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D. A. Ivanov

Publications and source records attributed to D. A. Ivanov.

At least 37 records · Page 2Linked to original sources
arXiv2007

Dimer phases in quantum antiferromagnets with orbital degeneracy

We study and solve the ground-state problem of a microscopic model for a family of orbitally degenerate quantum magnets. The orbital degrees of freedom are assumed to have directional character and are represented by static Potts-like variables. In the limit of vanishing Hund's coupling, the ground-state manifold of such a model is spanned by the hard-core dimer (spin singlet) coverings of the lattice. The extensive degeneracy of dimer coverings is lifted at a finite Hund's coupling through an order-out-of-disorder mechanism by virtual triplet excitations. The relevance of our results to several experimentally studied systems is discussed.

cond-mat.str-el↗
arXiv2007

Josephson current in a superconductor -- ferromagnet -- superconductor junction with in-plane ferromagnetic domains

We study a diffusive superconductor--ferromagnet--superconductor (SFS) junction with in-plane ferromagnetic domains. Close to the superconducting transition temperature, we describe the proximity effect in the junction with the linearized Usadel equations. We find that properties of such a junction depend on the size of the domains relative to the magnetic coherence length. In the case of large domains, the junction exhibits transitions to the $π$ state, similarly to a single-domain SFS junction. In the case of small domains, the magnetization effectively averages out, and the junction is always in the zero state, similarly to a superconductor--normal metal--superconductor (SNS) junction. In both those regimes, the influence of domain walls may be approximately described as an effective spin-flip scattering. We also study the inhomogeneous distribution of the local current density in the junction. Close to the 0--$π$ transitions, the directions of the critical current may be opposite in the vicinity of the domain wall and in the middle of the domains.

cond-mat.mes-hall↗
arXiv2007

Josephson current in a superconductor-ferromagnet junction with two non-collinear magnetic domains

We study the Josephson effect in a superconductor--ferromagnet--superconductor (SFS) junction with ferromagnetic domains of non-collinear magnetization. As a model for our study we consider a diffusive junction with two ferromagnetic domains along the junction. The superconductor is assumed to be close to the critical temperature $T_c$, and the linearized Usadel equations predict a sinusoidal current-phase relation. We find analytically the critical current as a function of domain lengths and of the angle between the orientations of their magnetizations. As a function of those parameters, the junction may undergo transitions between 0 and $π$ phases. We find that the presence of domains reduces the range of junction lengths at which the $π$ phase is observed. For the junction with two domains of the same length, the $π$ phase totally disappears as soon as the misorientation angle exceeds $π/2$. We further comment on possible implication of our results for experimentally observable 0--$π$ transitions in SFS junctions.

cond-mat.supr-con↗
arXiv2006

Resonating-valence-bond structure of Gutzwiller-projected superconducting wave functions

Gutzwiller-projected (GP) wave functions have been widely used for describing spin-liquid physics in frustrated magnets and in high-temperature superconductors. Such wave functions are known to represent states of the resonating-valence-bond (RVB) type. In the present work I discuss the RVB structure of a GP singlet superconducting state with nodes in the spectrum. The resulting state for the undoped spin system may be described in terms of the "path integral" over loop coverings of the lattice, thus extending the known construction for RVB states. The problem of the topological order in GP states may be reformulated in terms of the statistical behavior of loops. The simple example of the projected d-wave state on the square lattice demonstrates that the statistical behavior of loops is renormalized in a nontrivial manner by the projection.

cond-mat.str-el↗
arXiv2006

Minigap in superconductor-ferromagnet junctions with inhomogeneous magnetization

We consider the minigap in a disordered ferromagnet (F) in contact with a superconductor (S) in the situation when the magnetization of the F layer is inhomogeneous in space and noncollinear. If the magnetization is strongly inhomogeneous, it effectively averages out, and the minigap survives up to the exchange field $h_c ~ (L/a) E_{Th}$, where $L$ is the thickness of the F layer, $a$ is the scale on which the magnetization varies, and $E_{Th}$ is the Thouless energy. Technically, we use the "triplet" version of the Usadel equations including both singlet and triplet components of the Green's functions. In many cases, the effect of disordered magnetization may be effectively included in the conventional Usadel equations as the spin-flip scattering term. In the case of low-dimensional magnetic inhomogeneities (we consider spiral magnetization as an example), however, the full set of "triplet" equations must be solved.

cond-mat.supr-con↗
arXiv2006

Quantum mechanics with a time-dependent random unitary Hamiltonian: A perturbative study of the nonlinear Keldysh sigma-model

We analyze the perturbative series of the Keldysh-type sigma-model proposed recently for describing the quantum mechanics with time-dependent Hamiltonians from the unitary Wigner-Dyson random-matrix ensemble. We observe that vertices of orders higher than four cancel, which allows us to reduce the calculation of the energy-diffusion constant to that in a special kind of the matrix ϕ^4 model. We further verify that the perturbative four-loop correction to the energy-diffusion constant in the high-velocity limit cancels, in agreement with the conjecture of one of the authors.

cond-mat.mes-hall↗
arXiv2005

Minigap in a SN junction with paramagnetic impurities

We study the effect of spin-flip scattering on the density of states in a long diffusive S-N junction with transparent interface. We calculate the critical value of the spin-flip scattering rate at which the minigap closes and give the dependence of the minigap on the spin-flip rate. For the system we consider, the minigap and the critical spin-flip rate have the scale of the Thouless energy, and not of the superconducting gap.

cond-mat.supr-con↗
arXiv2003

Landau-Zener interferometry for qubits

One may probe intrinsic coherence of a qubit by periodically sweeping its control parameter. The qubit is then excited by Landau-Zener (LZ) mechanism. The interference between multiple LZ transition leads to the unusual oscillatory dependence of the energy absorption rate on sweeping amplitude and period. This interference pattern allows to determine the decoherence time of the qubit. We introduce a simple phenomenological model describing this "interferometer", and find the form of the interference pattern.

cond-mat↗
arXiv2003

Antiferromagnetism and phase separation in the t-J model at low doping: a variational study

Using Gutzwiller-projected wave functions, I estimate the ground-state energy of the t-J model for several variational states relevant for high-temperature cuprate superconductors. The results indicate antiferromagnetism and phase separation at low doping both in the superconducting state and in the staggered-flux normal state proposed for the vortex cores. While phase separation in the underdoped superconducting state may be relevant for the stripe formation mechanism, the results for the normal state suggest that similar charge inhomogeneities may also appear in vortex cores up to relatively high doping values.

cond-mat.supr-con↗
arXiv2003

Staggered-flux normal state in the weakly doped t-J model

A normal (non-superconducting) ground state of the t-J model may be variationally approximated by a Gutzwiller-projected wave function. Within this approximation, at small hole doping near half-filling, the normal state favors staggered-flux ordering. Such a staggered-flux state may occur in vortex cores of underdoped high-temperature cuprate superconductors. From comparing the energies of the staggered-flux state and of the superconducting state, we numerically obtain the condensation energy. Extracting the superfluid density directly from the projected superconducting wave function, we can also estimate the coherence length at zero temperature.

cond-mat.supr-con↗
arXiv2002

Vortex viscosity in the moderately clean limit of layered superconductors

We present a microscopic calculation of the energy dissipation in the core of a vortex moving in a two-dimensional or layered superconductor in the moderately clean regime. In this regime, the quasiclassical Bardeen--Stephen result remains valid in spite of the strong correlations between the energy levels. We find that the quasiclassical expression applies both in the limit of fast vortex motion (with transitions between smeared levels) and in the limit of slow vortex motion (with nearly adiabatic dynamics). This finding can be related to the similar result known for the unitary random-matrix model.

cond-mat.supr-con↗
arXiv2002

Ground-state properties of the Rokhsar-Kivelson dimer model on the triangular lattice

We explicitly show that the Rokhsar-Kivelson dimer model on the triangular lattice is a liquid with topological order. Using the Pfaffian technique, we prove that the difference in local properties between the two topologically degenerate ground states on the cylinders and on the tori decreases exponentially with the system size. We compute the relevant correlation length and show that it equals the correlation length of the vison operator.

cond-mat.str-el↗
arXiv2002

Minigap in a long disordered SNS junction: analytical results

We review and refine analytical results on the density of states in a long disordered superconductor - normal-metal - superconductor junction with transparent interfaces. Our analysis includes the behavior of the minigap near phase differences zero and PI across the junction, as well as the density of states at energies much larger than the minigap but much smaller than the superconducting gap.

cond-mat.supr-con↗
arXiv2002

Projected wave functions for fractionalized phases of quantum spin systems

Gutzwiller projection allows a construction of an assortment of variational wave functions for strongly correlated systems. For quantum spin S=1/2 models, Gutzwiller-projected wave functions have resonating-valence-bond structure and may represent states with fractional quantum numbers for the excitations. Using insights obtained from field-theoretical descriptions of fractionalization in two dimensions, we construct candidate wave functions of fractionalized states by projecting specific superconducting states. We explicitly demonstrate the presence of topological order in these states.

cond-mat.str-el↗
arXiv2001

The supersymmetric technique for random-matrix ensembles with zero eigenvalues

The supersymmetric technique is applied to computing the average spectral density near zero energy in the large-N limit of the random-matrix ensembles with zero eigenvalues: B, DIII-odd, and the chiral ensembles (classes AIII, BDI, and CII). The supersymmetric calculations reproduce the existing results obtained by other methods. The effect of zero eigenvalues may be interpreted as reducing the symmetry of the zero-energy supersymmetric action by breaking a certain abelian symmetry.

cond-mat↗
arXiv2001

Random-matrix ensembles in p-wave vortices

In disordered vortices in p-wave superconductors the two new random-matrix ensembles may be realized: B and DIII-odd (of so(2N+1) and so(4N+2)/u(2N+1) matrices respectively). We predict these ensembles from an explicit analysis of the symmetries of Bogoliubov-deGennes equations in three examples of vortices with different p-wave order parameters. A characteristic feature of the novel symmetry classes is a quasiparticle level at zero energy. Class B is realized when the time-reversal symmetry is broken, and class DIII-odd when the time-reversal symmetry is preserved. We also suggest that the main contribution to disordering the vortex spectrum comes from the distortion of the order parameter around impurities.

cond-mat.supr-con↗
arXiv2001

Interference effects in isolated Josephson junction arrays with geometric symmetries

As the size of a Josephson junction is reduced, charging effects become important and the superconducting phase across the link turns into a periodic quantum variable. Isolated Josephson junction arrays are described in terms of such periodic quantum variables and thus exhibit pronounced quantum interference effects arising from paths with different winding numbers (Aharonov-Casher effects). These interference effects have strong implications for the excitation spectrum of the array which are relevant in applications of superconducting junction arrays for quantum computing. The interference effects are most pronounced in arrays composed of identical junctions and possessing geometric symmetries; they may be controlled by either external gate potentials or by adding/removing charge to/from the array. Here we consider a loop of N identical junctions encircling one half superconducting quantum of magnetic flux. In this system, the ground state is found to be non-degenerate if the total number of Cooper pairs on the array is divisible by N, and doubly degenerate otherwise (after the stray charges are compensated by the gate voltages).

cond-mat.supr-con↗