Biomedical subjects

Gergely Zaránd

Publications and source records attributed to Gergely Zaránd.

8 recordsLinked to original sources

PubMed2006

Quantum criticality in a double-quantum-dot system.

We discuss the realization of the quantum-critical non-Fermi-liquid state, originally discovered within the two-impurity Kondo model, in double-quantum-dot systems. Contrary to common belief, the corresponding fixed point is robust against particle-hole and various other asymmetries and is unstable only to charge transfer between the two dots. We propose an experimental setup where such charge transfer processes are suppressed, allowing a controlled approach to the quantum-critical state. We also discuss transport and scaling properties in the vicinity of the critical point.

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PubMed2006

Spectroscopy of the Kondo problem in a box.

Motivated by experiments on double quantum dots, we study the problem of a single magnetic impurity confined in a finite metallic host. We prove an exact theorem for the ground state spin, and use analytic and numerical arguments to map out the spin structure of the excitation spectrum of the many-body Kondo-correlated state, throughout the weak to strong coupling crossover. These excitations can be probed in a simple tunneling-spectroscopy transport experiment; for that situation we solve rate equations for the conductance.

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PubMed2004

Theory of inelastic scattering from magnetic impurities.

We use the numerical renormalization group method to calculate the single-particle matrix elements T of the many-body T matrix of the conduction electrons scattered by a magnetic impurity at T=0 temperature. Since T determines both the total and the elastic, spin-diagonal scattering cross sections, we are able to compute the full energy, spin, and magnetic field dependence of the inelastic scattering cross section sigma(inel)(omega). We find an almost linear frequency dependence of sigma(inel)(omega) below the Kondo temperature T(K), which crosses over to a omega(2) behavior only at extremely low energies. Our method can be generalized to other quantum impurity models.

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PubMed2003

Magnetoelectronic spin echo.

We predict a spin echo in electron transport through layered ferromagnetic-normal-ferromagnetic metal structures: whereas a spin current polarized perpendicular to the magnetization direction decays when traversing a single homogeneous ferromagnet on the scale of the ferromagnetic spin-coherence length, it partially reappears by adding a second identical but antiparallel ferromagnet. This reentrant transverse spin current resembles the spin-echo effect in the magnetization of nuclei under pulsed excitations. We propose an experimental setup to measure the spin echo.

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PubMed2003

Effective Hamiltonian for Ga1-x MnxAs in the dilute limit.

We derive an effective Hamiltonian for Ga(1-x)Mn(x)As in the dilute limit, where Ga(1-x)Mn(x)As can be described in terms of spin F=3/2 polarons hopping between the Mn sites and coupled to the local Mn spins. We determine the parameters of our model from microscopic calculations. Our approach treats the large Coulomb interaction in a nonperturbative way, captures the effects of spin-orbit coupling and disorder, and is appropriate for other p-doped magnetic semiconductors.

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PubMed2003

Quantum magnetic impurities in magnetically ordered systems.

We discuss the problem of a spin 1/2 impurity immersed in a spin S magnetically ordered background. We show that the problem maps onto a generalization of the dissipative two level system with two independent heat baths, associated with the Goldstone modes of the magnet, that couple to different components of the impurity spin operator. Using analytical perturbative renormalization group methods and accurate numerical renormalization group we show that contrary to other dissipative models there is quantum frustration of decoherence and quasiscaling even in the strong coupling regime. We make predictions for the behavior of the impurity magnetic susceptibility. Our results may also have relevance to quantum computation.

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PubMed2003

SU(4) Fermi liquid state and spin filtering in a double quantum dot system.

We study a symmetrical double quantum dot (DD) system with strong capacitive interdot coupling using renormalization group methods. The dots are attached to separate leads, and there can be a weak tunneling between them. In the regime where there is a single electron on the DD the low-energy behavior is characterized by an SU(4)-symmetric Fermi liquid theory with entangled spin and charge Kondo correlations and a phase shift pi/4. Application of an external magnetic field gives rise to a large magnetoconductance and a crossover to a purely charge Kondo state in the charge sector with SU(2) symmetry. In a four-lead setup we find perfectly spin-polarized transmission.

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PubMed2002

Ga(1-x)Mn(x)As: a frustrated ferromagnet.

Starting from a microscopic description of the exchange interaction in Ga(1-x)Mn(x)As, we derive an effective Mn-Mn interaction. Because of the strong spin-orbit coupling in the valence band, this effective interaction is highly anisotropic and has a spatial structure somewhat similar to dipolar interactions. The corresponding ground state has a finite magnetization but is intrinsically spin disordered even at zero temperature, explaining many features of the experimental data. Our results are relevant for many magnetic semiconductors.

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