arXiv subjects

O. P. Sushkov

Publications and source records attributed to O. P. Sushkov.

At least 91 records · Page 5Linked to original sources

arXiv2001

Phase Diagram of the Spin-Orbital model on the Square Lattice

We study the phase diagram of the spin-orbital model in both the weak and strong limits of the quartic spin-orbital exchange interaction. This allows us to study quantum phase transitions in the model and to approach from both sides the most interesting intermediate-coupling regime and in particular the SU(4)-symmetric point of the Hamiltonian. It was suggested earlier by Li et al [Phys.Rev.Lett. vol. 81, 3527 (1999)] that at this point the ground state of the system is a plaquette spin-orbital liquid. We argue that the state is more complex. There is plaquette order, but it is anisotropic: bonds in one direction are stronger than those in the perpendicular direction. This order is somewhat similar to that found recently in the frustrated J_1-J_2 Heisenberg spin model.

cond-mat↗

arXiv2001

Conductance anomalies in a one-dimensional quantum contact

Short length quantum wires (quantum contacts) exhibit a conductance structure at the value of conductance close to 0.7 \times 2e^2/h. The structure is also called the conductance anomaly. In longer contacts the structure evolves to the lower values of conductance. We demonstrate that this structure is related to the development of charge density waves within the contact. This is a precursor for Wigner crystallization. Many-body Hartree-Fock calculations of conductance are performed. The results are in agreement with experimental data.

cond-mat.mes-hall↗

arXiv2001

Charge ordering and antiferromagnetic exchange in layered molecular crystals of the theta type

We consider the electronic properties of layered molecular crystals of the type theta-D$_2$A, where A is an anion and D is a donor molecule such as BEDT-TTF [where BEDT-TTF is bis-(ethylenedithia-tetrathiafulvalene)] which is arranged in the theta type pattern within the layers. We argue that the simplest strongly correlated electron model that can describe the rich phase diagram of these materials is the extended Hubbard model on the square lattice at a quarter filling. In the limit where the Coulomb repulsion on a single site is large, the nearest-neighbour Coulomb repulsion, V, plays a crucial role. When V is much larger than the intermolecular hopping integral t the ground state is an insulator with charge ordering. In this phase antiferromagnetism arises due to a novel fourth-order superexchange process around a plaquette on the square lattice. We argue that the charge ordered phase is destroyed below a critical non-zero value V, of the order of t. Slave boson theory is used to explicitly demonstrate this for the SU(N) generalisation of the model, in the large N limit. We also discuss the relevance of the model to the all-organic family beta''-(BEDT-TTF)$_2$SF$_5$YSO$_3$ where Y = CH$_2$CF$_2$, CH$_2$, CHF.

cond-mat.str-el↗

arXiv2000

Breit interaction correction to the hyperfine constant of an external s-electron in many-electron atom

Correction to the hyperfine constant $A$ of an external s-electron in many-electron atom caused by the Breit interaction is calculated analytically: $δA/A =0.68 Zα^2$. Physical mechanism for this correction is polarization of the internal electronic shells (mainly $1s^2$ shell) by the magnetic field of the external electron. This mechanism is similar to the polarization of vacuum considered by Karplus and Klein long time ago. The similarity is the reason why in both cases (Dirac sea polarization and internal atomic shells polarization) the corrections have the same dependence on the nuclear charge and fine structure constant. In conclusion we also discuss $Zα^2$ corrections to the parity violation effects in atoms.

physics.atom-ph↗

arXiv2000

New quantum phase transitions in the two-dimensional J1-J2 model

We analyze the phase diagram of the frustrated Heisenberg antiferromagnet, the J1-J2 model, in two dimensions. Two quantum phase transitions in the model are already known: the second order transition from the Neel state to the spin liquid state at (J_2/J_1)_{c2}=0.38, and the first order transition from the spin liquid state to the collinear state at (J_2/J_1)_{c4}=0.60. We have found evidence for two new second order phase transitions: the transition from the spin columnar dimerized state to the state with plaquette type modulation at (J_2/J_1)_{c3}=0.50(2), and the transition from the simple Neel state to the Neel state with spin columnar dimerization at (J_2/J_1)_{c1}=0.34(4). We also present an independent calculation of (J_2/J_1)_{c2}=0.38 using a new approach.

cond-mat.str-el↗

arXiv2000

Spin 1/2 Magnetic Impurity in a 2D Magnetic System Close to Quantum Critical Point

We consider a magnetic impurity in a spin liquid state of a magnetic system which is close to the quantum phase transition to the magnetically ordered state. There is similarity between this problem and the Kondo problem. We derive the impurity Green's function, consider renormalizations of the magnetic moments of the impurity, calculate critical indexes for the magnetic susceptibilities and finally consider specific heat and magnetic interaction of two impurities.

cond-mat.str-el↗

arXiv1999

Spontaneous spin stripe dimerization in the doped t-J model

To investigate spin dimerization in the t-J model we consider the extended t-J-δmodel with explicit spin dimerization introduced via parameter δ. At zero doping the dimerized spin liquid is unstable at δ< δ_c = 0.3. We demonstrate that the doping stabilizes the dimerized stripe phase: the δ_c(x) decreases when doping x increases. At doping larger than critical, x > x_c, the dimerized phase is stable even without explicit dimerization, i.e. at δ=0.

cond-mat.str-el↗

arXiv1999

Double-layer Heisenberg antiferromagnet at finite temperature: Brueckner Theory and Quantum Monte Carlo simulations

The double-layer Heisenberg antiferromagnet with intra- and inter-layer couplings $J$ and $J_\perp$ exhibits a zero temperature quantum phase transition between a quantum disordered dimer phase for $g>g_c$ and a Neel phase with long range antiferromagnetic order for $g<g_c$, where $g=J_\perp/J$ and $g_c \approx 2.5$. We consider the behavior of the system at finite temperature for $g \ge g_c$ using two different and complementary approaches; an analytical Brueckner approximation and numerically exact quantum Monte Carlo simulations. We calculate the temperature dependent spin excitation spectrum (including the triplet gap), dynamic and static structure factors, the specific heat, and the uniform magnetic susceptibility. The agreement between the analytical and numerical approaches is excellent. For $T \to 0$ and $g \to g_c$, our analytical results for the specific heat and the magnetic susceptibility coincide with those previously obtained within the nonlinear $σ$ model approach for $N\to \infty$. Our quantum Monte Carlo simulations extend to significantly lower temperatures than previously, allowing us to obtain accurate results for the asymptotic quantum critical behavior. We also obtain an improved estimate for the critical coupling: $g_c = 2.525 \pm 0.002$.

cond-mat.supr-con↗

arXiv1999

Quantum spin ladder with dynamic phonons. Bound states of magnons and phonons

We consider the two-chain Heisenberg ladder with antiferromagnetic interactions. Our approach is based on the description of spin excitations as triplets above a strong-coupling singlet ground state. It is shown that interaction with dynamic phonons drastically changes the dispersion of the elementary triplet excitation (magnon) and the spin structure factor. It is also shown that a new triplet excitation appears in the spectrum: the bound state of a magnon and a phonon.

cond-mat.str-el↗

arXiv1999

One and Two Dimensional Spin Systems in the Regime Close to Deconfinement of Spinons

Based on the Majumdar-Ghosh chain we construct several spin models which allow us to investigate spinon dynamics in the regime close to deconfinement of spinons. We consider the J_1 - J_2 - δmodel, two coupled J_1 - J_2 chains (ladder), and a 2D array of coupled J_1 - J_2 chains. Using the picture of two spinons interacting with a string confining potential we calculate the singlet- triplet splitting, magnetic structure factor, tunneling amplitude of two spinons and the excitation spectra for the ladder and the array.

cond-mat↗

arXiv1999

Spectrum of elementary and collective excitations in the dimerized S=1/2 Heisenberg chain with frustration

We have studied the low-energy excitation spectrum of a dimerized and frustrated antiferromagnetic Heisenberg chain. We use an analytic approach, based on a description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. Additional singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of the elementary triplets. We have also calculated the contributions of the elementary and collective excitations into the spin structure factor. Our results are in excellent agreement with exact diagonalizations and dimer series expansions data as long as the dimerization parameter $δ$ is not too small ($δ>0.1$), i.e. while the elementary triplets can be treated as localized objects.

cond-mat.supr-con↗

arXiv1998

Bruekner approach to the spin-wave gap critical index for the two-layer Heisenberg antiferromagnet

We consider the two-layer Heisenberg antiferromagnet near a zero temperature quantum phase transition from a disordered dimer phase to a collinear Neel state. At approaching the transition point the spin-wave gap vanishes as $Δ\propto (J_\perp-J_{\perp c})^ν$. To account for strong correlations between the S=1 elementary excitations we apply the Brueckner diagram approach which gives the critical index $ν\approx 0.5$. We demonstrate also that the linearized in density Brueckner equations give the mean field result $ν=1$. Finally an expansion of the Brueckner equations in powers of the density, combined with the scaling hypothesis, give $ν\approx 0.67$. This value reasonably agrees with that of the nonlinear O(3) $σ$-model. Our approach demonstrates that for other quantum spin models the critical index can be different from that in the nonlinear $σ$-model. We discuss the conditions for this to occur.

cond-mat.stat-mech↗

arXiv1998

Single hole dynamics in dimerized spin liquids

The dynamics of a single hole in quantum antiferromagnets is influenced by magnetic fluctuations. In the present work we consider two situations. The first one corresponds to a single hole in the two leg t-J spin ladder. In this case the wave function renormalization is relatively small and the quasiparticle residue of the S=1/2 state remains close to unity. However at large t/J there are higher spin (S=3/2,5/2,..) bound states of the hole with the magnetic excitations, and therefore there is a crossover from quasiparticles with S=1/2 to quasiparticles with higher spin. The second situation corresponds to a single hole in two coupled antiferromagnetic planes very close to the point of antiferromagnetic instability. In this case the hole wave function renormalization is very strong and the quasiparticle residue vanishes at the point of instability.

cond-mat.str-el↗

arXiv1998

Excitation spectrum of the S=1/2 quantum spin ladder with frustration: elementary quasiparticles and many-particle bound states

We study the excitation spectrum of the two-chain S=1/2 Heisenberg spin ladder with additional inter-chain second-neighbor frustrating interactions. The one and two-particle excitations are analyzed by using a mapping of the model onto a Bose gas of hard-core triplets. We find that low-lying singlet and triplet two-particle bound states are present and their binding energy increases with increasing frustration. In addition, many-particle bound states are found by a combination of variational and exact diagonalization techniques. We prove that the larger the number of bound quasiparticles the larger the binding energy. Thus the excitation spectrum has a complex structure and consists of elementary triplets and collective many-particle singlet and triplet excitations which generally mix with the elementary ones. The model exhibits a quantum phase transition from an antiferromagnetic ladder phase (small frustration) into Haldane phase (effectively ferromagnetic ladder for large frustration). We argue that near the transition point the spectrum in both triplet and singlet channels becomes gapless. The excitation wave function is dominated by large-size bound states which leads to the vanishing of the quasiparticle residue.

cond-mat.str-el↗

arXiv1998

Spin-spin interaction and magnetic state of 2-D Wigner crystal

It is demonstrated that there is anti-ferromagnetic spin-spin interaction between nearest electrons in the 2D Wigner crystal. This is also valid for the Wigner liquid - the state with destroyed long-range order but preserved short-range one. We calculate the value of the anti-ferromagnetic interaction (both analytically and numerically) and discuss a possible magnetic state of the Wigner crystal. This state can be strongly influenced by the spin-Peierls mechanism.

cond-mat.str-el↗

arXiv1998

Bound states of magnons in the S=1/2 quantum spin ladder

We study the excitation spectrum of the two-leg antiferromagnetic S=1/2 Heisenberg ladder. Our approach is based on the description of the excitations as triplets above a strong-coupling singlet ground state. The quasiparticle spectrum is calculated by treating the excitations as a dilute Bose gas with infinite on-site repulsion. We find singlet (S=0) and triplet (S=1) two-particle bound states of the elementary triplets. We argue that bound states generally exist in any dimerized quantum spin model.

cond-mat.str-el↗

arXiv1997

Polarizabilities and parity non-conservation in the Cs atom and limits on the deviation from the standard electroweak model

A semi-empirical calculation of the 6s - 7s Stark amplitude $α$ in Cs has been performed using the most accurate measurements and calculations of the electromagnetic amplitudes available. This is then used to extract the parameters of the electroweak theory from experimental data. The results are: $α= 269.0 (1.3) a_0^3$, weak charge of Cs $Q_W = -72.41(25)_{exp} (80)_{theor}$, deviation from the Standard model $S = -1.0(.3)_{exp} (1.0)_{theor}$ and limit on the mass of the extra Z-boson in SO(10) model $M_{Z_x} > 550 GeV$.

hep-ph↗

arXiv1997

Single Hole Green's Functions in Insulating Copper Oxides at Nonzero Temperature

We consider the single hole dynamics in a modified $t-J$ model at finite temperature. The modified model includes a next nearest ($t'$) and next-next nearest ($t''$) hopping. The model has been considered before in the zero temperature limit to explain angle resolved photo-emission measurements. We extend this consideration to the case of finite temperature where long-range anti-ferromagnetic order is destroyed, using the self-consistent Born approximation. The Dyson equation which relates the single hole Green's functions for a fixed pseudo-spin and for fixed spin is derived. The Green's function with fixed pseudo-spin is infrared stable but the Green's function with fixed spin is close to an infrared divergency. We demonstrate how to renormalize this Green's function in order to assure numerical convergence. At non-zero temperature the quasi-particle peaks are found to shift down in energy and to be broadened.

cond-mat↗