arXiv subjects

M. E. Zhitomirsky

Publications and source records attributed to M. E. Zhitomirsky.

102 records · Page 6Linked to original sources

arXiv1999

Instability of antiferromagnetic magnons in strong fields

We predict that spin-waves in an ordered quantum antiferromagnet (AFM) in a strong magnetic field become unstable with respect to spontaneous two-magnon decays. At zero temperature, the instability occurs between the threshold field $H^*$ and the saturation field $H_c$. As an example, we investigate the high-field dynamics of a Heisenberg antiferromagnet on a square lattice and show that the single-magnon branch of the spectrum disappears in the most part of the Brillouin zone.

cond-mat.str-el↗

arXiv1999

Excitonic ferromagnetism in the hexaborides

A ferromagnet with a small spontaneous moment but with a high Curie temperature can be obtained by doping an excitonic insulator made from a spin triplet exciton condensate. Such a condensate can occur in a semimetal with a small overlap or a semiconductor with a small bandgap. We propose that it is responsible for the unexpected ferromagnetism in the doped hexaboride material Ca_{1-x}La_xB_6.

cond-mat.str-el↗

arXiv1999

Phenomenology of the superconducting state in Sr2RuO4

The symmetry of the superconducting phase of Sr2RuO4 is identified as the odd-parity pairing state d(k)=\hat{z}(k_x \pm i k_y) based on recent experiments. The experimental evidence for the so-called orbital dependent superconductivity leads to a single-band description of superconductivity based on spin fluctuation mechanism. It is shown that the state \hat{z}(k_x \pm i k_y) can be stabilized by the spin fluctuation feedback mechanism analogous to the A-phase in 3He and by spin-orbit coupling effects.

cond-mat.supr-con↗

arXiv1998

Effect of spatial variations of superconducting gap on suppression of the transition temperature by impurities

We calculate correction to the critical temperature of a dirty superconductor, which results from the local variations of the gap function near impurity sites. This correction is of the order of T_c/E_F and becomes important for short-coherence length superconductors. It generally reduces a pair-breaking effect. In s-wave superconductors small amounts of nonmagnetic impurities can increase the transition temperature.

cond-mat.supr-con↗

arXiv1997

Thermal conductivity across a twin boundary in d-wave superconductor

We consider excitation spectrum near a twin boundary in an orthorhombic $d+s$ superconductor. The low-energy spectrum is highly sensitive to the presence of the small amount of s-wave component. Robustness of the bound states at the Fermi level with respect to impurities and an extra boundary potential is investigated. The role of Andreev transmission process for the low-temperature thermal conductivity across twin boundaries is studied for an impure superconductors. At very low temperatures the bulk part and the twin boundaries part of the thermal conductivity have similar linear-$T$ dependences, whereas at intermediate temperatures the two contributions behave like $T^3$ and $T^2$, respectively.

cond-mat.supr-con↗

arXiv1997

Electronic states on a twin boundary of a d-wave superconductor

We show that an induced $s$-wave harmonic in the superconducting gap of an orthorhombic $d_{x^2-y^2}$ superconductor strongly affects the excitation spectrum near a twinning plane. In particular, it yields bound states of zero energy with areal density proportional to the relative weight of the $s$-wave component. An unusual scattering process responsible for the thermal conductivity across the twin boundary at low temperatures is also identified.

cond-mat.supr-con↗

arXiv1996

Nearly critical ground state of LaCuO2.5

Using a combination of analytical techniques and Quantum Monte Carlo simulations we investigate the coupled spin ladder system LaCuO2.5. At a critical ratio of the interladder to intraladder coupling (J'/J)_c \approx 0.11 we find a quantum phase transition between a Neel ordered and a disordered state. At criticality the uniform susceptibility behaves as χ(T)=aT^2 with a universal prefactor. At intermediate temperatures the system crosses over to a ``decoupled ladders regime'' with pseudo-gap type behavior, similar to uncoupled ladders. This can explain the gap-like experimental data for the magnetic susceptibility of LaCuO2.5 despite the presence of the long range Neel order.

cond-mat↗

arXiv1996

Effect of Quantum Fluctuations on Magnetic Ordering in CaV$_3$O$_7$

We present a theoretical model for CaV$_3$O$_7$: the $1/4$-depleted square spin-$1/2$ Heisenberg model which includes both the nearest-neighbor coupling ($J$) and the next-nearest-neighbor coupling ($J'$), where $J$ and $J'$ are antiferromagnetic. Recent experiments of the neutron diffraction by Harashina et.al. report the magnetic ordering at low temperatures, which may be called as a stripe phase. It is shown that the observed spin structure is not stable in the classical theory. By employing the modified spin wave theory, we show that the stripe phase is stabilized by the quantum fluctuations for $J'/J > 0.69$. In CaV$_3$O$_7$, the coupling constants are estimated as $J \sim J'$ by comparing the theoretical and experimental results.

cond-mat↗

arXiv1996

New valence bond crystal phase of a frustrated spin-1/2 square lattice antiferromagnet

We propose a new type of magnetically disordered ground state for a frustrated quantum antiferromagnet. This disordered state is an array of spin singlets spontaneously formed on four spin plaquettes. Both perturbation results and bond-operator calculations show that this phase has lower energy than the columnar dimer state. Analysis of available numerical data on finite clusters also supports the conclusion that this state is realized at intermediate frustrations.

cond-mat↗

arXiv1996

Origin of spin-gap in CaV$_4$O$_9$: effect of frustration and lattice distortion

We study the origin of spin-gap in recently discovered material CaV$_4$O$_9$. We analyze the spin-$1/2$ Heisenberg model on the $1/5$ depleted square lattice with nearest neighbor (nn) and next nearest neighbor (nnn) interactions, in terms of the singlet and triplet states of the 4-spin plaquettes and 2-spin dimers. Phase diagram of the model is obtained within a linear ``spin-wave"-like approximation, and is shown to agree well with the earlier results of QMC simulations for nn interactions. We further propose that the special lattice structure of CaV$_4$O$_9$ naturally leads to lattice distortions, which enhances the spin-gap via a spin-Peierls mechanism.

cond-mat↗

arXiv1995

Static properties of a quasi one-dimensional antiferromagnet in magnetic field

We calculate the effect of zero-point fluctuations on the magnetostatic properties of a quasi one-dimensional antiferromagnetic helimagnet with and without in-plane anisotropy. We find substantial reduction of the uniform susceptibility and the sublattice magnetization from their classical values and appreciable renormalization of the spin-flop field at $T=0$. The magnetization curve varies nonlinearly at small fields and is described by a universal formula at high fields. The results are compared with numerical simulations on one-dimensional systems and measurements in the CsNiCl$_3$-type compounds.

cond-mat↗

arXiv1995

MAGNETIC PROPERTIES OF UNCONVENTIONAL SUPERCONDUCTORS

The article reviews recent developments on magnetic properties of superconductors with anisotropic Cooper pairing. In particular, we show how the concept of broken symmetries is applied to the investigation of the mixed state in superconductors with a multicomponent order parameter. Starting from the phenomenological description in the framework of the generalized Ginzburg-Landau theory, we discuss different types of quantized vortices appearing at $H_{c1}$ in states with and without time-reversal breaking. General classification of superconducting phase transitions in a uniform magnetic field at $H_{c2}$ is constructed. Vortex lattices of different forms are found in the vicinity of the upper critical field. Symmetry arguments are used to classify phase transitions inside the mixed state. Special attention is given to results which can be obtained analytically. Also special emphasis is put on the open questions of the theory.

cond-mat↗