arXiv subjects

A. V. Balatsky

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

At least 145 records · Page 8Linked to original sources

arXiv2002

Impurity states and interlayer tunneling in high temperature superconductors

We argue that the Scanning Tunneling Microscope (STM) images of resonant states generated by doping Zn or Ni impurities into Cu-O planes of BSCCO are the result of quantum interference of the impurity signal coming from several distinct paths. The impurity image seen on the surface is greatly affected by interlayer tunneling matrix elements. We find that the optimal tunneling path between the STM tip and the metal (Cu, Zn, or Ni) $d_{x^2 - y^2}$ orbitals in the Cu-O plane involves intermediate excited states. This tunneling path leads to the four-fold nonlocal filter of the impurity state in Cu-O plane that explains the experimental impurity spectra. Applications of the tunneling filter to the Cu vacancy defects and ``direct'' tunneling into Cu-O planes are also discussed.

cond-mat.supr-con↗

arXiv2002

Unified Description of the Resonance Peak and Incommensuration in High-$T_c$ Superconductors

We present a unified description of the resonance peak and low-energy incommensurate response observed in high-$T_c$ cuprate superconductors. We argue that both features have a purely magnetic origin and they represent universal features of an incommensurate spin state both below and above the superconducting transition temperature. In this description the resonance peak is the reflection of commensurate antiferromagnetism. Our theoretical scenario gives an account of the main features observed in various families of superconductors and predicts those not yet observed, like a resonance peak in La$_2$NiO$_{4+x}$.

cond-mat.supr-con↗

arXiv2002

Critical magnetic fluctuations induced superconductivity and residual density of states in $CeRhIn_5$ superconductor

We propose the multiband extension of the spin-fermion model to address the superconducting d-wave pairing due to magnetic interaction near critical point. We solve the unrestricted gap equation with a general d-wave symmetry gap and find that divergent magnetic correlation length $ξ$ leads to the very unharmonic shape of the gap function with shallow gap regions near nodes. These regions are extremely sensitive to disorder. Small impurity concentration induces substantial residual density of states. We argue that we can understand the large $N_{res}(0) = \lim_{T\to 0} C_p(T)/T$ value and its pressure dependence of the recently discovered $CeRhIn_5$ superconductor under pressure within this approach.

cond-mat.str-el↗

arXiv2001

Field induced d_x^2-y^2+id_xy state in d-density-wave metals

We argue that the d_{xy} component of the order parameter can be generated to form the d_x^2-y^2+id_xy-density wave state by the external magnetic field. The driving force for this transition is the coupling of the magnetic field with the orbital magnetism. The fully gapped particle spectrum and the magnetically active collective mode of the condensate are discussed as a possible signature of the d+id' density wave state.

cond-mat.supr-con↗

arXiv2001

High Angular Momentum Core States and Impurity Effects in the Mixed State of d-Wave Superconductors

The local quasiparticle spectra around the vortex core of a d-wave superconductor in the mixed state is studied by solving the lattice Bogoliubov-de Gennes equations self-consistently. It is shown that in addition to the zero-energy states, there also exist core states of high angular momentum. A nonmagnetic unitary impurity sitting at the core center is nondestructive to these core states and the zero-energy resonant state induced by itself is still visible in the local quasiparticle spectrum. The calculated imaging shows a fourfold ``star'' shape of the local density of states whose orientation is energy dependent. It is also found that, although the zero-energy states are extended, the core states of finite energy level could be localized.

cond-mat.supr-con↗

arXiv2000

Proximity to a Nearly Superconducting Quantum Critical Liquid

The coupling between superconductors and a quantum critical liquid that is nearly superconducting provides natural interpretation for the Josephson effect over unexpectedly long junctions, and the remarkable stripe-spacing dependence of the critical temperature in LSCO and YBCO superconductors.

cond-mat.supr-con↗

arXiv2000

Clapping modes in unconventional superconductors

We consider a superconducting state with a mixed symmetry order parameter components, e.g. $d+is$ or $d+id'$ with $d'= d_{xy}$. We argue for the existence of the new orbital magnetization mode which corresponds to the oscillations of relative phase $ϕ$ between two components around an equilibrium value of $ϕ= \fracπ{2}$. It is similar to the so called ``clapping'' mode in superfluid $^3He-A$. We estimate the frequency of this mode $ω_0(B,T)$ depending on the field and temperature for the specific case of magnetic field induced $d'$ state. We find that this mode is {\em tunable} with an applied magnetic field with $ω_0(B,T) \propto B \0$, where $\0$ is the magnitude of the d-wave order parameter. We argue also that similar filed induced clapping mode should be present in an organic p-wave superconductors.

cond-mat.supr-con↗

arXiv2000

Competing Quantum Orderings in Cuprate Superconductors: A Minimal Model

We present a minimal model for cuprate superconductors. At the unrestricted mean-field level, the model produces homogeneous superconductivity at large doping, striped superconductivity in the underdoped regime and various antiferromagnetic phases at low doping and for high temperatures. On the underdoped side, the superconductor is intrinsically inhomogeneous and global phase coherence is achieved through Josephson-like coupling of the superconducting stripes. The model is applied to calculate experimentally measurable ARPES spectra.

cond-mat.supr-con↗

arXiv2000

Identifying the pairing symmetry in the Sr2RuO4 superconductor

We have analyzed heat capacity and thermal conductivity measurements of Sr2RuO4 in the normal and superconducting state and come to the conclusion that an order parameter with nodal lines on the Fermi surface is required to account for the observed low-temperature behavior. A gapped order parameter is inconsistent with the reported thermodynamic and transport data. Guided by a strongly peaked dynamical susceptibility along the diagonals of the Brillouin zone in neutron scattering data, we suggest a spin-fluctuation mechanism that would favor the pairing state with the gap maxima along the zone diagonals (such as for a d_{xy} gap). The most plausible candidates are an odd parity, spin-triplet, f-wave pairing state, or an even parity, spin-singlet, d-wave state. Based on our analysis of possible pairing functions we propose measurements of the ultrasound attenuation and thermal conductivity in the magnetic field to further constrain the list of possible pairing states.

cond-mat.supr-con↗

arXiv2000

Impurity induced resonant state in a pseudogap state of a high temperature superconductor

We predict a resonance impurity state generated by the substitution of one Cu atom with a nonmagnetic atom, such as Zn, in the pseudogap state of a high-T_c superconductor. The precise microscopic origin of the pseudogap is not important for this state to be formed, in particular this resonance will be present even in the absence of superconducting fluctuations in the normal state. In the presence of superconducting fluctuations, we predict the existence of a counterpart impurity peak on a symmetric bias. The nature of impurity resonance is similar to the previously studied resonance in the d-wave superconducting state.

cond-mat.supr-con↗

arXiv2000

Photoemission Spectroscopy from Inhomogeneous Models of Cuprates

We investigate the electronic dynamics in the underdoped cuprates focusing on the effects of one-dimensional charge stripes. We address recent experimental Angular-Resolved Photoemission Spectra results on (La$_{1.28}$Nd$_{0.6}$Sr$_{0.12}$)CuO$_4$. We find that various inhomogeneous models can account for the distribution of quasiparticle weights close to momentum ${\bf k}=(π,0)$ and symmetry related points. The observed flat dispersion region around the same ${\bf k}$ point can only be addressed by certain classes of those inhomogeneous models which locally break spin symmetry. Homogeneous models including hopping elements up to second neighbors cannot reproduce the experimental quasiparticle weight, since most of it is centered around ${\bf k}=(\frac π{2},\frac π {2})$.

cond-mat.supr-con↗

arXiv2000

Interlayer Tunneling of Quasiparticles and Cooper Pairs in Bi-2212 Single Crystal Whiskers

The interlayer tunneling has been studied on high quality Bi-2212 stacks of micron to the submicron lateral size. We found that low temperature and low voltage tunneling I-V characteristics can be self-consistently described by Fermi-liquid model for a d-wave superconductor with a significant contribution from coherent interlayer tunneling. The gap and pseudogap interplay with variation of temperature and magnetic field has been extracted from the I-V characteristics. We consider also the role of charging effects for submicron stacks.

cond-mat.supr-con↗

arXiv2000

Superconducting $d_{x^2-y^2} \pm id_{xy}$ phase glass

We discuss the effects of magnetic impurities on d-wave superconductors. We calculate the electron mediated RKKY interaction between the impurity spins in a d-wave superconductor and find that it decays as $r^{-3}$ at large distances. We argue that this interaction leads to the formation of a spin glass at low temperature $T \ll T_c$. It was previously shown that a local complex $Δ^1 \sim d_{xy}$ order parameter is induced around each impurity spin. We consider the pair tunneling resulting in the Josephson interaction between different patches of induced $d_{xy}$ order parameter. Due to the local coupling between impurity spins and the superconducting order parameter the Josephson coupling favors a ferromagnetic phase at low temperatures. The competition between the Josephson coupling and the RKKY interaction gives rise to an interesting phase diagram. At low impurity concentrations we find an unusual supercondcting phase glass, where the impurity spins $S^z$ and $d_{xy}$ component are disordered and yet the product of these two develops a true long range order $ $. This phase has no analog in purely magnetic spin glasses and arises as a result of the direct coupling of the impurity spin to the phase of $Δ^1$. At high impurity concentrations it is possible that a ferromagnetic phase will form.

cond-mat.supr-con↗

arXiv2000

Striped superconductors in the extended Hubbard model

We present a minimal model of a doped Mott insulator that simultaneously supports antiferromagnetic stripes and d-wave superconductivity. We explore the implications for the global phase diagram of the superconducting cuprates. At the unrestricted mean-field level, the various phases of the cuprates, including weak and strong pseudogap phases, and two different types of superconductivity in the underdoped and the overdoped regimes, find a natural interpretation. We argue that on the underdoped side, the superconductor is intrinsically inhomogeneous -- striped coexistence of of superconductivity and magnetism -- and global phase coherence is achieved through Josephson-like coupling of the superconducting stripes. On the overdoped side, the state is overall homogeneous and the superconductivity is of the classical BCS type.

cond-mat.supr-con↗

arXiv2000

Microscopic Scenario for Striped Superconductors

We argue that the superconducting state found in high-$T_c$ cuprates is inhomogeneous with a corresponding inhomogeneous superfluid density. We introduce two classes of microscopic models which capture the magnetic and superconducting properties of these strongly correlated materials. We start from a generalized t-J model, in which appropriate inhomogeneous terms mimic stripes. We find that inhomogeneous interactions that break magnetic symmetries are essential to induce substantial pair binding of holes in the thermodynamic limit. We argue that this type of model reproduces the ARPES and neutron scattering data seen experimentally.

cond-mat.supr-con↗

arXiv2000

Inhomogeneity-Induced Superconductivity?

A t-J-like model for inhomogeneous superconductivity of cuprate oxides is presented, in which local anisotropic magnetic terms are essential. We show that this model predicts pairing, consistent with experiments, and argue how the macroscopic phase-coherent state gradually grows upon lowering of the temperature. We show that appropriate inhomogeneities are essential in order to have significant pair binding in the thermodynamic limit. Particularly, {\it local} breaking of SU(2) spin symmetry is an efficient mechanism for inducing pairing of two holes, as well as explaining the magnetic scattering properties. We also discuss the connection of the resulting inhomogeneity-induced superconductivity to recent experimental evidence for a linear relation between magnetic incommensurability and the superconducting transition temperature, as a function of doping.

cond-mat.supr-con↗

arXiv2000

Pairing in Inhomogeneous Superconductors

Starting from a t-J model, we introduce inhomogeneous terms to mimic stripes. We find that if the inhomogeneous terms break the SU(2) spin symmetry the binding between holes is tremendously enhanced in the thermodynamic limit. In any other model (including homogeneous models) the binding in the thermodynamic limit is small or neglible. By including these inhomogeneous terms we can reproduce experimental neutron scattering data. We also discuss the connection of the resulting inhomogeneity-induced superconductivity to recent experimental evidence for a linear relation between magnetic incommensurability and the superconducting transition temperature, as a function of doping.

cond-mat.str-el↗

arXiv2000

Collective Mode in a d_{x^2-y^2} + id_xy Superconductor

We consider a superconducting state with a mixed symmetry order parameter components, e.g. d+is or d+id' with d'= d_{xy}. We argue for the existence of the new orbital magnetization mode which corresponds to the oscillations of relative phase ϕbetween two components around an equilibrium value of ϕ= π/2. It is similar to the so called ``clapping'' mode in superfluid 3He-A. We estimate the frequency of this mode ω_0(B,T) depending on the field and temperature for the specific case of magnetic field induced d'= d_{xy} state. We find that this mode is tunable with an applied magnetic field with ω_0(B,T) \propto B D_0, where D_0 is the magnitude of the d-wave order parameter. We also estimate the velocity s(B,T) of this mode.

cond-mat.supr-con↗