Time-reversal Breaking State Research Articles

Schwinger boson mean-field theory of the kagome Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interactions

We have studied the effect of the Dzyaloshinskii-Moriya interaction on kagome Heisenberg anti-ferromagnet using Schwinger boson mean field theory (SBMFT). Within SBMFT framework, Messio et al had argued that the ground state of kagome antiferromagnet is possibly a chiral topological spin liquid (Phys. Rev. Lett. 108, 207204 (2012)). Thus, we have computed zero-temperature ground state phase diagram considering the time-reversal breaking states as well as fully symmetric Ans\"atze. We discuss the relevance of these results in experiments and other studies. Finally, we have computed the static and dynamic spin structure factors in relevant phases.

Berry phase and anomalous Hall effect in a three-orbital tight-binding Hamiltonian

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.

Doping-driven transition to a time-reversal breaking state in the phase diagram of the cuprates

Motivated by recent tunneling and Andreev-reflection experiments, we study the conditions for a quantum transition within the superconducting phase of the cuprates, in which a bulk imaginary (time-reversal breaking) ${\mathrm{id}}_{\mathrm{xy}}$ component appears in addition to the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ order parameter. We examine in detail the role of some important physical features of the cuprates. In particular we show that a closed Fermi surface, a bilayer splitting, an orthorhombic distortion, and the proximity to a quantum critical point around optimal doping favor the appearance of the imaginary component. These findings could explain why the mixed ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}{+id}_{\mathrm{xy}}$ order parameter is observed in ${\mathrm{Y}}_{1\ensuremath{-}y}{\mathrm{Ca}}_{y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}x}$ and ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4},$ and suggest that it could appear also in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8}.$ We also predict that, in all cuprates, the mixed state should be stable only in a limited doping region all contained beneath the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ dome. The behavior of the specific heat at the secondary transition is discussed.

TIME REVERSAL BREAKING SUPERCONDUCTING STATE IN THE PHASE DIAGRAM OF THE CUPRATES

We review and extend a previous study1 on the symmetry of the superconducting state, stimulated by recent tunneling and Andreev reflection measurements giving robust evidences for the existence of a dx2-y2 + idxy order parameter in the overdoped regime of two different cuprates. Looking for a possible second-order phase transition from a standard dx2-y2 to a mixed and time reversal breaking state, we confirm the results of our previous analysis on La 2-x Sr x CuO 4. In the case of Y 1-y Ca y Ba 2 Cu 3 O 7-x as well, among all the allowed symmetries, the dx2 - y2 + idxy is the most favored one and the unconventional state is likely to occur in a small dome of the phase diagram located in the optimal-overdoped region and at very low temperatures.

Detection and implications of a time-reversal breaking state in underdoped cuprates.

We present general symmetry considerations on how a time-reversal breaking state may be detected by angle-resolved photoemission using circularly polarized photons as has been proposed earlier. Results of recent experiments utilizing the proposal in underdoped cuprates are analyzed and found to be consistent in their symmetry and magnitude with a theory of the copper oxides. These experiments if correct, together with evidence of a quantum critical point and marginal Fermi-liquid properties near optimum doping, suggest that the essentials of a valid microscopic theory of the phenomena in the cuprates may have been found.

Superfluid properties of the extended Hubbard model with intersite electronpairing

We study the superfluid properties of short-coherence-length superconductorsdescribed by the extended Hubbard model with on-site repulsive and intersiteattractive interaction. We have analysed the superfluid stiffness and thermodynamicproperties of the model versus electron concentration and pairing interaction on atwo-dimensional square lattice with nearest- and next-nearest-neighbour hopping. Theeffects of phase fluctuations on anisotropic superconductivity of extended s-and dx2 −y2-wavetype were examined within the Kosterlitz–Thouless (KT) scenario. The KTcritical temperatures were determined and compared with those fromthe BCS–Hartree–Fock approximation. The Uemura-type plots, i.e.the critical temperature versus zero-temperature phase stiffness, wereobtained for both extended s- and d-wave pairings. The states with mixed(s∗ and dx2 −y2)symmetry, and in particular the time-reversal breaking state of s + id symmetry, wereanalysed and the phase diagrams taking into account these states are presented. Wealso briefly discuss the crossover from BCS to local pair superconductivity for the dx2 −y2-wavepairing. A comparison of the theoretical results with the experimental data, in particular thoseof Tc versusλ−2 (0),for high-Tccuprate superconductors is made.

Quasiclassical Theory of Twin Boundaries in High-TcSuperconductors

We investigate the electronic structure of twin boundaries in orthorhombically distorted high-T$_c$ materials using the quasiclassical theory of superconductivity. At low temperatures we find a local instability to a time-reversal symmetry breaking state at the twin boundary. This state yields spontaneous currents along the twin boundary that are microscopically explained by the structure of the quasiparticle bound states. We calculate the local density of states and find a splitting in the zero-bias peak once the spontaneous currents set in. This splitting is measurable by STM techniques and is a unique signature of time-reversal breaking states at a twin boundary.

Time-Reversal Symmetry Breaking at Grain Boundaries in d-Wave Superconductors

Crystal lattice distortion can generate a superconducting state which breaks locally time-reversal symmetry by yielding a coupling between order parameters of different symmetry. The problem of a twist in the crystal lattice at a boundary between differently oriented grains is investigated by means of a generalized Ginzburg–Landau theory. It is shown that under certain conditions a time-reversal breaking state appears at the grain boundary. This state leads to a nontrivial phase shift in the Josephson effect across the grain boundary. This phase shift can be observed through a modified flux quantization (fractional fluxes) and phase slip effects.

Penetration depth measurements as a test for time-reversal breaking states in unconventional superconductors

We examine the possibility of nonmagnetic impurity-induced time-reversal breaking in unconventional superconductors. We determine the phase diagram for impurity-induced broken-T-symmetry states, in a model in which a dominant unconventional order parameter is present along with a subdominant conventional one. We find that, under such conditions, a time-reversal breaking state is always possible in a wide range of impurity concentrations at sufficiently small temperatures. We determine the effect that the transition into such a phase has on the temperature dependence of the penetration depth. Our results suggest that such bulk, broken T-symmetry states could be detected by penetration depth measurements in very dirty samples.

Enforced Time-Reversal Breakdownin Unconventional Superconductors

In time-reversal symmetric unconventional superconductors time-reversal breaking structures can occur under certain conditions. This effect is demonstrated on two examples: a domain wall and a thin film for a superconductor with an order parameter consisting of two components. It is shown that the occurrence of time-reversal breaking states is compatible with the general stability conditions of a Ginzburg-Landau theory.

Critical current and domain formation of s+id superconductors

The authors discuss the Ginzburg-Landau (GL) theory of a mixed s- and d-wave pairing state as a model for high Tc superconductors. In particular, the domain wall formation between two degenerate time reversal breaking states (s-id and s+id), which seem to be consistent with the experimentally observed optical circular dichroism, is considered. The Ginzburg-Landau equations are solved for the domain wall structure. The solutions reveal that a weak superconducting region is formed near the domain wall. The critical current diminishes in the presence of domain wall formation. Experimental implications of the mixed s- and d-wave ordering are discussed: the domain wall regions can act like pinning centres or as weak links. The latter would result in percolation, rather than rectilinear conduction, of supercurrent.