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Search Results: 1 - 10 of 237 matches for " Boldizsar Janko "
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Theory of Scanning Tunneling Spectroscopy of Magnetic-Field-Induced Discrete Nodal States in a D-Wave Superconductor
Boldizsar Janko
Physics , 1998, DOI: 10.1103/PhysRevLett.82.4703
Abstract: In the presence of an external magnetic field, the low lying elementary excitations of a d-wave superconductor have quantized energy and their momenta are locked near the node direction. It is argued that these discrete states can most likely be detected by a local probe, such as a scanning tunneling microscope. The low temperature local tunneling conductance on the Wigner-Seitz cell boundaries of the vortex lattice is predicted to show peaks spaced as $\pm \sqrt{n}, n ={0,1,2, ...}$. The $n=0$ peak is anomalous, and it is present only if the superconducting order parameter changes sign at certain points on the Fermi surface. Away from the cell boundary, where the superfluid velocity is nonzero, each peak splits, in general, into four peaks, corresponding to the number of nodes in the order parameter.
Thermodynamic Constraints on the Magnetic Field Dependence of the Neutron Resonance in Cuprate Superconductors
Boldizsar Janko
Physics , 1999,
Abstract: In order to test the recently proposed connection between the neutron resonance and the specific heat anomaly in cuprates, the experimental specific heat data on $YBa_2Cu_3O_{6.93}$ and a theoretical estimate of the single particle fermionic contribution to the specific heat is used to provide an upper bound for the intensity of the neutron peak. The deduced peak intensity is similar in magnitude and temperature dependence to that observed in neutron scattering experiments, and it is constrained to decrease strongly under the influence of moderate magnetic fields oriented along the c-axis of the crystal. An explanation is proposed for the predicted suppression, based on the observation that the resonance intensity is very sensitive to superconducting phase correlations.
Parity Fluctuations Between Coulomb Blockaded Superconducting Islands
Boldizsar Janko,Vinay Ambegaokar
Physics , 1995, DOI: 10.1103/PhysRevLett.75.1154
Abstract: We find that if two superconducting islands of different number parity are linked by a tunnel junction the unpaired electron in the odd island has a tendency to tunnel into the even island. This process leads to fluctuations in time of the number parity of each island, giving rise to a random telegraph noise spectrum with a characteristic frequency that has an unusual temperature dependence. This new phenomenon should be observable in a Cooper-pair pump and similar single-electron tunneling devices.
Nanoscale Zeeman localization of charge carriers in diluted magnetic semiconductor-permalloy hybrids
Mona Berciu,Boldizsar Janko
Physics , 2003, DOI: 10.1103/PhysRevLett.90.246804
Abstract: We investigate the possibility of charge carrier localization in magnetic semiconductors due to the presence of a highly inhomogeneous external magnetic field. As an example, we study in detail the properties of a magnetic semiconductor-permalloy disk hybrid system. We find that the giant Zeeman respose of the magnetic semiconductor in conjuction with the highly non-uniform magnetic field created by the vortex state of a permalloy disk can lead to Zeeman localized states at the interface of the two materials. These trapped state are chiral, with chirality controlled by the orientation of the core magnetization of the permalloy disk. We calculate the energy spectrum and the eigenstates of these Zeeman localized states, and discuss their experimental signatures in spectroscopic probes.
Magnetic Semiconductors are Frustrated Ferromagnets
Gergely Zarand,Boldizsar Janko
Physics , 2001,
Abstract: Starting from microscopic and symmetry considerations, we derive the Hamiltonian describing the exchange interaction between the localized Mn spins and the valence band holes in $Ga_{1-x}Mn_x As$. We find that due to the strong spin-orbit coupling in the valence band, this exchange interaction has a rather complex structure and generates a highly anisotropic effective interaction between the Mn spins. The corresponding ground state has a finite ferromagnetic magnetization but is intrinsically spin-disordered even at zero temperature.
Electromagnetic response of a static vortex line in a type-II superconductor : a microscopic study
Boldizsar Janko,Joel D. Shore
Physics , 1992, DOI: 10.1103/PhysRevB.46.9270
Abstract: The electromagnetic response of a pinned Abrikosov fluxoid is examined in the framework of the Bogoliubov-de Gennes formalism. The matrix elements and the selection rules for both the single photon (emission - absorption) and two photon (Raman scattering) processes are obtained. The results reveal striking asymmetries: light absorption by quasiparticle pair creation or single quasiparticle scattering can occur only if the handedness of the incident radiation is opposite to that of the vortex core states. We show how these effects will lead to nonreciprocal circular birefringence, and also predict structure in the frequency dependence of conductivity and in the differential cross section of the Raman scattering.
BCS Superconductivity with Fixed Number Parity
Boldizsar Janko,Anders Smith,Vinay Ambegaokar
Physics , 1994, DOI: 10.1103/PhysRevB.50.1152
Abstract: We investigate superconductivity in a grand canonical ensemble with {\it fixed number parity} (even or odd). In the low temperature limit we find small corrections to the BCS gap equation and energy spectrum $E(k)$. The even-odd free energy difference in the same limit decreases linearly with temperature, in accordance with the behavior observed experimentally and previously arrived at from a quasiparticle model. The theory yields deviations from the BCS predictions for the specific heat, ultrasound attenuation, NSR relaxation rate, and electromagnetic absorption.
Superconductivity from a pseudogapped normal state: a mode coupling approach to precursor superconductivity
Jiri Maly,Boldizsar Janko,K. Levin
Physics , 1997,
Abstract: We derive a phase diagram for the pseudogap onset temperature $T^*$ (associated with the breakdown of the Fermi liquid state, due to strong pairing correlations) and the superconducting instability, $T_c$, as a function of variable pairing strength. Our diagrammatic approach to the BCS - Bose-Einstein cross-over problem self consistently treats the coupling between the single particle and pair propagators, and leads to a continuous evolution of these propagators into the standard $TT_c$.
Pseudogap effects induced by resonant pair scattering
Boldizsar Janko,Jiri Maly,K. Levin
Physics , 1997, DOI: 10.1103/PhysRevB.56.R11407
Abstract: We demonstrate how resonant pair scattering of correlated electrons above T_c can give rise to pseudogap behavior. This resonance in the scattering T-matrix appears for superconducting interactions of intermediate strength, within the framework of a simple fermionic model. It is associated with a splitting of the single peak in the spectral function into a pair of peaks separated by an energy gap. Our physical picture is contrasted with that derived from other T-matrix schemes, with superconducting fluctuation effects, and with preformed pair (boson-fermion) models. Implications for photoemission and tunneling experiments in the cuprates are discussed.
Scaling theory of magneto-resistance in disordered local moment ferromagnets
Gergely Zarand,Catalin Pascu Moca,Boldizsar Janko
Physics , 2004, DOI: 10.1103/PhysRevLett.94.247202
Abstract: We present a scaling theory of magneto-transport in Anderson-localized disordered ferromagnets. Within our framework a pronounced magnetic-field-sensitive resistance peak emerges naturally for temperatures near the magnetic phase transition. We find that the resistance anomaly is a direct consequence of the change in localization length caused by the magnetic transition. For increasing values of the external magnetic field, the resistance peak is gradually depleted and pushed towards higher temperatures. Our results are in good agreement with magneto-resistance measurements on a variety of disordered magnets.
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