arXiv subjects

M. V. Eremin

Publications and source records attributed to M. V. Eremin.

35 records · Page 2Linked to original sources

arXiv2007

Binding energy of a Cooper pairs with non-zero center of mass momentum in d-wave superconductors

The binding energy of Cooper pairs has been calculated for the case of d-wave symmetry of the superconducting gap in layered cuprate superconductors. We assume that Cooper pairs are formed by the short range potential and then derive the binding energy in the form Delta_kq = Δ_x(q)cos(k_xa) + Δ_y(q)cos(k_ya) + Ω_x(q)sin(k_xa) + Ω_y(q)sin(k_ya), where q is a total momentum of the pair. Numerical solutions of the self-consistent system of the integral equations for quantities Δ_x(q), Δ_y(q) and Ω_x(q), Ω_y(q) along different lines in q_x, q_y plane have been obtained. Anisotropy of the depairing total momentum (or depairing current) has been calculated.

cond-mat.supr-con↗

arXiv2006

Spin dynamics in the low-dimensional magnet TiOCl

We present detailed ESR investigations on single crystals of the low-dimensional quantum magnet TiOCl. The anisotropy of the g-factor indicates a stable orbital configuration below room temperature, and allows to estimate the energy of the first excited state as 0.3(1) eV ruling out a possible degeneracy of the orbital ground state. Moreover, we discuss the possible spin relaxation mechanisms in TiOCl and analyze the angular and temperature dependence of the linewidth up to 250 K in terms of anisotropic exchange interactions. Towards higher temperatures an exponential increase of the linewidth is observed, indicating an additional relaxation mechanism.

cond-mat.str-el↗

arXiv2005

Unconventional anisotropic superexchange in NaV2O5

The strong line broadening observed in electron spin resonance on NaV$_2$O$_5$ is found to originate from an unusual type of the symmetric anisotropic exchange interaction with simultaneous spin-orbit coupling on both sites. The microscopically derived anisotropic exchange constant is almost two orders of magnitude larger than the one obtained from conventional estimations. Based on this result we systematically evaluate the anisotropy of the ESR linewidth in terms of the symmetric anisotropic exchange, only, and we find microscopic evidence for precursor effects of the charge ordering already below 150 K.

cond-mat.str-el↗

arXiv2004

Anisotropy of the paramagnetic susceptibility in LaTiO$_{3}$: The electron-distribution picture in the ground state

The energy-level scheme and wave functions of the titanium ions in LaTiO$_{3}$ are calculated using crystal-field theory and spin-orbit coupling. The theoretically derived temperature dependence and anisotropy of the magnetic susceptibility agree well with experimental data obtained in an untwinned single crystal. The refined fitting procedure reveals an almost isotropic molecular field and a temperature dependence of the van Vleck susceptibility. The charge distribution of the 3d--electron on the Ti positions and the principle values of the quadrupole moments are derived and agree with NMR data and recent measurements of orbital momentum $ $ and crystal-field splitting. The low value of the ordered moment in the antiferromagnetic phase is discussed.

cond-mat.str-el↗

arXiv2004

Impact of Optical Modes on the Pairing Potential in Bilayer Cuprates

Superconducting transition temperature is calculated for differently doped bilayer cuprates. Superexchange is assumed to be the dominating mechanism of high-temperature superconductivity, but the contribution from the phonon potential is not negligible, which qualitatively explains the observed weak isotopic effect. The calculated value 2$Δ_{max}/k_B T_C \simeq 4.5$ is close to the experiment in the case of optimum doping.

cond-mat.supr-con↗

arXiv2004

Possible isotope effect on the resonance peak formation in high-T$_c$ cuprates

Starting from the three-band $p-d$ Hubbard Hamiltonian we derive an effective $t-J$ model including electron-phonon interaction of quasiparticles with optical phonons. Within the effective Hamiltonian we analyze the influence of electronic correlations and electron-phonon interaction on the dynamical spin susceptibility in layered cuprates. We find a huge isotope effect on the resonance peak in the magnetic spin susceptibility, ${Im}χ({\bf q},ω)$, seen by inelastic neutron scattering. It results from both the electron-phonon coupling and the electronic correlation effects taken into account beyond random phase approximation(RPA) scheme. We find at optimal doping the isotope coeffiecient $α_{res} \approx 0.35$ which can be further tested experimentally.

cond-mat.supr-con↗

arXiv2002

Magnetic Anisotropy in La_0.8Sr_0.2MnO_3: Electron Spin Resonance

We report on Ferromagnetic-Resonance experiments in a single crystal of La_0.8Sr_0.2MnO_3 in the temperature range from 4 to 300 K. The observed anisotropy of the resonance line changes on crossing the transition from the orthorhombic O-phase to the rhombohedral R-phase at T~100 K and indicates a reorientation of the spins at about 130 K.

cond-mat.str-el↗

arXiv2002

Polaron Effects on Superexchange Interaction: Isotope Shifts of $T_N$, $T_C$, and $T^*$ in Layered Copper Oxides

A compact expression has been obtained for the superexchange coupling of magnetic ions via intermediate anions with regard to polaron effects at both magnetic ions and intermediate anions. This expression is used to analyze the main features of the behavior of isotope shifts for temperatures of three types in layered cuprates: the Neel temperatures ($T_N$), critical temperatures of transitions to a superconducting state ($T_C$), and characteristic temperatures of the pseudogap in the normal state ($T^*$).

cond-mat.supr-con↗

arXiv2002

Charge Density Waves and $x=1/8$ anomaly in La$_{2-x-y}$(Nd,Eu)$_y$(Ba,Sr)$_x$CuO$_4$

We adopt a $t_1-t_2-t_3-J-G$ model for explanation of $x=1/8$ anomaly in La$_{2-x}$Sr$_x$CuO$_4$ family compound. The calculated charge susceptibility shows a maximum near $(π,π)$ at intermediate temperatures and near $(π,π/2)$ as temperature approaches zero, in agreement with neutron scattering experiments. Coulomb repulsion $G$ between the first neighbors turns out to be the source of Charge Density Waves (CDW) in narrow band $t_1^{\mathit{eff}}, t_2^{\mathit{eff}}, t_3^{\mathit{eff}}<G$. For physically realistic hopping values we obtain the CDW amplitude $e_Q=x$. The in-phase domain structure as a candidate for "stripe" picture is proposed.

cond-mat.supr-con↗

arXiv2002

Anisotropic Exchange in LiCuVO$_4$ probed by ESR

We investigated the paramagnetic resonance in single crystals of LiCuVO$_4$ with special attention to the angular variation of the absorption spectrum. To explain the large resonance linewidth of the order of 1 kOe, we analyzed the anisotropic exchange interaction in the chains of edge-sharing CuO$_6$ octahedra, taking into account the ring-exchange geometry of the nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The exchange parameters, which can be estimated from theoretical considerations, nicely agree with the parameters obtained from the angular dependence of the linewidth. The anisotropy of this magnetic ring exchange is found to be much larger than it is usually expected from conventional estimations which neglect the bonding geometry. Hence, the data yield the evidence that in copper oxides with edge-sharing structures the role of the orbital degrees of freedom is strongly enhanced. These findings establish LiCuVO$_4$ as one-dimensional compound at high temperatures. PACS: 76.30.-v, 76.30.Fc, 75.30.Et

cond-mat.str-el↗

arXiv2001

Crystal Field and Dzyaloshinsky-Moriya Interaction in orbitally ordered La_{0.95}Sr_{0.05}MnO_3: An ESR Study

We present a comprehensive analysis of Dzyaloshinsky-Moriya interaction and crystal-field parameters using the angular dependence of the paramagnetic resonance shift and linewidth in single crystals of La_{0.95}Sr_{0.05}MnO_3 within the orthorhombic Jahn-Teller distorted phase. The Dzyaloshinsky-Moriya interaction (~ 1K) results from the tilting of the MnO_6 octahedra against each other. The crystal-field parameters D and E are found to be of comparable magnitude (~ 1K) with D ~= -E. This indicates a strong mixing of the |3z^2-r^2> and |x^2-y^2> states for the real orbital configuration.

cond-mat.str-el↗

arXiv2001

Temperature Dependence of the Cu(2) NQR Line Width in YBa$_2$Cu$_3$O$_{7-y}$

Systematic measurements of the $^{63}$Cu(2) NQR line width were performed in underdoped YBa$_2$Cu$_3$O$_{7-y}$ samples over the temperature range 4.2 K $<T<300$ K. It was shown that the copper NQR line width monotonically increases upon lowering temperature in the below-critical region, resembling temperature behavior of the superconducting gap. The observed dependence is explained by the fact that the energy of a condensate of sliding charge-current states of the charge-density-wave type depends on the phase of order parameter. Calculations show that this dependence appears only at $T<T_c$. Quantitative estimates of the line broadening at $T<T_c$ agree with the measurement results.

cond-mat.supr-con↗

arXiv2001

Phase diagram of YBa$_2$Cu$_3$O$_{7-y}$ at T$<$T$_c$ based on Cu(2) transverse nuclear relaxation

Two maxima in transverse relaxation rate of Cu(2) nuclei in YBa$_2$Cu$_3$O$_{7-y}$ are observed, at T = 35 K and T = 47 K. Comparison of the $^{63}$Cu(2) and $^{65}$Cu(2) rates at T = 47 K indicates the magnetic character of relaxation. The enhancement at T = 47 K of fluctuating local magnetic fields perpendicular to the CuO$_2$ planes is connected with the critical fluctuations of orbital currents. Maximum at T = 35 K is connected with the appearance of inhomogeneous supeconducting phase. Together with data published to date, our experimental results allow to suggest a qualitatively new phase diagram of the superconducting phase.

cond-mat.supr-con↗

arXiv2001

Characteristic features of d pairing in bilayer cuprates under conditions of Peierls instability of the normal phase

A system of self-consistent integral equations for the superconducting gap is formulated and solved taking account of the instability of the normal phase of bilayered cuprates against charge-density waves. The critical parameters are calculated as a function of the wave vector, temperature, and doping index. It is found that the region in which superconductivity coexists with d-type charge-density waves depends strongly on the doping index. The effective energy-gap parameter, determined as the interval between the peaks of the density of states, can have a local minimum at temperatures T$<$T$_C$.

cond-mat.supr-con↗

arXiv2000

Optical studies of Cr$^{3+}$-Cr$^{2+}$ pair center in KZnF$_{3}$ crystal

Optical absorption spectra of Cr$^{3+}$-Cr$^{2+}$ pair center in KZnF$_{3}$:Cr$^{3+}$,Cr$^{2+}$ crystal were investigated in wide temperature range. Broad band at 30800 cm$^{-1}$ is attributed to cation-cation (e_g)-electron transfer transition. Narrow lines with maxima at 16720 cm$^{-1}$ and 19880 cm$^{-1}$ have been assigned to purely electronic exchange-induced electric-dipole transitions from the ground (Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^5E_g)) state to excited (Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^a)) and (Cr$^{3+}$,(^4A_{2g});Cr$^{2+}$,(^3E_g^b)) states, respectively. It's vibronic satellites corresponding to (a_{1g}) local mode of Cr$^{3+}$ fluorine octahedron of the pair are also observed. Energy of the local mode for the ground and mentioned excited states are 580, 540 and 530 cm$^{-1}$. Instead of expected double exchange for mixed valence pair ferromagnetic superexchange for Cr$^{3+}$-Cr$^{2+}$ pair in KZnF$_{3}$ crystal is realized. Exchange integral (J=-14.9\pm0.4) cm$^{-1}$ and Jahn-Teller splitting (Δ_{JT}=340\pm40) cm$^{-1}$ for the ground state of the pair were obtained by analysis of the temperature dependence of absorption lines. Important features of the crossover double exchange - ferromagnetic superexchange are discussed.

cond-mat.str-el↗

arXiv2000

Charge Order in NaV2O5 studied by EPR

We present angular dependent EPR measurements in NaV2O5 at X-band frequencies in the temperature range 4.2 K < T < 670 K. A detailed analysis in terms of the antisymmetric Dzyaloshinski-Moriya and the anisotropic exchange interactions yields the following scheme of charge order: On decreasing temperature a quarter-filled ladder with strong charge disproportions, existing for T > 100 K, is followed by zig-zag charge-order fluctuations which become long-range and static below T_SP=34 K.

cond-mat.str-el↗

arXiv1999

ESR study in lightly doped La_(1-x)Sr_(x)MnO_(3)

This is the first systematic ESR investigation of La_(1-x)Sr_(x)MnO_(3) single crystals for Sr concentrations x < 0.2. The rich phase diagram of this compound, which is dominated by the Jahn-Teller effect of Mn^(3+), is a matter of actual interest in connection with giant magnetoresistance phenomena. The ESR signal, which is due to both Mn^(3+) and Mn^(4+), directely probes the suppression of the Jahn-Teller distortion with increasing Sr concentration and reveals all characteristic structural and magnetic phase transitions.

cond-mat.str-el↗