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B. Mitrovic

Publications and source records attributed to B. Mitrovic.

15 recordsLinked to original sources
arXiv2013

Absence of phase transition in the XY-model on Menger sponge

We have performed a Monte Carlo study of the classical XY-model on a Menger sponge with the Wolff cluster algorithm (U. Wolff, 1989). The Menger sponge is a fractal object with infinite order of ramification and fractal dimension D = log(20)/log(3) = 2.7268. From the dependence of the helicity modulus on system size and on boundary conditions, we conclude that there is no phase transition in the system at any finite temperature.

cond-mat.stat-mech↗
arXiv2013

Monte Carlo study of the XY-model on Sierpnski carpet

We have performed a Monte Carlo study of the classical XY-model on a Sierpi\' nski carpet, which is a planar fractal structure with infinite order of ramification and fractal dimension 1.8928. We employed the Wolff cluster algorithm in our simulations and our results, in particular those for the susceptibility and the helicity modulus, indicate the absence of finite-temperature Berezinskii-Kosterlitz-Thouless (BKT) transition in this system.

cond-mat.stat-mech↗
arXiv2007

Divergence of the orbital nuclear magnetic relaxation rate in metals

We analyze the nuclear magnetic relaxation rate $(1/T_1)_{orb}$ due to the coupling of nuclear spin to the orbital moment of itinerant electrons in metals. In the clean non--interacting case, contributions from large--distance current fluctuations add up to cause a divergence of $(1/T_1)_{orb}$. When impurity scattering is present, the elastic mean free time $τ$ cuts off the divergence, and the magnitude of the effect at low temperatures is controlled by the parameter $\ln(μτ)$, where $μ$ is the chemical potential. The spin--dipolar hyperfine coupling, while has the same spatial variation $1/r^3$ as the orbital hyperfine coupling, does not produce a divergence in the nuclear magnetic relaxation rate.

cond-mat.supr-con↗
arXiv2006

Effect of disorder on the NMR relaxation rate in two-band superconductors

We calculate the effect of nonmagnetic impurity scattering on the spin-lattice relaxation rate in two-band superconductors with the s-wave pairing symmetry. It is found that for the interaction parameters appropriate for MgB2 the Hebel-Slichter peak is suppressed by disorder in the limit of small interband impurity scattering rate. In the limit of strong impurity scattering, when the gap functions in the two bands become nearly equal, the single-band behavior is recovered with a well-defined coherence peak just below the transition temperature.

cond-mat.supr-con↗
arXiv2005

Classical phase fluctuations in d-wave superconductors

We study the effects of low-energy nodal quasiparticles on the classical phase fluctuations in a two-dimensional d-wave superconductor. The singularities of the phase-only action at T\to 0 are removed in the presence of disorder, which justifies using an extended classical XY-model to describe phase fluctuations at low temperatures.

cond-mat.supr-con↗
arXiv2005

NMR relaxation time in a clean two-band superconductor

We study the spin-lattice relaxation rate of nuclear magnetic resonance in a two-band superconductor. Both conventional and unconventional pairing symmetries for an arbitrary band structure in the clean limit are considered. The importance of the inter-band interference effects is emphasized. The calculations in the conventional case with two isotropic gaps are performed using a two-band generalization of Eliashberg theory.

cond-mat.supr-con↗
arXiv2004

On the rate of Tc suppression by the interband impurity scattering in MgB2

We calculate the change in the superconducting transition temperature Tc of MgB2 caused by interband nonmagnetic impurity scattering using the Eliashberg theory for the two-band model of this compound. Much slower rate of Tc suppression is obtained compared to the prediction based on the BCS treatment of the two-band model which ignores renormalization and damping associated with the electron-phonon interaction. Hence, the interband impurity scattering rates deduced from experiments on MgB2 using the formula which results from the BCS approach to the two-band model are underestimated. We generalize the BCS treatment of the two-band model to include renormalization effects of the electron-phonon interaction and find an excellent agreement with the full strong coupling calculation.

cond-mat.supr-con↗
arXiv2004

Functional derivatives of Tc for a two-band superconductor: application to MgB2

We address the question of how phonons of various frequencies contribute to the superconducting transition temperature $T_c$ of a multi-band superconductor by considering the functional derivatives of $T_c$ with respect to various intraband and interband electron-phonon coupling functions. A general scheme for computing such functional derivatives is developed. The proofs are given that the functional derivatives which are diagonal in band indices are linear in phonon energy $Ω$ at small $Ω$, while the functional derivatives which are off-diagonal in band indices diverge at $Ω$ = 0 as 1/$Ω$. The case of a two-band model for MgB$_{2}$ is treated numerically.

cond-mat.supr-con↗
arXiv2003

Anisotropy in the helicity modulus of a quantum 3D XY-model: application to YBCO

We present a variational study of the helicity moduli of an anisotropic quantum three-dimensional (3D) XY-model of YBCO in superconducting state. It is found that both the ab-plane and the c-axis helicity moduli, which are proportional to the inverse square of the corresponding magnetic field penetration depth, vary with temperature T as T to the fourth power in the zero temperature limit. Moreover, the c-axis helicity modulus drops with temperature much faster than the ab-plane helicity modulus because of the weaker Josephson couplings along the c-axis compared to those along the ab-plane. These findings are in disagreement with the experiments on high quality samples of YBCO.

cond-mat.supr-con↗
arXiv2003

Nodal quasiparticles and classical phase fluctuations in d-wave superconductors

We show that the nodal quasiparticles have significant effect on the classical phase fluctuations in a quasi-two-dimensional d-wave superconductor. They give rise to singularities in the temperature behavior of some of the coupling constants in the phase-only effective action. One of the consequences is that the classical XY-model is not adequate for the description of the superconducting fluctuations in d-wave superconductors at low temperatures.

cond-mat.supr-con↗
arXiv2003

Anisotropy in the helicity modulus of a 3D XY-model: application to YBCO

We present a Monte Carlo study of the helicity moduli of an anisotropic classical three-dimensional (3D) XY-model of YBCO in superconducting state. It is found that both the ab-plane and the c-axis helicity moduli, which are proportional to the inverse square of the corresponding magnetic field penetration depth, vary linearly with temperature at low temperatures. The result for the c-axis helicity modulus is in disagreement with the experiments on high quality samples of YBCO. Thus we conclude that purely classical phase fluctuations of the superconducting order parameter cannot account for the observed c-axis electrodynamics of YBCO.

cond-mat.supr-con↗
arXiv1996

Two-dimensional XY spin/gauge glasses on periodic and quasiperiodic lattices

Via Monte Carlo studies of the frustrated XY or classical planar model we demonstrate the possibility of a finite (nonzero) temperature spin/gauge glass phase in two dimensions. Examples of both periodic and quasiperiodic two dimensional lattices, where a high temperature paramagnetic phase changes to a spin/gauge glass phase with the lowering of temperature, are presented. The existence of the spin/gauge glass phase is substantiated by our study of the temperature dependence of the Edwards-Anderson order parameter, spin glass susceptibility, linear susceptibility and the specific heat. Finite size scaling analysis of spin glass susceptibility and order parameter yields a nonzero critical temperature and exponents that are in close agreement with those obtained by Bhatt and Young in their random ${\pm J}$ Ising model study on a square lattice. These results suggest that certain periodic and quasiperiodic two-dimensional arrays of superconducting grains in suitably chosen transverse magnetic fields should behave as superconducting glasses at low temperatures.

cond-mat↗
arXiv1996

Strong coupling theory of the interlayer tunneling model for high temperature superconductors

The interlayer pair tunneling model of Anderson et al. is generalized to include the strong coupling effects associated with in-plane interactions. The equations for the superconducting transition temperature T_{c} are solved numerically for several models of electron-optical phonon coupling. The nonmagnetic in-plane impurity scattering suppresses T_{c} in all cases considered, and it is possible to obtain a fair agreement with experiments for a reasonable choice of parameters. For the anisotropic electron-phonon coupling proposed by Song and Annett we find that the interlayer pair tunneling can stabilize the d_{x^2-y^2}-wave superconducting state with a high T_{c}. Moreover, in this case there is a possibility of an impurity induced crossover from the d$_{x^2-y^2}$-wave state stabilized by the interlayer tunneling to the s-wave state at a low impurity concentration. We also calculate the isotope effect associated with the in-plane oxygen optic mode and its dependence on the strength of the interlayer pair tunneling. Small positive values of the isotope exponent are obtained for strengths of pair tunneling that give high transition temperatures.

supr-con↗
arXiv1996

Spin glass behavior of frustrated 2-D Penrose lattice in the classical planar model

Via extensive Monte Carlo studies we show that the frustrated XY Hamiltonian on a 2-D Penrose lattice admits of a spin glass phase at low temperature. Studies of the Edwards-Anderson order parameter, spin glass susceptibility, and local (linear) susceptibility point unequivocally to a paramagnetic to spin glass transition as the temperature is lowered. Specific heat shows a rounded peak at a temperature above the spin glass transition temperature, as is commonly observed in spin glasses. Our results strongly suggest that the critical point exponents are the same as obtained by Bhatt and Young in the ${\pm}J$ Ising model on a square lattice. However, unlike in the latter case, the critical temperature is clearly finite (nonzero). The results imply that a quasiperiodic 2-D array of superconducting grains in a suitably chosen transverse magnetic field should behave as a superconducting glass at low temperature.

cond-mat↗