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A. V. Balatsky

Publications and source records attributed to A. V. Balatsky.

At least 163 records · Page 9Linked to original sources
arXiv1999

Local Time-Reversal Symmetry Breaking in d_(x2-y2) Superconductors

We show that an isolated impurity in a spin singlet d_{x2-y2} superconductor generates a d_{xy} order parameter with locally broken time-reversal symmetry. The origin of this effect is a coupling between the d_{x2-y2} and the d_{xy} order parameter induced by spin-orbit scattering off the impurity. The signature of locally broken time-reversal symmetry is an induced orbital charge current near the impurity, which generates a localized magnetic field in the vicinity of the impurity. We present a microscopic theory for the impurity induced d_{xy} component, discuss its spatial structure as well as the pattern of induced current and local magnetic field near the localized impurity spin.

cond-mat.supr-con↗
arXiv1999

Finite Density of States in a Mixed State of d_x^2-y^2+id_xy Superconductor

We have calculated the density of states of quasiparticles in a d_x^2-y^2+id_xy superconductor, and show that in the mixed state the quasiparticle spectrum remains gapless because of the Doppler shift by superflow. It was found that if the d_{xy} order gap $Δ_1\propto \sqrt{H}$ as suggested by experiments, then thermal conductivity $κ\propto \sqrt{H}$ in accord with experimental data at lowest temperatures. This is an appended version of the paper published in Phys. Rev. {\bf B 59}, 6024, (1999). We now also discuss the disorder effects and analyze the H log H crossover at small fields. We argue that H log H regime is present and disorder effect is dominant as the field-induced seconary gap is small at small fields.

cond-mat.supr-con↗
arXiv1999

Field induced $d_{x^2-y^2}+id_{xy}$ state and marginal stability of high-Tc superconductors

It is shown that the {\em complex} $d_{xy}$ component is generated in d-wave superconductor in the magnetic field. As one enters superconducting state at finite field the normal to superconducting transition occurs into bulk $d_{x^2-y^2}+i d_{xy}$ state . The driving force for the transition is the linear coupling between magnetic field and non zero magnetization of the $d_{x^2-y^2}+i d_{xy}$ condensate. The external magnetic field violates parity and time reversal symmetries and the nodal quasiparticle states respond by generating the $id_{xy}$ component of the order parameter, with the magnitude estimated to be on the order of few Kelvin. Parity (P) and time reversal (T) symmetries are violated in this state.

cond-mat.supr-con↗
arXiv1999

Interlayer Transport of Quasiparticles and Cooper pairs in Bi2Sr2CaCu2O8+y Superconductors

We study the c-axis transport of stacked, intrinsic junctions in Bi2Sr2CaCu2O8+y single crystals, fabricated by the double-sided ion beam processing technique from single crystal whiskers. Measurements of the I-V characteristics of these samples allow us to obtain the temperature and voltage dependence of the quasiparticle c-axis conductivity in the superconducting state, the Josephson critical current, and the superconducting gap. We show that the BCS d-wave model in the clean limit for resonant impurity scattering with a significant contribution from coherent interlayer tunneling, describes satisfactorily the low temperature and low energy c-axis transport of both quasiparticles and Cooper pairs.

cond-mat.supr-con↗
arXiv1999

Linear dependence of peak width in $χ(\bq, ω)$ vs T_c for YBCO superconductors

It is shown that the momentum space width of the peak in the spin susceptibility, Im$χ(q,ω)$, is linearly proportional to the superconducting $T_c$: $T_c = \hbar v^*Δq$ with $\hbar v^* \simeq 35 meV$Å. This relation is similar to the linear relation between incommensurate peak splitting and $T_c$ in LaSrCuO superconductors, as first proposed by Yamada et al. (Phys. Rev. B 57, 6165, (1998)). The velocity $\hbar v^*$ is smaller than Fermi velocity or the spin-wave velocity of the parent compound and remains the same for a wide doping range. This result points towards strong similarities in magnetic state of YBCO and LaSrCuO.

cond-mat.supr-con↗
arXiv1998

Marginal stability of d-wave superconductor: spontaneous P and T violation in the presence of magnetic impurities

We argue that the $d_{x^2-y^2}$-wave superconductor is marginally stable in the presence of external perturbations. Subjected to the external perturbations by magnetic impurities, it develops a secondary component of the gap, complex $d_{xy}$, to maximize the coupling to impurities and lower the total energy. The secondary $d_{xy}$ component exists at high temperatures and produces the full gap $\sim 20 K$ in the single particle spectrum around each impurity, apart from impurity induced broadening. At low temperatures the phase ordering transition into global $ d_{x^2-y^2} + i d_{xy}$ state occurs.

cond-mat.supr-con↗
arXiv1998

Effect of planar impurities on the superfluid density of striped cuprates

We propose a mechanism for superconductivity suppression in stripe-correlated cuprates based on the pinning of the stripes by impurities, such as Zn. The suppression of superfluid density occurs in the vicinity of the impurity due to the low dimensional character of superfluid carriers on the stripes. Simple geometric argument about the planar fraction of the carriers, affected by stripe pinning, leads to prediction for impurity critical concentration $z_c \sim T^2_c$ and for the linear $T_c$ suppression by Zn doping. We show that our results reproduce the Uemura relation in the pure system and we predict the behavior of the superfluidy density as a function of Zn concentration. We compare our results with available data.

cond-mat.supr-con↗
arXiv1997

Magnetic impurity induced id_{xy} component and spontaneous time reversal and parity breaking in a d_{x^2-y^2}-wave superconductor

It is shown that the patches of complex d_{xy} component are generated around magnetic impurity in the presence of the coupling between orbital moment of the condensate and impurity spin S. Locally induced d_{xy} gap leads to the fully gapped quasiparticle spectrum near impurity. The phase of the induced i d_{xy} component is determined by impurity spin and averages to zero at high temperatures. It is suggested that at low temperature the well defined patches of d_{xy} are formed and they can undergo a phase transition into phase locked state via Josephson effect. Violation of time-reversal symmetry and parity occurs spontaneously via the second order transition. In the ordered phase both impurity magnetization and d_{xy} component of the order parameter develop and are proportional to each other.

cond-mat.supr-con↗
arXiv1997

Spontaneous time reversal and parity breaking in a $d_{x^2-y^2}$-wave superconductor with magnetic impurities

It is argued that the $d_{x^2-y^2}$ superconductor exhibits the transition into $d_{x^2-y^2} + id_{xy}$ state in the presence of magnetic impurities in certain conditions. Linear coupling between impurity magnetization and the $d_{xy}$ component drives this transition. Violation of time-reversal symmetry and parity occurs spontaneously via the second order transition. In the ordered phase both impurity magnetization and $d_{xy}$ component of the order parameter develop and are proportional to each other.

cond-mat.supr-con↗
arXiv1997

Low-Temperature Phase Transition in Bi_2Sr_2Ca(Ni_xCu_{1-x})_20_8: Evidence for Unconventional Superconductivity

We report the discovery of a low-temperature phase transition in Bi_2Sr_2Ca(Ni_xCu_{1-x})_20_8 high temperature superconductor. This transition manifests itself as a sharp reduction of the thermal conductivity of the samples at a temperature of T_c* \approx 200 mK. The temperature dependence of the thermal conductivity changes dramatically from T^αwith αbetween 1.6 and 1.75 above the transition to T-linear behavior below the transition. Application of a small magnetic field suppresses the low-temperature phase. We interpret this behavior as a phase transition into a second bulk low-temperature superconducting state in which the time-reversal symmetry is likely to be broken. This discovery constitutes direct evidence for unconventional superconductivity in high temperature superconductors.

cond-mat.supr-con↗
arXiv1997

On the Lifshitz tail in the density of states of a superconductor with magnetic impurities

We argue that any superconductor with magnetic impurities is gapless due to a Lifshitz tail in the density of states extending to zero energy. At low energy the density of states $ν(E \to 0)$ remains finite. We show that fluctuations in the impurity distribution produce regions of suppressed superconductivity, which are responsible for the low energy density of states.

cond-mat.supr-con↗
arXiv1997

Superconductivity of a Metallic Stripe Embedded in an Antiferromagnet

We study a simple model for the metallic stripes found in $La_{1.6-x}Nd_{0.4}Sr_xCuO_4$: two chain Hubbard ladder embedded in a static antiferromagnetic environments. We consider two cases: a ``topological stripe'', for which the phase of the Neel order parameter shifts by $π$ across the ladder, and a ``non-topological stripe'', for which there is no phase shift across the ladder. We perform one-loop renormalization group calculations to determine the low energy properties. We compare the results with those of the isolated ladder and show that for small doping superconductivity is enhanced in the topological stripe, and suppressed in the non-topological one. In the topological stripe, the superconducting order parameter is a mixture of a spin singlet component with zero momentum and a spin triplet component with momentum $π$. We argue that this mixture is generic, and is due to the presence of a new term in the quantum Ginzburg-Landau action. Some consequences of this mixing are discussed.

cond-mat.str-el↗
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

Spectral Properties of Quasiparticle Excitations Induced by Magnetic Moments in Superconductors

The consequences of localized, classical magnetic moments in superconductors are explored and their effect on the spectral properties of the intragap bound states is studied. Above a critical moment, a localized quasiparticle excitation in an s-wave superconductor is spontaneously created near a magnetic impurity, inducing a zero-temperature quantum transition. In this transition, the spin quantum number of the ground state changes from zero to 1/2, while the total charge remains the same. In contrast, the spin-unpolarized ground state of a d-wave superconductor is found to be stable for any value of the magnetic moment when the normal-state energy spectrum possesses particle-hole symmetry. The effect of impurity scattering on the quasiparticle states is interpreted in the spirit of relevant symmetries of the clean superconductor. The results obtained by the non-self-consistent (T matrix) and the self-consistent mean-field approximations are compared and qualitative agreement between the two schemes is found in the regime where the coherence length is longer than the Fermi length.

cond-mat.supr-con↗
arXiv1997

Theory of Scanning Tunneling Microscopy Probe of Impurity States in a D-Wave Superconductor

Scanning tunneling microscopy can provide a probe for the detailed study of quasiparticle states in high-Tc superconductors. We propose that it can also be used to acquire specific information about impurity-induced quasiparticle states and the superconducting order-parameter structure. In particular, the local density of states is found to be sensitive to impurity-induced resonances and to the symmetry of the order parameter.

cond-mat.supr-con↗
arXiv1996

Impurity States and the Absence of Quasiparticle Localization in Disordered D-Wave Superconductors

The absence of localization of impurity-induced low-energy quasiparticle states in a two-dimensional $d$-wave superconductor is argued for any amount of disorder in the limit of unitary scatterers. This surprising result follows from the fact that a unitary impurity produces a marginally-bound state at zero energy which decays as a power-law along the nodes of the $d$-wave energy gap. Consequently, for finite density of impurities, the impurity-induced states are coupled by long-range overlaps yielding extended quasiparticle states below a characteristic energy scale $ω_c$. Simple scaling arguments suggest that $ω_c \propto e^{-c/n_{\rm imp}}$, where $n_{\rm imp}$ is the impurity density and $c$ is a positive constant.

cond-mat↗
arXiv1995

Dynamical Properties of Quantum Hall Edge States

We consider the dynamical properties of simple edge states in integer ($ν= 1$) and fractional ($ ν= 1/2m+1$) quantum Hall (QH) liquids. The influence of a time-dependent local perturbation on the ground state is investigated. It is shown that the orthogonality catastrophe occurs for the initial and final state overlap $| | \sim L^{-{1\over{2ν}}({δ\overπ})^2}$ with the phase shift $δ$. The transition probability for the x-ray problem is also found with the index, dependent on $ν$. Optical experiments that measure the x-ray response of the QH edge are discussed. We also consider electrons tunneling from one dimensional Fermi liquid into a QH fluid. It is argued that for any filling fraction the tunneling from a Fermi liquid to the QH edge is suppressed at low temperatures and we find the nonlinear $I-V$ characteristics $I\sim V^{1/ν}$.

cond-mat↗