Fermi surface and strong coupling superconductivity in single crystal NdFeAsO_{1-x}F_x
C. Liu, T. Kondo, M. E. Tillman, R. Gordon, G. D. Samolyuk, Y. Lee, C. Martin, J. L. McChesney, S. Bud'ko, M. A. Tanatar, E. Rotenberg, P. C. Canfield, R. Prozo… 展开作者
C. Liu, T. Kondo, M. E. Tillman, R. Gordon, G. D. Samolyuk, Y. Lee, C. Martin, J. L. McChesney, S. Bud'ko, M. A. Tanatar, E. Rotenberg, P. C. Canfield, R. Prozorov, B. N. Harmon, A. Kaminski
We use angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic properties of the newly discovered oxypnictide superconductor, NdFeAsO_{1-x}F_x. We find a well-defined Fermi surface that consists of a large hole pocket at the Brillouin zone center and a smaller electron pocket in each corner of the Brillouin zone. The overall location and shape of the Fermi surface agrees reasonably well with calculations. The band dispersion is quite complicated with many flat bands located just below the chemical potential. We observe a superconducting gap of 20 meV, which indicates that this system is in the strong coupling regime. The emergence of a coherent peak below the critical temperature Tc and diminished spectral weight at the chemical potential above Tc closely resembles the spectral characteristics of the cuprates.
Among the widely studied superconducting iron-pnictide compounds belonging to the Ln1111 family (with Ln a lanthanide), a systematic investigation of the crossover region between the superconducting and the antiferromagnetic phase for the Ln = Nd case has been missing. We fill this gap by focusing on the intermediate doping regime of NdFeAsO(1-x)F(x) by means of dc-magnetometry and muon-spin spectroscopy measurements. The long-range order we detect at low fluorine doping is replaced by short-range magnetic interactions at x = 0.08, where also superconductivity appears. In this case, longitudin…
We investigate the temperature dependence of the lower critical field $H_{c1}(T)$, the field at which vortices penetrate into the sample, of a high-quality fluorine-doped NdFeAsO single crystal under static magnetic fields $H$ parallel to the $c$-axis. The temperature dependence of the first vortex penetration field has been experimentally obtained and pronounced changes of the $H_{c1}$(T) curvature are observed, which is attributed to the multiband superconductivity. Using a two-band model with $s$-wave-like gaps, the temperature-dependence of the lower critical field $H_{c1}(T)$ can be well …
We investigated transport and magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$ single crystal under hydrostatic pressures up to 50\,GPa. The ambient pressure superconductivity at $T_{c} \sim$ 45.4\,K is fully suppressed at $P_{c} \sim$ 21 GPa. Upon further increase of the pressure, the ferromagnetism associated with the order of rare-earth subsystem is induced at the border of superconductivity. Our finding is supported by the hysteresis in the magnetization $M$($H$) loops and the strong increase in the field cooled data, $M$($T$), toward low temperatures. We also show that the temperature evol…
High-quality single crystals of NdFeAsO$_{0.65}$F$_{0.35}$ (the transition temperature $T_{\rm c} \simeq 30.6$~K) were studied in zero-field (ZF) and transverse-field (TF) muon-spin rotation/relaxation ($μ$SR) experiments. An upturn in muon-spin depolarization rate at $T\lesssim 3$~K was observed in ZF-$μ$SR measurements and it was associated with the onset of ordering of Nd electronic moments. Measurements of the magnetic field penetration depth ($λ$) were performed in the TF geometry. By applying the external magnetic field $B_{\rm ex}$ parallel to the crystallographic $c$-axis ($B_{\rm ex}\…
Anisotropic superconducting properties including the upper critical field Hc2, thermal activation energy U0, and critical current density Jc are systematically studied in a large Ca1-xLaxFeAs2 single crystal (x ~ 0.18). The obtained Hc2 bears a moderate anisotropy gamma of approximately 2-4.2, located between those of '122' Ba1-xKxFe2As2 (1 < gamma < 2) and '1111' NdFeAsO1-xFx (5 < gamma < 9.2). Both the magnitude of U0 and its field dependence are very similar to those of NdFeAsO1-xFx, also indicating anisotropic superconductivity. Moreover, high and anisotropic Jc's exceeding 10^5 A/cm2 have…
We report the results of angle dependent resistivity of NdFeAsO$_{0.82}$F$_{0.18}$ single crystals in the superconducting state. By doing the scaling of resistivity within the frame of the anisotropic Ginzburg-Landau theory, it is found that the angle dependent resistivity measured under different magnetic fields at a certain temperature can be collapsed onto one curve. As a scaling parameter, the anisotropy $Γ$ can be determined for different temperatures. It is found that $Γ(T)$ increases slowly with decreasing temperature, varying from $Γ\simeq$ 5.48 at T=50 K to $Γ\simeq$ 6.24 at T=44 K. T…
We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals. We find that the Gamma hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15 +- 1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value. This is consistent with an isotropic or anisotropic s-wave symmetry of the order parameter or exotic d-wave symmetry with nodes located off the Fermi surface sheets. This is a significant departure from the situation i…
We report an easy single step synthesis route of title compound NdFeAsO0.80F0.20 superconductor having bulk superconductivity below 50 K. The title compound is synthesized via solid-state reaction route by encapsulation in an evacuated (10-3 Torr) quartz tube. Rietveld analysis of powder X-ray diffraction data shows that compound crystallized in tetragonal structure with space group P4/nmm. R(T)H measurements showed superconductivity with Tc (R=0) at 48 K and a very high upper critical field (Hc2) of up to 345 Tesla. Magnetic measurements exhibited bulk superconductivity in terms of diamagneti…
The crystal structure of LnFeAsO$_{1-y}$ (Ln = La, Nd) has been studied by the powder neutron diffraction technique. The superconducting phase diagram of NdFeAsO$_{1-y}$ is established as a function of oxygen content which is determined by Rietveld refinement. The small As-Fe bond length suggests that As and Fe atoms are connected covalently. FeAs$_4$-tetrahedrons transform toward a regular shape with increasing oxygen deficiency. Superconducting transition temperatures seem to attain maximum values for regular FeAs$_4$-tetrahedrons.
We study the effect of the lattice structure on the spin-fluctuation mediated superconductivity in the iron pnictides adopting the five-band models of several virtual lattice structures of LaFeAsO as well as actual materials such as NdFeAsO and LaFePO obtained from the maximally-localized Wannier orbitals. Random phase approximation is applied to the models to solve the Eliashberg equation. This reveals that the gap function and the strength of the superconducting instability are determined by the cooperation or competition among multiple spin fluctuation modes arising from several nestings am…
Hall effect and magnetoresistance have been measured on single crystals of $NdFeAsO_{1-x}F_{x}$ with x = 0 ($T_c$ = 0 $ $K) and x = 0.18 ($T_c$ = 50 $ $K). For the undoped samples, strong Hall effect and magnetoresistance with strong temperature dependence were found below about 150 K. The magnetoresistance was found to be as large as 30% at 15 K at a magnetic field of 9 T. From the transport data we found that the transition near 155 K was accomplished in two steps: first one occurs at 155 K which may be associated with the structural transition, the second one takes place at about 140 K whic…
We explore a spin-fermion model with fermion-spin-quadrupolar interaction. In a nematic phase, this interaction reduces to a four-fermion interaction that is a basis of superconductivity. When the coupling constant is positive the superconductivity is p-wave with spin-parallel paired fermions. When it is negative the superconductivity is p-wave and fermions are spin-antiparallel paired. For a system with zero chemical potential, even a very small coupling can bind fermions into bound state that leads to the superconductivity. When the chemical potential is non-zero the system possesses quantum…
We show that hole-doped cuprates can harbor {\it Fermi-surface-free superconductivity} similar to the case of the pnictides. This occurs near the doping at which a new Fermi surface pocket appears in the antinodal region. The change in Fermi surface topology is accompanied by a characteristic rise in spectral weight. Our results support the presence of a trisected superconducting dome, and suggest that superconductivity is responsible for stabilizing the $(π,π)$ magnetic order.
We find new mechanism of superconductivity beyond the spin-fluctuation theory, the standard model for unconventional superconductivity in the weak coupling approach, where Kondo fluctuations result in multi-gap superconductivity around an antiferromagnetic quantum critical point of the slave-fermion theory. Fingerprints of the hybridization mechanism are two kinds of resonance modes in not only spin but also charge fluctuations, originating from $d-wave$ pairing of conduction electrons and spinless holons, respectively, thus differentiated from the spin-fluctuation mechanism. We show that the …
We study the condition for optimizing superconductivity in the iron pnictides from the lattice structure point of view. Studying the band structure of the hypothetical lattice structure of LaFeAsO, the hole Fermi surface multiplicity is found to be maximized around the Fe-As-Fe bond angle regime where the arsenic atoms form a regular tetrahedron. Superconductivity is optimized within this three hole Fermi surface regime, while the stoner factor of the antiferromagnetism has an overall tendency of increasing upon decreasing the bond angle. Combining also the effect of the varying the Fe-As bond…
The electronic structure of the high-Tc cuprates is studied in terms of "large-U" and "small-U" orbitals. A striped structure and three types of quasiparticles are obtained, polaron-like "stripons" carrying charge, "svivons" carrying spin, and "quasielectrons" carrying both. The anomalous properties are explained, and specifically the behavior of the resistivity, Hall constant, and thermoelectric power. High-temperature superconductivity results from transitions between pair states of quasielectrons and stripons.
Ca(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ undergoes successive phase transitions with increasing Rh doping in the $T$ $=$ 0 limit. The antiferromagnetic-metal phase with orthorhombic structure at 0.00 $\le$ $x$ $\le$ 0.020 is driven to a superconducting phase with uncollapsed-tetragonal (ucT) structure at 0.020 $\le$ $x$ $\le$ 0.024; a non-superconducting collapsed-tetragonal (cT) phase takes over at $x$ $\geq$ 0.024. The breakdown of Fermi-liquid transport is observed in the ucT phase above $T_{\rm c}$. In the adjacent cT phase, Fermi-liquid transport is restored along with a disappearance of supercondu…
Conventional superconductivity follows Bardeen-Cooper-Schrieffer(BCS) theory of electrons-pairing in momentum-space, while superfluidity is the Bose-Einstein condensation(BEC) of atoms paired in real-space. These properties of solid metals and ultra-cold gases, respectively, are connected by the BCS-BEC crossover. Here we investigate the band dispersions in FeTe$_{0.6}$Se$_{0.4}$($T_c$ = 14.5 K $\sim$ 1.2 meV) in an accessible range below and above the Fermi level($E_F$) using ultra-high resolution laser angle-resolved photoemission spectroscopy. We uncover an electron band lying just 0.7 meV …
We show that an in-plane magnetic field can drive two-dimensional spin-orbit-coupled systems under superconducting proximity effect into a gapless phase where parts of the normal state Fermi surface are gapped, and the ungapped parts are reconstructed into a small Fermi surface of Bogoliubov quasiparticles at zero energy. Charge distribution, spin texture, and density of states of such "partial Fermi surface" are discussed. Material platforms for its physical realization are proposed.
We have successfully observed quantum oscillation (QO) for FeTe_{1-x}Se_{x}. QO measurements were performed using non-superconducting and superconducting thin crystals of FeTe_{0.65}Se_{0.35} fabricated by the scotch-tape method. We show that the Fermi surfaces (FS) of the non-superconducting crystal are in good agreement with the rigid band shift model based on electron doping by excess Fe while that of the superconducting crystal is in good agreement with the calculated FS with no shift. From the FS comparison of both crystals, we demonstrate the change of the cross-sectional area of the FS,…
The idea of raising Tc in the spin-fluctuation mediated superconductivity on disconnected Fermi surfaces with the gap function changing sign across but not within the Fermi pockets, proposed by Kuroki and Arita for two dimensions (2D), is here extended to three-dimensional (3D) systems. Two typical cases of 3D disconnected Fermi surfaces (stacked bond-alternating lattice and stacked ladder layers) are considered. By solving Eliashberg's equation for Green's function obtained with the fluctuation exchange approximation (FLEX) for the repulsive Hubbard model on these structures, we have shown th…
Cooper's one pair problem is investigated for a 2D lattice in the background of both passive and active Fermi sea in a weakly correlated environment. Boson exchange mechanisms involving excitons as well as phonons are invoked for pairing in the s- wave channel. The important quantities calculated are pairing energy and coherence length as functions of bosonic energy, attracted coupling constant and band filling factor. Comparison of our theoretical results with those from experiments on overdoped cuprate superconductors and other type of theoretical calculations show electronic mechanism to be…
The mechanism of superconductivity in Pr$_2$Ba$_4$Cu$_7$O$_{15-δ}$ is studied using a quasi-one dimensional double chain model with appopriate hopping integrals, on-site $U$, and off-site repulsion $V_1$. Applying the fluctuation exchange method to this model and solving the Eliashberg equation, we obtain the doping dependence of superconductivity that is consistent with the experiments. The superconducting gap has an extended s-wave-like form, which gives a temperature dependence of the spin lattice relaxation rate that does not contradict with the experimental results.
As a half-metal is spin-polarized at its Fermi level by definition, it was conventionally thought to have little proximity effect to an s-wave superconductor. Here we show that, with interface spin-orbit coupling, $p_x +ip_y$ superconductivity without spin degeneracy is induced on the half-metal, and we give an estimate of its bulk energy gap. Therefore a single-band half-metal can give us a topological superconductor with a single chiral Majorana edge state. Our band calculation shows that two atomic layers of VTe or CrO$_2$ is a single-band half-metal for a wide range ($\sim$0.1eV) of Fermi …
The importance of introducing the SO(5) singlet into the new novel model of non-Fermi liquid proposed by LeClair and collaborators is pointed out. This SO(5) singlet order parameter leads to a phase transition between the normal metal and the pseudogap region in the phase diagram for the anti-ferromagnetic (AF) phase and the (d-wave) superconducting (SC) phase. A non-zero value for this order can increase substantially the critical temperature of the SC transition.
For two-dimensional single-valley quadratic band crossing systems with weak repulsive electron-electron interactions, we show that upon introducing a chemical potential, particle-hole order is suppressed and superconductivity becomes the leading instability. In contrast to the two-valley case realized in bilayer graphene, the single-valley quadratic band touching is protected by crystal symmetries, and the different symmetries and number of fermion flavors can lead to distinct phase instabilities. Our results are obtained using a weak-coupling Wilsonian renormalization group procedure on a low…
The intercalated graphite superconductor CaC6 with Tc ~ 11.5 K has been synthesized and characterized with magnetoresistance measurements. Above the transition, the resistivity follows a T^2 dependence up to 50 K, which suggests Fermi liquid behavior. Above 50 K, the data can be fit to the Bloch-Gruneisen model providing a Debye temperature of theta = 263 K. By using McMillan formula, we estimate the electron-phonon coupling constant of lambda = 0.85 which places this material in the intermediate-coupling regime. The upper critical field is determined parallel and perpendicular to the supercon…
We performed point-contact Andreev reflection spectroscopy measurements in Ca(Fe_{1-x}Co_x)2As2 single crystals with effective x=0.060 +- 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5-6 meV. Their fit with the three-dimensional Blonder-Tinkham-Klapwijk model (making use of a analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Ferm…
The density-of-states at the Fermi energy, $N(E_F)$, is low in doped superconducting semiconductors and high-$T_C$ cuprates. This contrasts with the common view that superconductivity requires a large electron-boson coupling $λ$ and therefore also a large $N(E_F)$. However, the generic Fermi surfaces (FS) of these systems are relatively simple. Here is presented arguments showing that going from a 3-dimensional multi-band FS to a 2-dimensional and simple FS is energetically favorable to superconductivity. Nesting and few excitations of bosons compensate for a low $N(E_F)$. The typical behavior…
The resonance, a collective boson mode, was usually thought to be a possible glue of superconductivity. We argue that it is rather a natural product of the \emph{d}-wave pairing and the Fermi surface topology. A universal scaling $E_{res}/2Δ^{H}_{S}\sim 1.0$ ($Δ_{S}^{H}$ the magnitude of superconducting gap at hot spot) is proposed for cuprates, irrespective of the hole-/electron-doping, low-/high-energy resonance, monotonic/nonmonotonic \emph{d}-wave paring, and the parameters selected. We reveal that there may exist two resonance peaks in the electron-doped cuprates. The low- and high- energ…
We have grown single-crystal BiSbTe3 3D TI sample and studied structural, TE as well as pressure dependent magneto-transport properties. Large positive Seebeck coefficient confirmed the p-type nature of BiSbTe3, which is consistent with Hall measurement. We have also studied the electronic band structure using Laser-based ARPES, which revealed the existence of a Dirac-cone like metallic surface state in BiSbTe3 with a Dirac Point situated exactly at the Fermi level. Additionally, superconductivity emerges under pressure of 8 GPa with a critical temperature of ~2.5 K. With further increase of p…
The correlations between the superconductivity in iron pnictides and their electronic structures are elusive and controversial so far. Here through angle-resolved photoemission spectroscopy measurements, we show that such correlations are rather distinct in AFe$_{1-x}$Co$_x$As (A=Li, Na), but only after one realizes that they are orbital selective. We found that the superconductivity is enhanced by the Fermi surface nesting, but only when it is between $d_{xz}/d_{yz}$ Fermi surfaces, while for the $d_{xy}$ orbital, even nearly perfect Fermi surface nesting could not induce superconductivity. M…
We used angle resolved photoemission spectroscopy and thermoelectric power to study the poorly explored, highly overdoped side of the phase diagram of Ba(Fe1-xCox)2As2 high temperature superconductor. Our data demonstrate that several Lifshitz transitions - topological changes of the Fermi surface - occur for large x. Tc starts to decrease with doping when the cylindrical, central hole pocket changes to ellipsoids centering at the Z point, and goes to zero before these ellipsoids disappear around x = 0.2. Changes in thermoelectric power occur at similar x-values. Beyond this doping level the c…
In the family of the iron-based superconductors, the $RE$FeAsO-type compounds (with $RE$ being a rare-earth metal) exhibit the highest bulk superconducting transition temperatures ($T_{\mathrm{c}}$) up to $55\ \textrm{K}$ and thus hold the key to the elusive pairing mechanism. Recently, it has been demonstrated that the intrinsic electronic structure of SmFe$_{0.92}$Co$_{0.08}$AsO ($T_{\mathrm{c}}=18\ \textrm{K}$) is highly nontrivial and consists of multiple band-edge singularities in close proximity to the Fermi level. However, it remains unclear whether these singularities are generic to th…
We study a model of a two-dimensional repulsive Fermi gas with Rashba spin-orbit coupling $α_R$, and investigate the superconducting instability using renormalization group approach. We find that in general superconductivity is enhanced as the dimensionless ratio 1/2mα_R^2/E_F$ increases, resulting in unconventional superconducting states which break time reversal symmetry.
We study an effective fermion model on a square lattice to investigate the cooperation and competition of superconductivity and anti-ferromagnetism. In addition to particle tunneling and on-site interaction, a bosonic excitation mediated attractive interaction is also included in the model. We assume that the attractive interaction is mediated by spin fluctuations and excitations of Bose-Einstein condensation (BEC) in electronic systems and Bose-Fermi mixtures on optical lattices, respectively. Using an effective mean-field theory to treat both superconductivity and anti-ferromagnetism at equa…
Divergencies appearing in perturbation expansions of interacting many-body systems can often be removed by expanding around a suitably chosen renormalized (instead of the non-interacting) Hamiltonian. We describe such a renormalized perturbation expansion for interacting Fermi systems, which treats Fermi surface shifts and superconductivity with an arbitrary gap function via additive counterterms. The expansion is formulated explicitly for the Hubbard model to second order in the interaction. Numerical soutions of the self-consistency condition determining the Fermi surface and the gap functio…
We analyze a mean-field model of electrons on a square lattice with two types of interaction: forward scattering favoring a d-wave Pomeranchuk instability and a BCS pairing interaction driving d-wave superconductivity. Tuning the interaction parameters a rich variety of phase diagrams is obtained. If the BCS interaction is not too strong, Fermi surface symmetry breaking is stabilized around van Hove filling, and coexists with superconductivity at low temperatures. For pure forward scattering Fermi surface symmetry breaking occurs typically via a first order transition at low temperatures. The …
We review magnetic, superconducting and non-Fermi-liquid properties of the structurally layered heavy-fermion compounds Ce$_n$M$_m$In$_{3n+2m}$ (M=Co, Rh, Ir). These properties suggest d-wave superconductivity and proximity to an antiferromagetic quantum-critical point.
In quantum materials, electrons that have strong correlations tend to localize, leading to quantum spins as the building blocks for low-energy physics. When strongly correlated electrons coexist with more weakly-correlated conduction electrons, multiple channels of effective interactions develop and compete with each other. The competition creates quantum fluctuations having a large spectral weight, with the associated entropies reaching significant fractions of $R\ln 2$ per electron. Advancing a framework to understand how the fluctuating local moments influence unconventional superconductivi…
Low-temperature thermodynamic properties of strongly interacting Fermi liquids with fermion condensate are investigated. We demonstrate that the spin susceptibility of these systems exhibits the Curie-Weiss law, and the entropy contains a temperature-independent term. The excessive entropy is released at the superconducting transition, enhancing the specific heat jump Delta C and rendering it proportional to the effective Curie constant. The theoretical results are favorably compared with the experimental data on the heavy fermion metal CeCoIn5, as well as He-3 films.
We succeeded in growing a single crystal of Ce2CoIn8 by the flux method. The results of specific heat and electrical resistivity measurements indicate that Ce2CoIn8 is a heavy-fermion superconductor below 0.4 K with an electronic specific heat coefficient gamma as large as 500 mJ/K^2mol-Ce.
We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase diagram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetisation. The twin peak DOS may be derived from a tight-binding, quasi-one-dimensional bandstructure, inspired by previous bandstructure calculations.
We study whether Fermi-surface (FS) nesting can give rise to high-temperature superconductivity in iron pnictides. Starting with ab initio construction of an effective four-orbital model, we employ the fluctuation-exchange approximation to show that FS does not necessarily favor the stripe antiferromagnetic order observed in experiments, especially for realistic electronic correlations. If superconductivity in iron pnictides is magnetically mediated and has fully-gapped sign-reversing s-wave symmetry, our results suggest that the pairing interaction does not arise only from FS nesting and exch…
Tracking unconventional superconductivity in the presence of strongly correlated Fermi arcs
Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；Department of Materials Science and Engineering and Materials Research Institute, The Pennsylvania State University（宾夕法尼亚州立大学材料科学与工程系及材料研究所）
Plaquette lattices with each unit cell containing multiple atoms are good candidates for disconnected Fermi surfaces, which are shown by Kuroki and Arita to be favorable for spin-flucutation mediated superconductivity from electron repulsion. Here we find an interesting example in a tetramer lattice where the structure within each unit cell dominates the nodal structure of the gap function. We trace its reason to the way in which a Cooper pair is formed across the hybridized molecular orbitals, where we still end up with a T_c much higher than usual.
Fractionalized Fermi liquids (FL$^\ast$) have been introduced as non-Fermi-liquid metallic phases, characterized by coexisting electron-like charge carriers and local moments which itself form a fractionalized spin liquid. Here we investigate a Kondo lattice model on the honeycomb lattice with compass interactions among the local moments, a concrete model hosting FL$^\ast$ phases based on Kitaev's $\mathbb{Z}_2$ spin liquid. We characterize the FL$^\ast$ phases via perturbation theory, and we employ a Majorana-fermion mean-field theory to map out the full phase diagram. Most remarkably we find…
We argue that the Fermi arc observed in angle resolved photoemission measurements in underdoped cuprates can be understood as a consequence of inelastic scattering in a d-wave superconductor. We analyze this phenomenon in the context of strong coupling Eliashberg theory, deriving a `single lifetime' model for describing the temperature evolution of the spectral gap as measured by single particle probes such as photoemission and tunneling.
Motivated by a scarcity of simple and analytically tractable models of superconductivity from strong repulsive interactions, we introduce a simple tight-binding lattice model of fermions with repulsive interactions that exhibits unconventional superconductivity (beyond BCS theory). The model resembles an idealized conductor-dielectric-conductor trilayer. The Cooper pair consists of electrons on opposite sides of the dielectric, which mediates the attraction. In the strong coupling limit, we use degenerate perturbation theory to show that the model reduces to a superconducting hard-core Bose-Hu…
We consider competition of Kondo effect and s-wave superconductivity in heavy fermion and mixed valence superconductors, using the phenomenological approach for the periodic Anderson model. Similar to the well known results for single-impurity Kondo effect in superconductors, we have found principal possibility of a re-entrant regime of the superconducting transition temperature, $T_c$, in heavy fermion superconductors in a narrow range of model parameters and concentration of f-electrons. Suppression of $T_c$ in mixed valence superconductors is much weaker. Our theory has most validity in the…
When the interface of a superconductor (SC) with Weyl semimetal (WSM) supports Fermi arcs, the chirality blockade eliminates the induction of superconductivity into the bulk. This leaves the Fermi arc states as the only low-energy degrees of freedom in the proximity problem. Therefore the SC|WSM system will be a platform to probe transport properties that only involve the Fermi arcs. With a boundary condition that flips the spin at the boundary, we find a $Z_2$ protected Bogoliubov Fermi contour (BFC) around which the Bogoliubov quasi-particles disperse linearly. The resulting BFC and excitati…
Fermi surface nesting, as a peculiar reciprocal space feature, is not only closely correlated with the real space superstructure, but also directly modulates the underlying electronic behavior. In this work, we elucidate the Fermi surface nesting feature of the IrSb compound with buckled-honeycomb-vacancy (BHV) ordering through Rh and Sn doping, and its correlation with structure and electronic state evolution. The advantageous substitution of atom sites (i.e., Rh on the Ir sites, Sn on the Sb sites, respectively), rather than the direct occupation of vacancies, induces the collapse of BHV ord…
Based on recent magnetic-quantum-oscillation, ARPES, neutron-scattering and other data, we propose that superconductivity in the cuprates occurs via a convenient matching of the spatial distribution of incommensurate spin fluctuations to the amplitude and phase of the $d_{x^2-y^2}$ Cooper-pair wavefunction; this establishes a robust causal relationship between the lengthscale of the fluctuations and the superconducting coherence length. It is suggested that the spin fluctuations are driven by the Fermi surface, which is prone to nesting; they couple to the itinerant holes via the on-site Coulo…
Nuclear quadrupole resonance measurements were performed on the heavy fermion superconductor Ce2PdIn8. Above the Kondo coherence temperature T_coh \simeq 30K, the spin-lattice relaxation rate 1/T_1 is temperature independent, whereas at lower temperatures, down to the onset of superconductivity at T_c = 0.64K, it is nearly proportional to T^{1/2}. Below T_c, 1/T_1 shows no coherence peak and decreases as T^3 down to 75mK. All these findings indicate that Ce2PdIn8 is close to the antiferromagnetic quantum critical point, and the superconducting state has an unconventional character with line no…
We study superconductivity on the surface of a topological insulator, mediated by magnetic fluctuations in an adjacent ferromagnetic or antiferromagnetic insulator. Superconductivity can arise from effective interactions between helical fermions induced by interfacial fermion-magnon interactions. For both ferromagnetic and antiferromagnetic insulators, these fermion-fermion interactions have the correct structure to facilitate pairing between particles located on the same side of the Fermi surface, also known as Amperean pairing. In antiferromagnets, the strength of the induced interactions ca…
We report a study on the interplay between antiferromagnetism (AFM) and superconductivity (SC) in a heavy-fermion compound CeRhIn$_5$ under pressure $P=1.75$ GPa. The onset of the magnetic order is evidenced from a clear split of $^{115}$In-NQR spectrum due to the spontaneous internal field below the Néel temperature $T_N=2.5$ K. Simultaneously, bulk SC below $T_c=2.0$ K is demonstrated by the observation of the Meissner diamagnetism signal whose size is the same as in the exclusively superconducting phase. These results indicate that the AFM coexists homogeneously with the SC at a microscopic…
A full Fermi surface exists in underdoped high-temperature superconductors if the temperature T lies above the pseudogap temperature T*. Below T* only arcs of Fermi surface survive, scaling with T/T* as T -> 0, with T* displaying strong doping dependence. There is no accepted explanation for this behavior. We show that generalizing the BCS theory of normal superconductivity to include d-wave pairs and antiferromagnetism leads to the origin and doping dependence of the T* scale, and a quantitative description of Fermi arcs. These results place strong constraints on viable theories of high-tempe…
