interference from trivial Fermi surface states is minimized. Here we apply our method of symmetry indicators to all suitable nonmagnetic compounds in all 230 possible space groups. A database search reveals thousands of candidate topological materials, of which we highlight 241 topological insulators and 142 topological crystalline insulators that have either noticeable full bandgaps or a considerable direct gap together with small trivial Fermi pockets. Furthermore, we list 692 topological semimetals that have band crossing points located near the Fermi level. These candidate materials open up the possibility of using topological materials in next-generation electronic devices.

Significant change in the electronic behavior associated with structural distortions in monocrystalline SrAg4As2

We report a combined study of transport and thermodynamic measurements on the layered pnictide material SrAg4As2. Upon cooling, a drop in electrical and Hall resistivity, a jump in heat capacity and an increase in susceptibility and magnetoresistance (MR) are observed around 110 K. These observations suggest that non-magnetic phase transitions emerge at around 110 K, that are likely associated with structural distortions. In sharp contrast with the first-principles calculations based on the crystal structure at room temperature, quantum oscillations reveal small Fermi pockets with light effective masses, suggesting a significant change in the Fermi surface topology caused by the low temperature structural distortion. No superconductivity emerges in SrAg$_4$As$_2$ down to 2 K and under pressures up to 2.13 GPa; instead, the low temperature structural distortion increases linearly with temperature at a rate of ~13 K/GPa above 0.89 GPa.

Theory of a fallen high Tc superconductivity in Ultra Low Tc Bismuth

Superconductivity with an ultra low Tc ∼ 0.5 mK was discovered recently in bismuth, a semimetal. We begin with a reference cubic lattice, of a cooperatively dimerized (cubic) A7 structure of Bi and develop a model. Using an earlier observation of Jones and Peierls of a quasi one dimensionality and half filled band character of all three 6p bands, we develop a theory for solid Bi. We suggest that electron correlation effects that gets enhanced in quasi one dimensional systems has profound consequences. We show that Bi is a failed high Tc superconductor, because of a strong competition from a valence bond dimerization instability that causes a cubic to A7 distortion. A consequent gap opening leaves the experimentally seen small fermi pockets. Signatures of the fallen high Tc superconductivity in the normal state properties of Bi and how to resurrect high Tc superconductivity are discussed. It is also likely that because of an appreciable coulomb driven inter band pair scattering, PT violating chiral superconductivity is possible. Our theory is also applicable to the A7 and cubic phases of Sb, As and P, all in the same column as Bi in the periodic table.

Quantum linear magnetoresistance in NbTe2

NbTe2 is a quasi-2D layered semimetal with charge density wave ground state showing a distorted-1T structure at room temperature. Here we report the anisotropic magneto-transport properties of NbTe2. An anomalous linear magnetoresistance up to 30% at 3 K in 9 T was observed, which can be well explained by a quantum linear magnetoresistance model. Our results reveal that a large quasi-2D Fermi surface and small Fermi pockets with linearly dispersive bands coexist in NbTe2. The comparison with the isostructural TaTe2 provides more information about the band structure evolution with charge density wave transitions in NbTe2 and TaTe2.

Charge and quadrupole fluctuations and gap anisotropy in BiS2 -based superconductors

Recent angle-resolved spectroscopy in BiS$_2$-based superconductors has indicated that the superconducting gap amplitude possesses remarkable anisotropy and/or a sign change on a small Fermi pocket around $X$ point. It implies a possibility of an unconventional pairing state. Here we study the gap anisotropy in superconductivity mediated by inherent charge/quadrupole fluctuations in an extended Hubbard model, which includes inter-site interaction between Bi and S atoms. The first-principles downfolded band structure is composed of Bi $6p_x/p_y$ and S $3p_x/p_y$ orbitals on a BiS$_2$ single layer. Evaluating the linearized gap equation, we find that the ferroic charge/quadrupole fluctuation driven by the inter-site interaction leads to a fully-gapped $d_{x^2-y^2}$-wave pairing state, in which the gap amplitude has sizable anisotropy on the Fermi surface.

Nontrivial Berry phase and type-II Dirac transport in the layered materialPdTe2

Here we report the evidence of the type II Dirac Fermion in the layered crystal PdTe2. The de Haas-van Alphen oscillations find a small Fermi pocket with a cross section of 0.077nm-2 with a nontrivial Berry phase. First-principal calculations reveal that it is originated from the hole pocket of a tilted Dirac cone. Angle Resolved Photoemission Spectroscopy demonstrates a type II Dirac cone featured dispersion. We also suggest PdTe2 is an improved platform to host the topological superconductors.

Large nonsaturating magnetoresistance and signature of nondegenerate Dirac nodes in ZrSiS

Whereas the discovery of Dirac- and Weyl-type excitations in electronic systems is a major breakthrough in recent condensed matter physics, finding appropriate materials for fundamental physics and technological applications is an experimental challenge. In all of the reported materials, linear dispersion survives only up to a few hundred millielectronvolts from the Dirac or Weyl nodes. On the other hand, real materials are subject to uncontrolled doping during preparation and thermal effect near room temperature can hinder the rich physics. In ZrSiS, angle-resolved photoemission spectroscopy measurements have shown an unusually robust linear dispersion (up to [Formula: see text]2 eV) with multiple nondegenerate Dirac nodes. In this context, we present the magnetotransport study on ZrSiS crystal, which represents a large family of materials (WHM with W = Zr, Hf; H = Si, Ge, Sn; M = O, S, Se, Te) with identical band topology. Along with extremely large and nonsaturating magnetoresistance (MR), [Formula: see text]1.4 [Formula: see text] 105% at 2 K and 9 T, it shows strong anisotropy, depending on the direction of the magnetic field. Quantum oscillation and Hall effect measurements have revealed large hole and small electron Fermi pockets. A nontrivial [Formula: see text] Berry phase confirms the Dirac fermionic nature for both types of charge carriers. The long-sought relativistic phenomenon of massless Dirac fermions, known as the Adler-Bell-Jackiw chiral anomaly, has also been observed.

Anisotropic magneto-transport and magnetic properties of low-temperature phase of TaTe2

TaTe2 is a quasi-2D charge density wave (CDW) compound with distorted-1T–type structure exhibiting double-zigzag chains. Here we report the Fermi surface topology of a low-temperature phase of TaTe2 (LT-TaTe2) by anisotropic magneto-transport and magnetic measurements on high-quality single crystals. An anomalous large linear magnetoresistance up to 140% at 3 K in 9 T was observed, suggesting the existence of a small Fermi pocket in the Dirac cone state in quantum transport models. Moreover, strong magnetic anisotropy was observed for B⊥(001) and B∥(001). Angle-dependent magnetoresistance and de Hass-van Alphen oscillations suggest the anisotropy of the normal Fermi surface and the small Fermi pocket in Dirac cone state.

Shubnikov-de Haas quantum oscillations reveal a reconstructed Fermi surface near optimal doping in a thin film of the cuprate superconductorPr1.86Ce0.14CuO4±δ

We study magnetotransport properties of the electron-doped superconductor Pr$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ with $x$ = 0.14 in magnetic fields up to 92~T, and observe Shubnikov de-Haas magnetic quantum oscillations. The oscillations display a single frequency $F$=255$\pm$10~T, indicating a small Fermi pocket that is $\sim$~1\% of the two-dimensional Brillouin zone and consistent with a Fermi surface reconstructed from the large hole-like cylinder predicted for these layered materials. Despite the low nominal doping, all electronic properties including the effective mass and Hall effect are consistent with overdoped compounds. Our study demonstrates that the exceptional chemical control afforded by high quality thin films will enable Fermi surface studies deep into the overdoped cuprate phase diagram.