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arXiv2014

Searching for a quantum critical point in YbCu5-xAux

Structural, magnetic, transport and thermal properties of YbCu5-xAux alloys with Au concentration between the limit of structural stability of AuBe5 type at x = 0.4 up to x = 0.7 are reported. The outstanding features of this system are: i) the constant and record high values of Cm /T 7J/molK^2 below a characteristic temperature T*, ranging between 150 mK and 350 mK. ii) A power law thermal dependence dependence Cm/T(T>T*)=A/T^q, with q = 1.3 +/- 0.1, and iii) an arising incoherent electronic scattering observed in the resistivity at T < 1K for x < 0.6 despite the fact that Yb magnetic atoms are placed in a lattice. Magnetic frustration, originated in the tetrahedral distribution of Yb atoms, appears as the responsible of the exotic behavior of this system.

cond-mat.str-el↗

arXiv2014

Comment to "Critical Exponents of the Superfluid-Bose Glass Transition in Three-Dimensions" by Z. Yao et al., arXiv:1402.5417v1

In this comment we address the preprint Z. Yao et al. (arXiv:1402.5417v1) concerning the conditions upon which the correct critical temperature exponent $ϕ$ can be observed (experimentally or numerically) close to the superfluid-Bose glass quantum phase transition in three dimensions. Yao et al. announce to have resolved the contradiction between the predictions of Fisher et al. [Phys. Rev. B 40, 546 (1989)] that $ϕ= νz \geq 2$ (where $ν$ and $z$ are critical exponents at the quantum phase transition) and recent experiments and simulations, showing an exponent $ϕ\sim 1.1$. Yao et al's resolution hinges on claiming that the $ϕ\sim 1.1$ observations, which were conducted by varying the chemical potential, were not made to sufficiently close to the critical point to observe universal behavior. Here we critically examine their criteria for observing universal behavior. We show that past simulations were in fact conducted closer to the quantum critical point than the new results of Yao et al.; and that Yao et al.'s own results for varying chemical potential are consistent with previous results showing $ϕ\sim 1.1$, and do not show a clear crossover to a different $ϕ$ value.

cond-mat.dis-nn↗

arXiv2014

Functional Renormalization Group Analysis of $η$-Pairing in Iron-based Superconductors

Using functional renormalization group (FRG) approach, we analyze the $η$ pairing in a five d-orbital model for iron-pncitides. We find that the $η$ pairing between two hole pockets is a leading instability at low energy when the two hole pockets are close to a nesting condition. The result suggests that the system can spontaneously enter into a superconducting state with both normal pairing and $η$ paring and the $η$ pairing is driven by the inter-orbital antiferromagnetic exchange coupling while the normal pairing is driven by the intra-orbital antiferromagnetic ones. An effective model at low energy is proposed to account for the $η$ pairing instability.

cond-mat.supr-con↗

arXiv2014

Construction of the B88 exchange-energy functional in two dimensions

We construct a generalized-gradient approximation for the exchange-energy density of finite two-dimensional systems. Guided by non-empirical principles, we include the proper small-gradient limit and the proper tail for the exchange-hole potential. The observed performance is superior to that of the two-dimensional local-density approximation, which underlines the usefulness of the approach in practical applications.

cond-mat.str-el↗

arXiv2014

Broad 1st Order Phase Transitions: phase coexistence, unusual hysteresis and glasslike kinetic arrest

We discuss the phenomenology of phase transitions studied with two control variables. Such measurements have become routine with magnetic field and temperature being varied for 1st order magnetic transitions with an ease not conceivable with pressure and temperature. Similar ease may be possible with electric field as the second control variable, as in current studies on multiferroics. We develop this phenomenology for broad 1st order transitions that occur over a range of the control variable, as is common in substitutional alloys with inherent quenched disorder, including the possibility of a glasslike arrest of the transition occurring within this range of either control variable. The interrupted transition then results in the two competing phases coexisting to the lowest temperature. The new experimental protocol with acronym CHUF, where the sample is cooled and heated under different values of the second control variable, allows a detailed study of glasslike arrest of kinetics of the phase transformation. The many experimental studies on 1st order magnetic transitions in a large number of materials, that provide the cause for this phenomenology, shall be discussed subsequently.

cond-mat.mtrl-sci↗

arXiv2014

Parity violation in ferromagnet-superconductor heterostructures with strong spin-orbit coupling

We study spectroscopic properties of ferromagnetic-superconductor heterostructures with strong spin-orbit coupling of the Rashba type and in the presence of exchange fields. The superconducting layer (film) experiences both an intrinsic spin-orbit field and an exchange field due to the proximity to ferromagnetic layers (films). We analyse the temperature dependence of the order parameter for superconductivity at various values of exchange field and spin-orbit coupling, and describe momentum-dependent properties that exhibit parity violation. Furthermore, we show that parity violation can be probed in tunneling experiments of the single-particle density of states and in photoemission experiments of the momentum distribution.

cond-mat.supr-con↗

arXiv2014

The Classification of Two Dimensional topological Field Theories

The goal of this paper is to introduce some of the major ideas behind extended topological quantum field theories with an emphasis on explicit examples and calculations. The statement of the Cobordism Hypothesis is explained and immediately used to classify framed and oriented extended two dimensional topological quantum field theories. The passage from framed theories to oriented theories is equivalent to giving homotopy fixed points of an $SO(n)$ action on the space of field theories. This paper then constructs extended two dimensional Dijkgraaf-Witten theory (also called finite gauge theory) as an example of a two dimensional extended field theory by assigning invariants at the level of points and extending up. Finally, it is concluded that Dijkgraaf-Witten theory is the only example of an extended framed two dimensional topological quantum field theory by showing that any field theory is equivalent to Dijkgraaf-Witen theory for some cyclic group.

math.AT↗

arXiv2014

Pressure Dependent Structural Changes and Predicted Electrical Polarization in Perovskite RMnO3

High pressure x-ray diffraction (XRD) measurements on RMnO3 (R=Dy, Ho and Lu) reveals that varying structural changes occurs for different R ions. Large lattice changes (orthorhombic strain) occur in DyMnO3 and HoMnO3 while the Jahn-Teller (JT) distortion remains stable. On the other hand, in LuMnO3, Mn-O bond distortions are observed in the region 4-8 GPa with the broad minimum in the JT distortion. High pressure IR measurements indicate that a phonon near 390 cm-1 corresponding to the complex motion of the Mn and O ions changes anomalously for LuMnO3. It softens in the 4-8 GPa region, which is consistent with the structural change in Mn-O bonds and then hardens at high pressures. By contrast, the phonons continuously harden with increasing pressure for DyMnO3 and HoMnO3. DFT calculations show that the E-phase LuMnO3 is the most stable phase up to the 10 GPa pressure examined. Simulations indicate that the distinct structural change under pressure in LuMnO3 can possibly be used to optimize the electric polarization by pressure/strain.

cond-mat.str-el↗

arXiv2014

Complex electronic structure of Ca(1-x)Sr(x)RuO(3)

We investigate the core level spectra of Ca(1-x)Sr(x)RuO(3) employing high resolution photoemission spectroscopy. Sample surface appears to be dominated by the contributions from Ru-O layers. Sr 3p core level spectra are sharp and asymmetric in SrRuO(3) as expected in a metallic system, and exhibit multiple features for the intermediate compositions that can be attributed to the difference in Ca-O and Sr-O covalency. The Ru core level spectra exhibit distinct signature of satellite features due to the finite electron correlations strength among Ru 4d electrons. The intensity of the satellite feature is weaker in the surface spectra compared to the bulk. The low temperature spectra exhibit enhancement of satellite intensity in the spectra corresponding to ferromagnetic compositions due to the inter-site exchange coupling induced depletion of the intensity at the Fermi level. The increase in x leads to a decrease in satellite intensity that has been attributed to the increase in hopping interaction strength due to the enhancement of the Ru-O-Ru bond angle. Evidently, the complex electronic properties of these materials are derived from the interplay between the electron correlation and hopping interaction strengths.

cond-mat.str-el↗

arXiv2014

Observation of quasiparticles for phase incoherent d-wave pairing in Bi2212

In contrast to a complex feature of antinodal state, suffering from competing order(s), the "pure" pairing gap of cuprates is detected in the nodal region, which therefore holds the key to the superconducting mechanism. The pairing gap has been viewed to be rather conventional, closing at the superconducting transition temperature (Tc). However, the density of states contributed from the nodal region was claimed to have a gap-like structure even above Tc. Here we present a missing evidence for a single-particle gap near the node signifying the realization of a phase incoherent d-wave superconductivity above Tc in the optimally doped Bi2Sr2CaCu2O8+d. We find that the pair formation is formulated by momentum-independent temperature evolutions of three parameters: a BCS-type energy gap (Delta), a single particle scattering rate (Gamma_single) and a pair breaking rate (Gamma_pair). The superconductivity occurs when the Gamma_pair value is suppressed smaller than Gamma_single on cooling, and the magnitude of Tc in cuprates is governed by a condition of Gamma_single(Tc)=Gamma_pair(Tc).

cond-mat.supr-con↗

arXiv2014

Notes on Entanglement in Abelian Gauge Theories

We streamline and generalize the recent progress in understanding entanglement between spatial regions in Abelian gauge theories. We provide an unambiguous and explicit prescription for calculating entanglement entropy in a $\mathbb Z_N$ lattice gauge theory. The main idea is that the lattice should be split into two disjoint regions of links separated by a buffer zone of plaquettes. We show that the previous calculations of the entanglement entropy can be realized as special cases of our setup, and we argue that the ambiguities reported in the previous work can be understood as basis choices for gauge-invariant operators living in the buffer zone. The proposed procedure applies to Abelian theories with matter and with continuous symmetry groups, both on the lattice and in the continuum.

hep-th↗

arXiv2014

Floquet Majorana Edge Mode and Non-Abelian Anyons in a Driven Kitaev Model

We theoretically study laser driven nonequilibrium states in the Kitaev honeycomb model with a magnetoelectric cross coupling. We show that a topological spin liquid with a gapless chiral edge mode emerges when we apply an elliptically or circularly polarized laser. This is a strongly correlated quantum spin version of the Floquet topological insulator. In the topological phase, the edge mode is made from Majorana fermions and the bulk has gapped non-Abelian anyon excitations.

cond-mat.str-el↗

arXiv2014

Stochastically Projecting Tensor Networks

We apply a series of projection techniques on top of tensor networks to compute energies of ground state wave functions with higher accuracy than tensor networks alone with minimal additional cost. We consider both matrix product states as well as tree tensor networks in this work. Building on top of these approaches, we apply fixed-node quantum Monte Carlo, Lanczos steps, and exact projection. We demonstrate these improvements for the triangular lattice Heisenberg model, where we capture up to 57 percent of the remaining energy not captured by the tensor network alone. We conclude by discussing further ways to improve our approach.

cond-mat.str-el↗

arXiv2014

A high Neel temperature 5$d$ oxide: NaOsO$_3$

The origin of a high Neel temperature in a 5$d$ oxide, NaOsO$_3$, has been analyzed within the mean-field limit of a multiband Hubbard model and compared with the analogous 4$d$ oxide, SrTcO$_3$. Our analysis shows that there are a lot of similarities in both these oxides on the dependence of the the effective exchange interaction strength ($J_0$) on the electron-electron interaction strength ($U$). However, the relevant value of $U$ in each system puts them in different portions of the parameter space. Although the Neel temperature for NaOsO$_3$ is less than that for SrTcO$_3$, our results suggest that there could be examples among other 5$d$ oxides which have a higher Neel temperature. We have also examined the stability of the G-type antiferromagnetic state found in NaOsO$_3$ as a function of electron doping within GGA+U calculations and find a robust G-type antiferromagnetic metallic state stabilized. The most surprising aspect of the doped results is the rigid band-like evolution of the electronic structure which indicates that the magnetism in NaOsO$_3$ is not driven by fermi surface nesting.

cond-mat.str-el↗

arXiv2014

Electron-Phonon Spectral Density of MgB2 from Optical Data, through Maximum Entropy

We use maximum entropy techniques to extract an electron-phonon density from optical data in the normal state at T = 45 K in MgB2. Limiting the analysis to a range of phonon energies below 110 meV which is sufficient to capture all phonon structures we find a spectral function which is in good agreement with that calculated for the quasi two-dimensional sigma-band. Extending the analysis to higher energies up to 160 meV we find no evidence for any additional contributions to the fluctuation spectrum but find that the data can only be understood if the density of states is taken to decrease with increasing energy.

cond-mat.str-el↗

arXiv2014

Gaps and pseudo-gaps at the Mott quantum Critical point in the perovskite rare earth nickelates

We report on tunneling measurements that reveal for the first time the evolution of the quasi-particle state density across the bandwidth controlled Mott metal to insulator transition in the rare earth perovskite nickelates. In this, a canonical class of transition metal oxides, we study in particular two materials close to the T=0 metal-insulator transition: NdNiO3 , an antiferromagnetic insulator, and LaNiO3, a correlated metal. We measure a sharp gap in NdNiO3, which has an insulating ground state, of ~ 30 meV. Remarkably, metallic LaNiO3 exhibits a pseudogap of the same order that presages the metal insulator transition. The smallness of both the gap and pseudogap suggests they arise from a common origin: proximity to a quantum critical point at or near the T=0 metal-insulator transition. It also supports theoretical models of the quantum phase transition in terms of spin and charge instabilities of an itinerant Fermi surface.

cond-mat.str-el↗

arXiv2014

Exact solutions of an extended Bose-Hubbard model with E2 symmetry

An extended Bose-Hubbard (BH) model with number-dependent multi-site and infinite-range hopping is proposed, which, similar to the original BH model, describes a phase transition between the delocalized super-fluid (SF) phase and localized Mott insulator (MI) phase. It is shown that this extended model with local Euclidean E2 symmetry is exactly solvable when on-site local potential or disorder is included, while the model without local potential or disorder is quasi-exactly solvable, which means only a part of the excited states including the ground state being exactly solvable. As applications of the exact solution for the ground state, phase diagram of the model in 1D without local potential and on-site disorder for filling factor rho = 1 with M = 6 sites and that with M = 10 are obtained. The probabilities to detect n particles on a single site, Pn, for n = 0, 1, 2 as functions of the control parameter U/t in these two cases are also calculated. It is shown that the critical point in Pn and in the entanglement measure is away from that of the SF-MI transition determined in the phase analysis. It is also shown that the the model-independent entanglement measure is related with Pn, which, therefore, may be practically useful because Pn is measurable experimentally.

cond-mat.str-el↗

arXiv2014

Bulk charge stripe order competing with superconductivity in La2-xSrxCuO4 (x=0.12)

We present a volume-sensitive high-energy x-ray diffraction study of the underdoped cuprate high temperature superconductor La2-xSrxCuO4 (x = 0.12, Tc=27 K) in applied magnetic field. Bulk short-range charge stripe order with propagation vector q_ch = (0.231, 0, 0.5) is demonstrated to exist below T_ch = 85(10) K and shown to compete with superconductivity. We argue that bulk charge ordering arises from fluctuating stripes that become pinned near boundaries between orthorhombic twin domains.

cond-mat.supr-con↗