arXiv subjects

Manfred Sigrist

Publications and source records attributed to Manfred Sigrist.

172 records · Page 10Linked to original sources

arXiv1998

Study on nonmagnetic impurities in the superconducting state of two-dimensional t-J model

We study the effect of nonmagnetic impurities in the superconducting state of the t-J model. The Bogoliubov de Gennes equation is derived using a slave-boson mean-field approximation, and we solve it numerically at T=0. The d_{x^2-y^2}-wave order parameter is suppressed near an impurity and its spatial variation induces the extended s-wave component. We also find the local charge density wave (CDW)-like feature due to the Friedel oscillations. This leads to the splitting of the expected zero-energy Andreev bound state. Therefore the system is unlikely introducing a localized state of other symmetry such as superconductivity with broken time reversal symmetry or antiferromagnetism.

cond-mat.supr-con↗

arXiv1998

Time-reversal symmetry breaking surface states in t-J model

Recently a phenomenological Ginzburg-Landau (GL) theory has been proposed to describe the occurrence of a locally time-reversal symmetry (T) breaking state near a Josephson junction between unconventional superconductors. In this paper we derive this type of GL free energy microscopically from the t-J model within a slave-boson mean-field approximation. The resulting GL free energy is shown to satisfy the conditions to have a T-violating surface state. The existence of this junction state may explain some of the recent experiments on High-Tc superconductors.

cond-mat.supr-con↗

arXiv1997

Andreev reflection in unitary and non-unitary triplet states

The quasiparticle reflection and transmission properties at normal conductor-superconductor interfaces are discussed for unitary and non-unitary spin triplet pairing states recently proposed for Sr_2RuO_4. We find resonance peaks in the Andreev reflection amplitude, which are related to surface bound states in the superconductor. They lead to conductance peak features below the quasiparticle gap in the superconductor. The position of the peaks depends on the angle of the incident quasiparticles in a different way for unitary and non-unitary states. Based on this observation we propose a possible experiment which allows to distinguish between the two kinds of superconducting states.

cond-mat.supr-con↗

arXiv1997

Lightly Doped t-J Three-Leg Ladders - an Analog for the Underdoped Cuprates

The three-leg ladder has one odd-parity and two even-parity channels. At low doping these behave quite differently. Numerical calculations for a t-J model show that the initial phase upon hole doping has two components - a conducting Luttinger liquid in the odd-parity channel, coexisting with an insulating (i.e. undoped) spin liquid phase in the even-parity channels. This phase has a partially truncated Fermi surface and violates the Luttinger theorem. This coexistence of conducting fermionic and insulating paired bosonic degrees of freedom is similar to the recent proposal of Geshkenbein, Ioffe, and Larkin for the underdoped spin-gap normal phase of the cuprates. A mean field approximation is derived which has many similarities to the numerical results. One difference however is an induced hole pairing in the odd-parity channel at arbitrary small dopings, similar to that proposed by Geshkenbein, Ioffe, and Larkin for the two-dimensional case. At higher dopings, we propose that a quantum phase transition will occur as holes enter the even-parity channels, resulting in a Luther-Emery liquid with hole pairing with essentially d-wave character. In the mean field approximation a crossover occurs which we interpret as a reflection of this quantum phase transition deduced from the numerical results.

cond-mat.str-el↗

arXiv1997

Time Reversal Symmetry Breaking and Spontaneous Currents in s-Wave / Normal Metal / d-Wave Superconductor Sandwiches

We study the physical properties of an $s$-wave -- normal metal -- $d$-wave junction in terms of the Andreev bound states in the normal metal layer. The phase dependence of bound states with different orientations leads to superconducting states with broken time reversal symmetry for generic orientations of the $d$-wave superconductor crystal. The occurrence of such a state and the associated spontaneous supercurrent along the junction is analyzed in the framework of Ginzburg-Landau theory and by the solution of the Bogolyubov-de Gennes equations.

cond-mat.supr-con↗

arXiv1997

Extended Gapless Regions in Disordered d_{x^2-y^2}-Wave Superconductors

A generalization of the Abrikosov-Gorkov equations for non-magnetic impurities in unconventional superconductors is proposed, including higher harmonics in the expansion of the momentum dependent gap function and a momentum dependent impurity scattering potential. This model is treated within a self-consistent calculation to obtain the electronic density of states, the optical conductivity, and the gap function in a two-dimensional d_{x^2-y^2}-wave superconductor. It is argued that momentum dependent scattering from the impurities may lead to extended gapless regions in the gap function centered around the nodes of the pure d_{x^2-y^2}-wave superconductor. The associated enhancement of the residual density of states may be responsible for the rapid decrease of T_c and the increase of the London penetration depth with hole doping observed in overdoped cuprate superconductors.

cond-mat.supr-con↗

arXiv1997

Low-Temperature Scaling Regime of Random Ferromagnetic-Antiferromagnetic Spin Chains

Using the Continuous Time Quantum Monte Carlo Loop algorithm, we calculate the temperature dependence of the uniform susceptibility, and the specific heat of a spin-1/2 chain with random antiferromagnetic and ferromagnetic couplings, down to very low temperatures. Our data show a consistent scaling behavior in both quantities and support strongly the conjecture drawn from the approximative real-space renormalization group treatment. A statistical analysis scheme is developed which will be useful for the search scaling behavior in numerical and experimental data of random spin chains.

cond-mat↗

arXiv1995

Plaquette Resonating-Valence-Bond Ground State of CaV$_4$O$_9$

A theoretical model is presented to explain the spin gap observed for CaV$_4$O$_9$. The underlying lattice of the 1/5-depleted square lattice favors a formation of plaquette resonating valence bond state. Inclusion of the frustrating second neighbor interaction enhances this tendency, leading to a quantum disordered state of a two dimensional spin-1/2 Heisenberg model with a sufficiently big spin gap compatible with experiments.

cond-mat↗

arXiv1995

Proximity-induced time-reversal symmetry breaking at Josephson junctions between unconventional superconductors

We argue that a locally time-reversal symmetry breaking state can occur at Josephson junctions between unconventional superconductors. Order parameters induced by the proximity effect can combine with the bulk order parameter to form such a state. This property is specifically due to the intrinsic phase structure of the pairing wave function in unconventional superconductors. Experimental consequences of this effect in high-temperature superconductors are examined.

cond-mat↗

arXiv1994

Test Experiment for Time-Reversal Symmetry Breaking Superconductivity

A new experiment is proposed to probe the time-reversal symmetry of a superconductor. It is shown that a time-reversal symmetry breaking superconductor can be identified by the observation of a fractional flux in connection with a Josephson junction in a special geometry.

cond-mat↗