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A. V. Balatsky

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

At least 127 records · Page 7Linked to original sources
arXiv2003

Effects of a Collective Spin Resonance Mode on the STM Spectra of D-Wave Superconductors

A high-energy spin resonance mode is known to exist in many high-temperature superconductors. Motivated by recent scanning tunneling microscopy (STM) experiments in superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we study the effects of this resonance mode on the local density of states (LDOS). The coupling between the electrons in a d-wave superconductor and the resonance mode produces high-energy peaks in the LDOS, which displays a two-unit-cell periodic modulation around a nonmagnetic impurity. This suggests a new means to not only detect the dynamical spin collective mode but also study its coupling to electronic excitations.

cond-mat.supr-con↗
arXiv2003

Atomic scale elastic textures coupled to electrons in superconductors

We present an atomic scale theory of lattice distortions using strain related variables and their constraint equations. Our approach connects constrained atomic length scale variations to continuum elasticity and describes elasticity at all length scales. We apply the general approach to a two-dimensional square lattice with a monatomic basis, and find the atomic scale elastic textures around a structural domain wall and a single defect, as exemplary textures. We clarify the microscopic origin of gradient terms, some of which are included phenomenologically in Landau-Ginzburg theory. The obtained elastic textures are used to investigate the effects of elasticity-driven lattice deformation on the nanoscale electronic structure in superconductor by solving the Bogliubov-de Gennes equations with the electronic degrees of freedom coupled to the lattice ones. It is shown that the order parameter is depressed in the regions where the lattice deformation takes place. The calculated local density of states suggests the electronic structure is strongly modulated as a response to the lattice deformation-- the elasticity propagates the electronic response over long distances. In particular, it is possible for the trapping of low-lying quasiparticle states around the defects. These predictions could be directly tested by STM experiments in superconducting materials.

cond-mat.supr-con↗
arXiv2003

Nuclear spin-lattice relaxation rate in the D+iD superconducting state: implications for CoO superconductor

We calculated the nuclear spin-lattice relaxation rate $1/T_1$ for the D+iD superconducting state with impurities. We found that small amount of unitary impurities quickly produces the residual density of states inside the gap. As a result, the T-linear behavior in 1/T$_1$ is observed at low temperatures. Our results show that the D+iD pairing symmetry of the superconducting state of Na$_{0.35}$CoO$_{2} \cdot y$H$_2$ O is compatible with recent $^{59}$Co 1/T$_1$ experiments of several groups.

cond-mat.str-el↗
arXiv2003

$1/T_1$ in the D-wave superconducting state with coexisting antiferromagnetism

We consider a general D-wave superconducting gap function in the presence of coexisting antiferromagnetic (AFM) long-rang order. We find that the D-wave order parameter develops additional nodes without lowering the symmetry of the state due to the interplay of the superconducting and AFM correlations. This AFM correlated D-wave gap has a small gap structure inside the generic D-wave gap. As a result of this additional structure it shows a quite different response to impurities compared to a standard D-wave gap. We calculate the nuclear spin-lattice relaxation rate $1/T_1$ of this gap in the presence of impurities and discuss the implications for the experiments of some AFM heavy- fermion superconductors and in particular for CeRhIn5.

cond-mat.supr-con↗
arXiv2003

Elasticity-Driven Nanoscale Electronic Structure in Superconductors

The effects of long-range anisotropic elastic deformations on electronic structure in superconductors are analyzed within the framework of the Bogoliubov-de Gennes equations. Cases of twin boundaries and isolated defects are considered as illustrations. We find that the superconducting order parameter is depressed in the regions where pronounced lattice deformation occurs. The calculated local density of states suggests that the electronic structure is strongly modulated in response to lattice deformations, and propagates to longer distances. In particular, this allows the trapping of low-lying quasiparticle states around defects. Some of our predictions can be directly tested by STM experiments.

cond-mat.supr-con↗
arXiv2003

Vortex Core Excitations in Superconductors with Frustrated Antiferromagnetism

Motivated by recent discovery of cobalt oxide and organic superconductors, we apply an effective model with strong antiferromagnetic and superconducting pairing interaction to a related lattice structure. It is found that the antiferromagnetism is highly frustrated and a broken-time-reversal-symmetry chiral $d+id^{\prime}$-wave pairing state prevails. In the mixed state, we have solved the local electronic structure near the vortex core and found no local induction of antiferromagnetism. This result is in striking contrast to the case of copper oxide superconductors. The calculated local density of states indicates the existence of low-lying quasiparticle bound states inside the vortex core, due to a fully gapped chiral pairing state. The prediction can be directly tested by scanning tunneling microscopy.

cond-mat.supr-con↗
arXiv2003

Vibration Mode Induced Shapiro Steps and Backaction in Josephson Junctions

A model of a superconducting tunnel junction coupled to a mechanical oscillator is studied at zero temperature in the case of linear coupling between the oscillator and tunneling electrons. It is found that the Josephson current flowing between two superconductors is modulated by the motion of the oscillator. Coupling to harmonic oscillator produces additional Shapiro steps in $I-V$ characteristic of Josephson junction whose position is tuned by the frequency of the vibration mode. We also find a new velocity dependent term originating from the backaction of the ac Josephson current. This term is periodic in time and vanishes at zero bias voltage.

cond-mat.mes-hall↗
arXiv2003

Inelastic Tunneling Spectroscopy in a D-wave Superconductor

We propose a mechanism to use inelastic tunneling spectrosopy STM to detect a single spin in a d-wave superconductor and in a pseudogap state, based on a direct exchange coupling J between the surface electrons and the local spin S in a magnetic field. This coupling will produce a kink in a dI/dV characteristic at Zeeman energy of the spin ω_0. We find that for relevant values of parameters signal scales as dI^2/dV^2 \simeq (JN_0)^2 Θ(eV - ω_0) and could be in the range of 10^{-2} of the bare density of states where N_0 is the density of states for surface electrons. Scattering in superconductor with the coherence peak at gap maximum Δleads also to strong features at Δ+ ω_0. This suggests a new technique for a detection of a local spin excitation with STM. We also consider a detection of a local vibrational mode as a simple extension of the spin case.

cond-mat.str-el↗
arXiv2003

Anomalous specific heat jump in the heavy fermion superconductor CeCoIn$_5$

We study the anomalously large specific heat jump and its systematic change with pressure in CeCoIn$_5$ superconductor. Starting with the general free energy functional of the superconductor for a coupled electron boson system, we derived the analytic result of the specific heat jump of the strong coupling superconductivity occurring in the coupled electron boson system. Then using the two component spin-fermion model we calculate the specific heat coefficient $C(T)/T$ both for the normal and superconducting states and show a good agreement with the experiment of CeCoIn$_5$. Our result also clearly demonstrated that the specific heat coefficient $C(T)/T$ of a coupled electron boson system can be freely interpreted as a renormalization either of the electronic or of the bosonic degrees of freedom.

cond-mat.str-el↗
arXiv2002

Tail states in superconductors with weak magnetic impurities

We analyse the behavior of the density of states in a singlet s-wave superconductor with weak magnetic impurities in the clean limit by using the method of optimal fluctuation. We show that the density of states varies as $\ln N(E)\propto -|E-Δ_0|^{(7-d)/4}$ near the mean field gap edge $Δ_0$ in a d-dimensional superconductor. The optimal fluctuation in d>1 is strongly anisotropic. We compare the density of states with that obtained in other recent approaches.

cond-mat.supr-con↗
arXiv2002

Superconductivity and Quantum Criticality in CeCoIn_5

Electrical resistivity measurements on a single crystal of the heavy-fermion superconductor CeCoIn_5 at pressures to 4.2 GPa reveal a strong crossover in transport properties near P^* \approx 1.6 GPa, where T_c is a maximum. The temperature-pressure phase diagram constructed from these data provides a natural connection to cuprate physics, including the possible existence of a pseudogap.

cond-mat.str-el↗
arXiv2002

Theory of Current and Shot Noise Spectroscopy in Single-Molecular Quantum Dots with Phonon Mode

Using the Keldysh nonequilibrium Green function technique, we study the current and shot noise spectroscopy of a single molecular quantum dot coupled to a local phonon mode. It is found that in the presence of electron-phonon coupling, in addition to the resonant peak associated with the single level of the dot, satellite peaks with the separation set by the frequency of phonon mode appear in the differential conductance. In the ``single level'' resonant tunneling region, the differential shot noise power exhibit two split peaks. However, only single peaks show up in the ``phonon assisted'' resonant-tunneling region. An experimental setup to test these predictions is also proposed.

cond-mat.mes-hall↗
arXiv2002

Exchange Based Noise Spectroscopy of a Single Precessing Spin with STM

ESR-STM is an emerging technique which is capable of detecting the precession of a single spin. We discuss the mechanism of ESR-STM based on a direct exchange coupling between the tunneling electrons and the local precessing spin S. We claim that since the number of tunneling electrons in a single precessing period is small (~20) one may expect a net temporary polarization within this period that will couple via exchange interaction to the localized spin. This coupling will randomly modulate the tunneling barrier and create a dispersion in the tunneling current which is a product of a Larmor frequency component due to the precession of the single spin and the dispersion of the spin of the tunneling electrons. This noise component is spread over the whole frequency range for random white noise spin polarization of electrons. In opposite case the power spectrum of the spins of the tunneling electrons has a peak at zero frequency an elevated noise in the current at omega_L will appear. We discuss the possible source of this spin polarization. We find that for relevant values of parameters signal to noise ratio in the spectral characteristic is 2-4 and is comparable to the reported signal to noise ratio. The magnitude of the current fluctuation is a relatively weak increaing function of the DC current and the magnetic field. The linewidth produced by the back action effect of tunneling electrons on the precessing spin is also discussed.

cond-mat.mes-hall↗
arXiv2002

Josephson Current in the Presence of a Precessing Spin

The Josephson current in the presence of a precessing spin between various types of superconductors is studied. It is shown that the Josephson current flowing between two spin-singlet pairing superconductors is not modulated by the precession of the spin. When both superconductors have equal-spin-triplet pairing state, the flowing Josephson current is modulated with twice of the Larmor frequency by the precessing spin. It was also found that up to the second tunneling matrix elements, no Josephson current can occur with only a direct exchange interaction between the localized spin and the conduction electrons, if the two superconductors have different spin-parity pairing states.

cond-mat.supr-con↗
arXiv2002

Theory of single spin detection with STM

We propose a mechanism for detection of a single spin center on a non-magnetic substrate. In the detection scheme, the STM tunnel current is correlated with the spin orientation. In the presence of magnetic field, the spin precesses and the tunnel current is modulated at the Larmor frequency. The mechanism relies on the effective spin-orbit interaction between the injected unpolarized STM current and the local spin center, which leads to the nodal structure of the spatial signal profile. Based on the proposed mechanism, the strongest spin-related signal can be expected for the systems with large spin-orbit coupling and low carrier concentration.

cond-mat.mes-hall↗
arXiv2002

Quantum Electronic Transport through a Precessing Spin

The conductance through a local nuclear spin precessing in a magnetic field is studied by using the equations-of-motion approach. The characteristics of the conductance is determined by the tunneling matrix and the position of equilibrium chemical potential. We find that the spin flip coupling between the electrons on the spin site and the leads produces the conductance oscillation. When the spin is precessing in the magnetic field at Larmor frequency ($ω_{L}$), the conductance develops the oscillation with the frequency of both $ω_{L}$ and 2$ω_{L}$ components, the relative spectrum weight of which can be tuned by the chemical potential and the spin flip coupling.

cond-mat.mes-hall↗
arXiv2002

Quantum Nyquist Temperature Fluctuations

We consider the temperature fluctuations of a small object. Classical fluctuations of the temperature have been considered for a long time. Using the Nyquist approach, we show that the temperature of an object fluctuates when in a thermal contact with a reservoir. For large temperatures or large specific heat of the object $C_v$, we recover standard results of classical thermodynamic fluctuations $<ΔT^2> = \frac{k_B T^2}{C_v}$. Upon decreasing the size of the object, we argue, one necessarily reaches the quantum regime that we call quantum temperature fluctuations. At temperatures below $T^{*}\sim \hbar/k_{B}τ$, where $τ$ is the thermal relaxation time of the system, the fluctuations change the character and become quantum. For a nano-scale metallic particle in a good thermal contact with a reservoir, $T^{*}$ can be on a scale of a few Kelvin.

cond-mat.stat-mech↗