Search

We observe the magnetic quantum oscillation in the heat capacity of the Kondo insulator YbB$_{12}$. The frequency of these oscillations $F = 670$ T, aligns with findings from magnetoresistance and torque magnetometry experiments for $\mu_0 H > 35$ T in the Kondo insulating phase. Remarkably, the quantum oscillation amplitudes in the heat capacity are substantial, with $\Delta \tilde{C}/T \approx$ 0.5 $\rm{mJ}$ $\rm{mol^{-1}K^{-2}}$ at 0.8 K, accounting for 13$\%$ of the known linear heat capacity coefficient $\gamma$. Double-peak structures of quantum-oscillation amplitudes due to the distribution function of fermions were identified and used to determine the value of the effective mass from the heat capacity, which agrees well with that from torque magnetometry. These observations support charge-neutral fermions contributing to the quantum oscillations in YbB$_{12}$., Comment: 4 figures in the main text, and 3 more figures

We analyze a model for electronic structure and interactions in twisted transition metal chalcogenide WSe$_2$ for superconductivity. In this material, spin-orbit scattering locks the z-components of spins of low-energy fermions near the Dirac ${\bf K}$ and ${\bf K}'$ points of the hexagonal Brillouin zone, reducing the symmetry of spin-spin interactions to that of an xy model. We show that a nominally repulsive 4-fermion interaction gives rise to an attraction for pairing in a two-component $E^{-}$ channel, which is a hexagonal lattice representation of the $\ell =1$ channel. The gap function is inversion-odd and a linear combination of spin singlet and spin triplet. At weak coupling superconductivity emerges via the Kohn-Luttinger mechanism; we compute $T_c$ for the Fermi-level lying close to the van Hove singularity. At strong coupling, the pairing is mediated by XY magnetic fluctuations peaked at momenta ${\bf K}-{\bf K}' = 2 {\bf K}$ and we estimate $T_c$ using the form of the quantum-critical XY fluctuations, displaying $\omega/T$ scaling., Comment: 23 pages, 5 figures

Transport experiments in twisted bilayer graphene (TBG) show a fan-like region near integer fillings with a resistivity linear in temperature down to the lowest temperature measured. This suggests quantum-critical points at the boundaries to long-range ordered phases. The particular order proposed by Blutinck et al. for twisted bi-layer graphene (TBG) is a loop-current order at the carbon length-scale together with modulations on the moir\'e length scale. This is shown to be the ground state of a xy model with translational symmetry and time-reversal broken. Here, this is extended to derive a model for quantum-critical properties. We derive the quantum xy model coupled to fermions in this situation. The kinetic energy operator for the model and the coupling of fermions to the fluctuations of the xy model are derived. The previously derived universal properties in the quantum-critical region of such a model, leading to a marginal Fermi-liquid, irrespective of the underlying microscopics is briefly reviewed. The properties include the resistivity and various other transport properties with and without applying a magnetic field and the instability of the quantum fluctuating state to superconductivity in d-wave symmetry.

A fluctuating-valence impurity in a metal is quantum-critical unlike a Kondo impurity which has the properties of a local Fermi-liquid. A systematic theory for the fluctuating-valence lattice is constructed, based on the hybridization and pairing of itinerant d-orbitals with localized f-orbitals both of which are essential parts of the solution of the impurity problem. It also uses the fact that the single-particle excitations at the Fermi-surface in any dimension can be written as orthogonal Majoranas and those with linear departures from the Fermi-surface as linear combination of bare particles and holes with the same spin. The calculations on the lattice give four spin-degenerate one-particle excitations of fractionalized fermions; two sets disperse across the chemical potential and the other two have gaps. The former are shown to be dark to any linear electro-magnetic probes of their charge and spin and observable only through probes of their free-energy such as a Fermi-liquid specific heat and magneto-oscillations characteristic of a Fermi-surface but without a Zeeman splitting. The excitations with the gaps behave as in insulators but with renormalized amplitudes. The superfluid density is zero. A magnetic field $H$ turns the insulator to a metal with a singularity in magnetization proportional to $\sqrt{H - H_c}$, with $H_c$ related to the gap. Beyond $H_c$, the usual Zeeman splitting appears in the magneto-oscillations. The properties and predictions are compared to the momentous recent discoveries in fluctuating-valence insulators. Similar excitations may be expected in transition metal chalcogenide layers at fluctuating-valence, and quite likely for Kagome lattices, and twisted multi-layer graphene near specific fillings., Comment: Extended version with clear exposition of the starting basis, calculations, figures and derivation of results in comparison with experiments

A physical model for a mixed-valence impurity in a metal must satisfy the Friedel screening theorem for both valences. Such a model is shown, following earlier work which showed low energy singularities in it, to be supersymmetric, leading to a free Majorana and a phase-shifted Majorana excitation. The theory extended approximately to a lattice of mixed-valence ions at appropriate filling gives, without fine-tuning the parameters, a protected gapless Majorana fermion band across the chemical potential, besides the mixed-valence particle and hole bands separated by gaps. In this situation the system is electrically neutral in linear response but has de Haas-van Alphen oscillations. This is used to explain the recently observed magneto-oscillations in mixed-valence insulators as well as their accompanying low energy thermodynamic and relaxation rate anomalies. Some predictions to test the validity of the theoretical results are provided, the most striking of which is that there should be extensive ground state entropy in such compounds.

We examine recent magnetic torque measurements in two compounds, $\gamma$-Li$_2$IrO$_3$ and RuCl$_3$, which have been discussed as possible realizations of the Kitaev model. The analysis of the reported discontinuity in torque, as an external magnetic field is rotated across the $c-$axis in both crystals, suggests that they have a translationally-invariant chiral spin-order of the from $<{\bf S}_i. ({\bf S}_j ~\times ~ {\bf S}_k)> \ne 0$ in the ground state and persisting over a very wide range of magnetic field and temperature. An extra-ordinary $|B|B^2$ dependence of the torque for small fields, beside the usual $B^2$ part, is predicted due to the chiral spin-order, and found to be consistent with experiments upon further analysis of the data. Other experiments such as inelastic scattering and thermal Hall effect and several questions raised by the discovery of chiral spin-order, including its topological consequences are discussed., Comment: Clearer figures of the experimental data provided. Also clearer exposition and comment on related recent work

We re-examine the experimental results for the magnetic response function $\chi''({\bf q}, E, T)$, for ${\bf q}$ around the anti-ferromagnetic vectors ${\bf Q}$, in the quantum-critical region, obtained by inelastic neutron scattering, on an Fe-based superconductor, and on a heavy Fermion compound. The motivation is to compare the results with a recent theory, which shows that the fluctuations in a generic anti-ferromagnetic model for itinerant fermions map to those in the universality class of the dissipative quantum-XY model. The quantum-critical fluctuations in this model, in a range of parameters, are given by the correlations of spatial and of temporal topological defects. The theory predicts a $\chi''({\bf q}, E, T)$ (i) which is a separable function of $({\bf q -Q})$ and of ($E$,$T$), (ii) at crticality, the energy dependent part is $\propto \tanh (E/2T)$ below a cut-off energy, (iii) the correlation time departs from its infinite value at criticality on the disordered side by an essential singularity, and (iv) the correlation length depends logarithmically on the correlation time, so that the dynamical critical exponent $z$ is $\infty$ . The limited existing experimental results are found to be consistent with the first two unusual predictions from which the linear dependence of the resistivity on T and the $T \ln T$ dependence of the entropy also follow. More experiments are suggested, especially to test the theory of variations on the correlation time and length on the departure from criticality.

Quasi-two dimensional itinerant fermions in the Anti-Ferro-Magnetic (AFM) quantum-critical region of their phase diagram, such as in the Fe-based superconductors or in some of the heavy-fermion compounds, exhibit a resistivity varying linearly with temperature and a contribution to specific heat or thermopower proportional to $T \ln T$. It is shown here that a generic model of itinerant AFM can be canonically transformed such that its critical fluctuations around the AFM-vector $Q$ can be obtained from the fluctuations in the long wave-length limit of a dissipative quantum XY model. The fluctuations of the dissipative quantum XY model in 2D have been evaluated recently and in a large regime of parameters, they are determined, not by renormalized spin-fluctuations but by topological excitations. In this regime, the fluctuations are separable in their spatial and temporal dependence and have a dynamical critical exponent $z =\infty.$ The time dependence gives $\omega/T$-scaling at criticality. The observed resistivity and entropy then follow directly. Several predictions to test the theory are also given., Comment: A supplementary section providing the derivation of the marginal fermi liquid single particle self energy is given and some other improvements made

The formation of heavy fermion bands can occur by means of the conversion of a periodic array of local moments into itinerant electrons via the Kondo effect and the huge consequent Fermi-liquid renormalizations. Leggett predicted for liquid $^3$He that Fermi-liquid renormalizations change in the superconducting state, leading to a temperature dependence of the London penetration depth~$\Lambda$ quite different from that in the BCS theory. Using Leggett's theory, as modified for heavy fermions, it is possible to extract from the measured temperature dependence of $\Lambda$ in high quality samples both Landau parameters $F_0^s$ and $F_1^s$; this has never been accomplished before. A modification of the temperature dependence of the specific heat $C_\mathrm{el}$, related to that of $\Lambda$, is also expected. We have carefully determined the magnitude and temperature dependence of $\Lambda$ in CeCoIn$_5$ by muon spin relaxation rate measurements to obtain $F_0^s = 36 \pm 1$ and $F_1^s = 1.2 \pm 0.3$, and find a consistent change in the temperature dependence of electronic specific heat $C_\mathrm{el}$. This, the first determination of $F_1^s$ with a value~$\ll F_0^s$ in a heavy fermion compound, tests the basic assumption of the theory of heavy fermions, that the frequency dependence of the self-energy is much more important than its momentum dependence., Comment: Accepted by Phys. Rev. Lett

The order parameter and its variations in space and time in many different states in condensed matter physics at low temperatures are described by the complex function $\Psi({\bf r}, t)$. These states include superfluids, superconductors, and a subclass of antiferromagnets and charge-density waves. The collective fluctuations in the ordered state may then be categorized as oscillations of phase and amplitude of $\Psi({\bf r}, t)$. The phase oscillations are the {\it Goldstone} modes of the broken continuous symmetry. The amplitude modes, even at long wavelengths, are well defined and decoupled from the phase oscillations only near particle-hole symmetry, where the equations of motion have an effective Lorentz symmetry as in particle physics, and if there are no significant avenues for decay into other excitations. They bear close correspondence with the so-called {\it Higgs} modes in particle physics, whose prediction and discovery is very important for the standard model of particle physics. In this review, we discuss the theory and the possible observation of the amplitude or Higgs modes in condensed matter physics -- in superconductors, cold-atoms in periodic lattices, and in uniaxial antiferromagnets. We discuss the necessity for at least approximate particle-hole symmetry as well as the special conditions required to couple to such modes because, being scalars, they do not couple linearly to the usual condensed matter probes., Comment: 39 pages, 12 figures. Replaced with published version; Annual Reviews of Condensed Matter Physics Volume 6 (2015)

The experimental study of the modulation of the envelope of spin-echo signals due to internal and external fields is an important spectroscopic tool to detect very small internal magnetic fields. We derive the free induction decay and the frequency spectrum and amplitude of spin-echo signals for arbitrary orientation of fields with respect to crystalline axis for nuclei in a crystal of orthorhombic symmetry. Results reproduce the results that no modulation should be observed in tetragonal crystals for fields either along the c-axis or any direction in the basal plane and give details of the signal as a function of the orthorhombicity parameter. They are used to discuss recent experimental results and provide guidelines for future experiments.

The optical effects due to the loop-current order parameter in under-doped cuprates are studied in order to understand the recent observation of unusual birefringence in electromagnetic propagation in under-doped cuprates. It is shown why birefringence occurs even in multiple domains of order with size of domains much smaller than the wave-length and in twinned samples. Not only is there a rotation of polarization of incident light but also a rotation of the principal optical axis from the crystalline axes. Both are calculated in relative agreement with experiments in terms of the same parameters. The magnitude of the effect is orders of magnitude larger than the unusual Kerr effect observed in under-doped cuprates earlier. The new observations, including their comparison with the Kerr effect, test the symmetry of the proposed order decisively and confirm the conclusions from polarized neutron scattering.

We study the free energy landscape of a minimal model for relaxor ferroelectrics. Using a variational method which includes leading correlations beyond the mean-field approximation as well as disorder averaging at the level of a simple replica theory, we find metastable paraelectric states with a stability region that extends to zero temperature. The free energy of such states exhibits an essential singularity for weak compositional disorder pointing to their necessary occurrence. Ferroelectric states appear as local minima in the free energy at high temperatures and become stable below a coexistence temperature $T_c$. We calculate the phase diagram in the electric field-temperature plane and find a coexistence line of the polar and non-polar phases which ends at a critical point. First-order phase transitions are induced for fields sufficiently large to cross the region of stability of the metastable paraelectric phase. These polar and non-polar states have distinct structure factors from those of conventional ferroelectrics. We use this theoretical framework to compare and to gain physical understanding of various experimental results in typical relaxors., Comment: 10 pages, 7 figures

The loop-current state discovered in under-doped cuprates is characterized by a vector ${\bf \Omega}$ which has four possible orientations which correspond to different domains of order in a perfect sample. Since translational symmetry remains unchanged in the pure limit, no gap occurs at the chemical potential. On the other hand Scanning tunneling microscopy (STM) has revealed that the magnitude of the pseudo-gap in under-doped cuprates varies spatially and is correlated with disorder. For disorder coupling also to the direction of ${\bf \Omega}$, there can only be a finite temperature dependent static correlation length for the loop-current state below the ordering temperature of the pure problem. It is shown that, in this situation, singular forward scattering of fermions for large correlation lengths induces an angle dependent pseudo-gap in the single-particle spectral function near the chemical potential. The peaks in the spectral function at the fermi-vectors are away from the chemical potential proportionally to the square of the average loop order parameter measurable by polarized neutron scattering. This result is tested. Due to the finite correlation length there always exist low frequency excitations at long wavelength at all temperatures in the "ordered" phase. Such fluctuations motionally average over the shifts in frequencies of local probes such as NMR and muon resonance expected for a truly static order., Comment: Improved version

We show that a finite Hall effect in zero applied magnetic field occurs for partially filled bands in certain time-reversal violating states with zero net flux per unit-cell. These states are the Magneto-chiral states with parameters in the effective one-particle Hamiltonian such that they do not satisfy the Haldane-type constraints for topological electronic states. The results extend an earlier discussion of the Kerr effect observed in the cuprates but may be applicable to other experimental situations., Comment: published version

The quantum critical Antiferromagnetic (AFM) fluctuation spectra measured by inelastic neutron scattering recently in two heavy fermion superconductors are used together with their other measured properties to calculate their D-wave superconducting transition temperatures $T_{\rm c}$. To this end, the linearized Eliashberg equations for D-wave superconductivity induced by AFM fluctuations are solved in models of fermions with various levels of nesting. The results for the ratio of $T_{\rm c}$ to the characteristic spin-fluctuation energy are well parametrized by a dimensionless coupling constant and the AFM correlation length. Comparing the results with experiments suggests that one may reasonably conclude that superconductivity in these compounds is indeed caused by AFM fluctuations. This conclusion is strengthened by a calculation with the same parameters of the measured coefficient of the normal state quantum-critical resistivity $\propto T^{3/2}$ characteristic of {\it gaussian} AFM quantum-critical fluctuations. The calculations give details of the superconducting coupling as a function of the correlation length and the integrated fluctuation spectra useful in other compounds.

We study a minimal model for a relaxor ferroelectric including dipolar interactions, and short-range harmonic and anharmonic forces for the critical modes as in the theory of pure ferroelectrics together with quenched disorder coupled linearly to the critical modes. We present the simplest approximate solution of the model necessary to obtain the principal features of the correlation functions. Specifically, we calculate and compare the structure factor measured by neutron scattering in different characteristic regimes of temperature in the relaxor PbMg$_{1/3}$Nb$_{2/3}$O$_3$., Comment: 7 pages, 5 figures

A brief summary of collective mode excitations that can exist in singlet superconductors with irreducible representation $L$ is given. Such excitations may be classified as the coupled excitations of the charge density $\rho$ and the phase $\phi $ of the order parameter, or of the amplitude $\Delta$ of order parameter. Each of these classes may be further characterized in the long wavelength limit by the irreducible representation $\ell$ of the excitation, which may or may not be the same as the ground state $L$., Comment: An addendum to our earlier paper titled "Higgs modes in D-wave Superconductors"

Super-high resolution laser-based angle-resolved photoemission measurements have been carried out on Bi2Sr2CaCu2O8+d (Bi2212) superconductors to investigate momentum dependence of electron coupling with collective excitations (modes). Two coexisting energy scales are clearly revealed over a large momentum space for the first time in the superconducting state of an overdoped Bi2212 superconductor. These two energy scales exhibit distinct momentum dependence: one keeps its energy near 78 meV over a large momentum space while the other changes its energy from $\sim$40 meV near the antinodal region to $\sim$70 meV near the nodal region. These observations provide a new picture on momentum evolution of electron-boson coupling in Bi2212 that electrons are coupled with two sharp modes simultaneously over a large momentum space in the superconducting states. Their unusual momentum dependence poses a challenge to our current understanding of electron-mode-coupling and its role for high temperature superconductivity in cuprate superconductors., Comment: 14 Pages, 4 Figures

A d-wave superconducting phase with coexisting intra-unit-cell orbital current order has the remarkable property that it supports finite size Fermi pockets of Bougoliubov quasiparticles. Experimentally detectable consequences of this include a residual $T$-linear term in the specific heat in the absence of disorder and residual features in the thermal and microwave conductivity in the low disorder limit., Comment: 4 pages, 2 figures

The concept of "broken symmetry", that the symmetry of the vacuum may be lower than the Hamiltonian of a quantum theory, plays an important role in modern physics. A manifestation of this phenomena is the Higgs boson in particle physics whose long awaited discovery is imminent. An equivalent mode in superconductors is implicit in the early theories of their collective fluctuations. Spurred by some mysterious experimental results, the theory of the oscillation of the amplitude of superconductivity order parameter, which is the equivalent to the Higgs modes in s-wave superconductors and its identification in the experiments, was explicitly provided. It was also shown that a necessary condition for this to occur is the emergent Lorentz invariance in the superconducting state while the metallic state and the region just below $T_c$ is manifestly non-Lorentz invariant. Here we show that d-wave superconductors, such as the high temperature Cuprate superconductors, should have a rich assortment of Higgs bosons, each in a different irreducible representation of the point-group symmetries of the lattice. We also show that these modes have a characteristic singular spectral structure which can be discovered in Raman scattering experiments.

The collective modes observed in the loop-current ordered state in under-doped cuprates by polarized neutron scattering require that the ground state is a linear combination in each unit-cell of the four basis states which are the possible classical magnetic moment configurations in each unit-cell. The direction of such moments is in the c-axis of the crystals. The basis states are connected by both time-reversal as well as spatial rotations about the center of the unit-cells. Several new features arise in the theory of polarized neutron scattering cross-section in this situation which appear not to have been encountered before. An important consequence of these is that a finite component transverse to the classical magnetic moment directions is detected in the experiments. We show that this transverse component is of purely quantum-mechanical origin and that its direction in the plane normal to the c-axis is not detectable, even in principle, in experiments, at least in the quantum-mechanical model we have adopted. We estimate the direction of the "tilt" in the moment, i.e. the ratio of the transverse component to the c-axis component, using parameters of the ground state obtained by fitting to the observed dispersion of the collective modes in the ordered state. We can obtain reasonable agreement with experiments but only by introducing a parameter for which only an approximate magnitude can be estimated. Approximate calculations of the form-factors are also provided., Comment: 12 pages, 3 figures

Recently two branches of weakly dispersive collective modes have been discovered in under-doped cuprates by inelastic neutron scattering. Polarization analysis reveals that the modes are magnetic excitations. They are only visible for temperatures below the transition temperature to a broken symmetry phase which was discovered earlier and their intensity increases as temperature is further decreased. The broken symmetry phase itself has symmetries consistent with ordering of orbital current loops within a unit-cell without breaking translational symmetry. In order to calculate the collective modes of such a state we add quantum terms to the Ashkin-Teller (AT) model with which the classical loop current order has been described. We derive that the mean field ground state of the quantum model is a product over all unit-cells of linear combination of the four possible classical configurations of the loop current order in each unit-cell. The collective modes are calculated by using a generalized Holstein-Primakoff boson representation of orbital moment operators and lead to three branches of gapped weakly dispersive collective modes. The experimental results are consistent with the two lower energy branches; the third mode is at a higher energy than looked for by present neutron scattering experiments and might also be over-damped. Implications of the discovery of the collective modes are discussed., Comment: 16 pages, 6 figures

We describe a variational calculation for the problem of screening of a point charge in a layered correlated metal for dopings close to the Mott transition where the screening is non-linear due to the proximity to the incompressible insulating state. We find that external charge can induce locally incompressible regions and that the non-linear dependence of the screening on density can induce overscreening in the nearest nearby layers while preserving overall charge neutrality., Comment: 7 pages, 1 figure, final version as published

We consider the Anomalous Hall (AH) state induced by interactions in a three-orbital per unit-cell model. To be specific we consider a model appropriate for the Copper-Oxide lattice to highlight the necessary conditions for time-reversal breaking states which are AH states and which are not. We compare the singularities of the wave-functions of the three-orbital model, which are related to the nonzero Berry curvature, and their variation with a change of gauge to those in the two-orbital model introduced in a seminal paper by Haldane. Explicit derivation using wave-functions rather than the more powerful abstract methods may provide additional physical understanding of the phenomena.

In the Eliashberg integral equations for d-wave superconductivity, two different functions $(\alpha^2 F)_n(\omega, \theta)$ and $(\alpha^2 F)_{p,d}(\omega)$ determine, respectively, the "normal" and the "pairing" self-energies. We present a quantitative analysis of the high-resolution laser based ARPES data on the compound Bi-2212 to deduce the function$(\alpha^2 F)_n(\omega, \theta)$. Besides its detailed $\omega$ dependence, we find the remarkable result that this function is nearly independent of $\theta$ between the ($\pi,\pi$)-direction and 25 degrees from it. Assuming that the same fluctuations determine both the normal and the pairing self-energy, we ask what theories give the function $(\alpha^2 F)_{p,d}(\omega)$ required for the d-wave pairing instability at high temperatures as well as the deduced $(\alpha^2 F)_n(\theta, \omega)$. We show that the deduced $(\alpha^2 F)_n(\theta, \omega)$ can only be obtained from Antiferromagnetic (AFM) fluctuations if their correlation length is smaller than a lattice constant. Using $(\alpha^2 F)_{p,d}(\omega)$ consistent with such a correlation length and the symmetry of matrix-elements scattering fermions off AFM fluctuations, we calculate $T_c$ an show that AFM fluctuations are excluded as the pairing mechanism for d-wave superconductivity in cuprates. We also consider the quantum-critical fluctuations derived microscopically as the fluctuations of the observed loop-current order discovered in the under-doped cuprates. We show that their frequency dependence and the momentum dependence of their matrix-elements to scatter fermions are consistent with the $\theta$ and $\omega$ dependence of the deduced $(\alpha^2 F)_n(\omega, \theta)$. The pairing kernel $(\alpha^2 F)_{p,d}(\omega)$ calculated using the experimental values in the Eliashberg equation gives $d-wave$ instability at $T_c$ comparable to the experiments.

The flat bands in bilayer graphene(BLG) are sensitive to electric fields E\bot directed between the layers, and magnify the electron-electron interaction effects, thus making BLG an attractive platform for new two-dimensional (2D) electron physics[1-5]. Theories[6-16] have suggested the possibility of a variety of interesting broken symmetry states, some characterized by spontaneous mass gaps, when the electron-density is at the carrier neutrality point (CNP). The theoretically proposed gaps[6,7,10] in bilayer graphene are analogous[17,18] to the masses generated by broken symmetries in particle physics and give rise to large momentum-space Berry curvatures[8,19] accompanied by spontaneous quantum Hall effects[7-9]. Though recent experiments[20-23] have provided convincing evidence of strong electronic correlations near the CNP in BLG, the presence of gaps is difficult to establish because of the lack of direct spectroscopic measurements. Here we present transport measurements in ultra-clean double-gated BLG, using source-drain bias as a spectroscopic tool to resolve a gap of ~2 meV at the CNP. The gap can be closed by an electric field E\bot \sim13 mV/nm but increases monotonically with a magnetic field B, with an apparent particle-hole asymmetry above the gap, thus providing the first mapping of the ground states in BLG., Comment: 4 figures

Super-high resolution laser-based angle-resolved photoemission measurements have been performed on a high temperature superconductor Bi_2Sr_2CaCu_2O_8. The band back-bending characteristic of the Bogoliubov-like quasiparticle dispersion is clearly revealed at low temperature in the superconducting state. This makes it possible for the first time to experimentally extract the complex electron self-energy and the complex gap function in the superconducting state. The resultant electron self-energy and gap function exhibit features at ~54 meV and ~40 meV, in addition to the superconducting gap-induced structure at lower binding energy and a broad featureless structure at higher binding energy. These information will provide key insight and constraints on the origin of electron pairing in high temperature superconductors., Comment: 4 pages, 4 figures

We show that the two branches of collective modes discovered recently in under-doped Cuprates with huge spectral weight are a necessary consequence of the loop-current state. Such a state has been shown in earlier experiments to be consistent with the symmetry of the order parameter competing with superconductivity in four families of Cuprates. We also predict a third branch of excitations and suggest techniques to discover it. Using parameters to fit the observed modes, we show that the direction of the effective moments in the ground state lies in a cone at an angle to the c-axis as observed in experiments.

The dissipative XY model in two spatial dimensions belongs to a new universality class of quantum critical phenomena with the remarkable property of the decoupling of the critical fluctuations in space and time. We have shown earlier that the quantum critical point is driven by proliferation in time of topological configurations that we termed warps. We show here that a warp may be regarded as a configuration of a monopoles surrounded symmetrically by anti-monopoles so that the total charge of the configuration is zero. Therefore the interaction with other warps is local in space. They however interact with other warps at the same spatial point logarithmically in time. As a function of dissipation warps unbind leading to a quantum phase transition. The critical fluctuations are momentum independent but have power law correlations in time.

Motivated by the experimental measurement of electrical and hall conductivity, thermopower and Nernst effect, we calculate the longitudinal and transverse electrical and heat transport in graphene in the presence of unitary scatterers as well as charged impurities. The temperature and carrier density dependence in this system display a number of anomalous features that arise due to the relativistic nature of the low energy fermionic degrees of freedom. We derive the properties in detail including the effect of unitary and charged impurities self-consistently, and present tables giving the analytic expressions for all the transport properties in the limit of small and large temperature compared to the chemical potential and the scattering rates. We compare our results with the available experimental data. While the qualitative variations with temperature and density of carriers or chemical potential of all transport properties can be reproduced, we find that a given set of parameters of the impurities fits the Hall conductivity, Thermopower and the Nernst effect quantitatively but cannot fit the conductivity quantitatively. On the other hand a single set of parameters for scattering from Coulomb impurities fits conductivity, hall resistance and thermopower but not Nernst.

One of the canons of condensed matter physics is the Onsager Reciprocity principle in systems in which the Hamiltonian commutes with the time-reversal operator. Recent results of measurements of the Nernst coefficient in underdoped YBa_2Cu_30_{6+x}, together with the measurements of the anisotropy of conductivity and the inferred anisotropy of the thermopower, imply that this principle is violated. The probable violation and its temperature dependence are shown to be consistent with the Loop-current phase which has been directly observed in other experiments. The violation is related directly to the magneto-electric symmetry of such a phase in which an applied electric field generates an effective magnetic field at right angle to it and to the order parameter vector, and vice versa., Comment: Conclusions of the article are incorrect

We consider the thermal expansion, change of sound velocity with pressure and temperature, and the Poisson ratio of lattices which have rigid units (polyhedra very large stiffness to change in bond-length and to bond-angle variations) connected to other such units through relatively compressible polyhedra. We show that in such a model, the potential energy for rotations of the rigid units can occur only as a function of the combination ${\boldsymbol \Theta}_i \equiv ({\boldsymbol \theta}_i - (\nabla \times {\bf u}_i)/2)$, where ${\boldsymbol \theta_i} $ are the orthogonal rotation angles of the rigid unit $i$ and ${\bf u}_i$ is its displacement. Given such new invariants in the theory of elasticity and the hierarchy of force constants of the model, a negative thermal expansion coefficient as well as a decrease in the elastic constants of the solid with temperature and pressure is shown to follow. These are consistent with the observations., Comment: 8 pages, 1 figure

An overview of the momentum and frequency dependence of effective electron-electron interactions which favor electronic instability to a superconducting state in the angular-momentum channel $\ell$ and the properties of the interactions which determine $T_c$ is provided. Both interactions induced through exchange of phonons as well as purely electronic fluctuations of spin density, charge density or current density are considered. Special attention is paid to the role of quantum critical fluctuations including pairing due to their virtual exchange as well as de-pairing due to inelastic scattering. In light of the above, empirical data and theory specific to phonon induced superconductivity, in cold atoms, superfluidity in liquid $He^3$, superconductivity in some of the heavy fermion compounds, in Cuprates, in pncitides and the valence skipping compound, is reviewed. The physical basis for the following observation is provided: The universal ratio of s-wave $T_c$ to Fermi-energy for fermions at the unitarity limit with attractive interactions is about 0.15, the ratio of the maximum $T_c$ to the typical phonon frequency in phonon induced s-wave superconductivity is of the same order; the ratio of p-wave $T_c$ to the renormalized Fermi-energy in liquid $He^3$, a very strongly correlated Fermi-liquid near its melting pressure, is only $O(10^{-3})$; in the Cuprates and the heavy-fermions where d-wave superconductivity occurs in a region governed by a special class of quantum-critical fluctuations, this ratio rises to $O(10^{-2})$. These discussions also suggest factors important for obtaining higher $T_c$.

The loop-current state discovered in the pseudogap phase of cuprates breaks time reversal symmetry and lowers the point group symmetry of the crystal. The order parameter and the magnetic structure within each unit cell which is associated with it can be described by a toroidal moment parallel to the copper-oxide planes. We discuss lattice point group symmetry of the magnetic structure. As an application, we discuss a few effects that necessarily accompany order parameter in the pseudogap phase. The magnitude estimated for these specific effects makes them hard to observe because they rely on the small magnetic fields associated with the order parameter. Effects, associated with the electronic energies are much larger. Some of them have already been discussed., Comment: 7 pages

The coupling between localized spins and phonons can lead to shifts in the dielectric constant of insulating materials at magnetic ordering transitions. Studies on isostructural SeCuO3 (ferromagnetic) and TeCuO3 (antiferromagnetic) illustrate how the q-dependent spin-spin correlation function couples to phonon frequencies leading to a shift in the dielectric constant. A model is discussed for this spin-phonon coupling. The magnetodielectric coupling in multiferroic materials can be very large at a ferroelectric transition temperature. This coupling is investigated in the recently identified multiferroic Ni3V2O8., Comment: 4 pages, 3 figures. Originally appeared in: The Twelfth US-Japan Seminar on Dielectric and Piezoelectric Ceramics, p. 215 (2005) (unpublished conference proceedings). The manuscript has not been modified or updated in any way from this version

The conventional interpretation of the recent magneto-oscillation experiments in underdoped Cuprates, requires that there be a state of altered translational symmetry in the pseudogap state which is not supported by structural and Angle Resolved Photoemission Spectroscopy (ARPES) experiments. I show here that the observed oscillations may be reconciled with the conclusion arrived in ARPES experiments that the fermi-surface, suitably defined, has the shape of four arcs which shrink to four points as $T \to 0$. Experiments, including infrared absorption in a magnetic field, are suggested to distinguish between such a state from that obtained by the conventional interpretation of the magneto-oscillations.