arXiv subjects

Anton Ramsak

Publications and source records attributed to Anton Ramsak.

14 recordsLinked to original sources

arXiv2020

Thermal effects on a nonadiabatic spin-flip protocol of spin-orbit qubits

We study the influence of a thermal environment on a non-adiabatic spin-flip driving protocol of spin-orbit qubits. The driving protocol operates by moving the qubit, trapped in a harmonic potential, along a nanowire in the presence of a time-dependent spin-orbit interaction. We consider the harmonic degrees of freedom to be weakly coupled to a thermal bath. We find an analytical expression for the Floquet states and derive the Lindblad equation for a strongly non-adiabatically driven qubit. The Lindblad equation corrects the dynamics of an isolated qubit with Lamb shift terms and a dissipative behaviour. Using the Lindblad equation, the influence of a thermal environment on the spin-flip protocol is analysed.

cond-mat.mes-hall↗

arXiv2020

Rashba controlled two-electron spin-charge qubits as building blocks of a quantum computer

The spin-sensitive charge oscillation, controlled by an external magnetic field, was recently proposed as a mechanism of transformations of qubits, defined as two-electron spin-charge Wannier molecules in a square quantum dot. The paper expands this idea by including the effects of Rashba type spin-orbit coupling. The problem is studied theoretically by mapping the system to an analytic effective Hamiltonian for 8 low energy states, comprising singlet and triplet on each dot diagonal. The validity of the mapping is confirmed by comparing the energy and spin of full and mapped system, and also by the reproduction of charge-oscillation dynamics in the presence of magnetic flux. The newly introduced Rashba coupling significantly enriches the system dynamics, affecting the magnitude of charge oscillations and allowing the controlled transitions between singlet and triplet states due to the spin rotations, induced by spin-orbit coupling. The results indicate the possibility for use of the studied system for quantum information processing, while possible extensions of the system to serve as a qubit in a universal quantum computer, fulfilling all five DiVincenzo criteria, are also discussed.

cond-mat.mes-hall↗

arXiv2019

Effects of noise on fidelity in spin-orbit qubit transformations

We analyse non-adiabatic non-Abelian holonomic transformations of spin-qubits confined to a linear time dependent harmonic trap with time dependent Rashba interaction. For this system exact results can be derived for spin rotation angle which also enables exact treatment of white gate-noise effects. We concentrate in particular on the reliability of cyclic transformations quantified by fidelity defined by the probability that the qubit after one full cycle remains in the ground-state energy manifold. The formalism allows exact analysis of spin transformations that optimise final fidelity. Various examples of time dependent fidelity probability distributions are presented and discussed.

cond-mat.mes-hall↗

arXiv2013

Spin thermopower in the overscreened Kondo model

We study the spin thermopower in the two-channel spin-1/2 Kondo model which exhibits the phenomenon of impurity spin overscreening and non-Fermi-liquid properties. While magnetic field lower than the Kondo temperature does not strongly polarize the impurity spin, we show that it nevertheless strongly affects the low-energy part of the spectral function. In turn, this leads to characteristic saturation of the spin Seebeck coefficient at the value of 0.388kB/|e| at T ~ T*, where T* ~ B^2/TK is the scale of the crossover between the intermediate-temperature non-Fermi-liquid regime and the low-temperature Fermi-liquid regime. We show that measuring the spin thermopower at low magnetic fields would provide a sensitive test for distinguishing regular Fermi liquid, singular Fermi liguid, and non-Fermi liquid behaviour in nanodevices.

cond-mat.str-el↗

arXiv2012

Spin-thermopower in interacting quantum dots

Using analytical arguments and the numerical renormalization group method we investigate the spin-thermopower of a quantum dot in a magnetic field. In the particle-hole symmetric situation the temperature difference applied across the dot drives a pure spin current without accompanying charge current. For temperatures and fields at or above the Kondo temperature, but of the same order of magnitude, the spin-Seebeck coefficient is large, of the order of k_B/e. Via a mapping, we relate the spin-Seebeck coefficient to the charge-Seebeck coefficient of a negative-U quantum dot where the corresponding result was recently reported by Andergassen et al. in Phys. Rev. B 84, 241107 (2011). For several regimes we provide simplified analytical expressions. In the Kondo regime, the dependence of the spin-Seebeck coefficient on the temperature and the magnetic field is explained in terms of the shift of the Kondo resonance due to the field and its broadening with the temperature and the field. We also consider the influence of breaking the particle-hole symmetry and show that a pure spin current can still be realized provided a suitable electric voltage is applied across the dot. Then, except for large asymmetries, the behavior of the spin-Seebeck coefficient remains similar to that found in the particle-hole symmetric point.

cond-mat.mes-hall↗

arXiv2009

Kondo effect in oscillating molecules

We consider electronic transport through break-junctions bridged by a single molecule in the Kondo regime. We describe the system by a two-channel Anderson model. We take the tunneling matrix elements to depend on the position of the molecule. It is shown, that if the modulation of the tunneling by displacement is large, the potential confining the molecule to the central position between the leads is softened and the position of the molecule is increasingly susceptible to external perturbations that break the inversion symmetry. In this regime, the molecule is attracted to one of the leads and as a consequence the conductance is small. We argue on semi-classical grounds why the softening occurs and corroborate our findings by numerical examples obtained by Wilson's numerical renormalization group and Schoenhammer-Gunnarsson's variational method.

cond-mat.mes-hall↗

arXiv2007

Vibrational effects on low-temperature properties of molecular conductors

We calculate characteristic correlation functions for the Anderson model with additional phonon-assisted coupling to the odd conduction channel. This model describes, for example, the behavior of a molecule embedded between two electrodes in linear transport experiments where the position of the molecule with respect to the leads affects the tunneling amplitudes. We use variational projection-operator method and numerical renormalization group (NRG) method. The spin is Kondo screened either by even or odd conduction channel depending on the gate voltage and electron-phonon coupling. However, in all regimes the gate-voltage dependence of the zero temperature conductance is found to be qualitatively the same as in the model with no coupling to the vibrational mode.

cond-mat.mes-hall↗

arXiv2007

SU(2) and SU(4) Kondo effect in double quantum dots

We investigate serial double quantum dot systems with on-site and inter-site interaction by means of Schönhammer-Gunnarsson projection-operator method. The ground state is established by the competition between extended Kondo phases and localized singlet phases in spi$ degrees of freedom. We present and discuss different phases, as discerned by characteristic correlation functions. We discuss also how different phases would be seen in linear transport measurements.

cond-mat.mes-hall↗

arXiv2006

Kondo effect in double quantum dots with inter-dot repulsion

We investigate a symmetrical double quantum dot system serially attached to the leads. The emphasis is on the numerical analysis of finite inter-dot tunneling in the presence of inter-dot repulsive capacitive coupling. The results reveal the competition between extended Kondo phases and local singlet phases in spin and charge degrees of freedom. The corresponding phase diagram is determined quantitatively.

cond-mat.mes-hall↗

arXiv2003

Conductance of nanosystems with interaction

The zero-temperature linear response conductance through an interacting mesoscopic region attached to noninteracting leads is investigated. We present a set of formulas expressing the conductance in terms of the ground-state energy of an auxiliary system, namely a ring threaded by a magnetic flux and containing the correlated electron region. We prove that the formalism is exact if the ground state of the system is a Fermi liquid. We show that in such systems the ground-state energy is a universal function of the magnetic flux, where the conductance is the relevant parameter. The method is illustrated with results for the transport through an interacting quantum dot and a simple Aharonov-Bohm ring with Kondo-Fano resonance physics.

cond-mat.mes-hall↗

arXiv2003

Formulae for zero-temperature conductance through a region with interaction

The zero-temperature linear response conductance through an interacting mesoscopic region attached to noninteracting leads is investigated. We present a set of formulae expressing the conductance in terms of the ground-state energy or persistent currents in an auxiliary system, namely a ring threaded by a magnetic flux and containing the correlated electron region. We first derive the conductance formulae for the noninteracting case and then give arguments why the formalism is also correct in the interacting case if the ground state of a system exhibits Fermi liquid properties. We prove that in such systems, the ground-state energy is a universal function of the magnetic flux, where the conductance is the only parameter. The method is tested by comparing its predictions with exact results and results of other methods for problems such as the transport through single and double quantum dots containing interacting electrons. The comparisons show an excellent quantitative agreement.

cond-mat.mes-hall↗

arXiv2003

Conductance of interacting Aharonov-Bohm systems

A simple formula for the zero-temperature linear response conductance of an interacting mesoscopic region, threaded by magnetic flux, and attached to noninteracting single-channel leads is presented. The formula is valid for a general interacting system exhibiting Fermi liquid properties. As an example of the efficiency of the formula the results for the conductance of a simple Aharonov-Bohm ring with Kondo-Fano resonance physics are presented and compared with numerical renormalization group results.

cond-mat.mes-hall↗

arXiv2000

Electron momentum distribution of a single mobile hole in the t-J model

We investigate the electron momentum distribution function (EMDF) for the two-dimensional t-J model. The results are based on the self-consistent Born approximation (SCBA) for the self-energy and the wave function. In the Ising limit of the model we give the results in a closed form, in the Heisenberg limit the results are obtained numerically. An anomalous momentum dependence of EMDF is found and the anomaly is in the lowest order in number of magnons expressed analitycally. We interpret the anomaly as a fingerprint of an emerging large Fermi surface coexisting with hole pockets.

cond-mat.str-el↗

arXiv2000

Pseudo gap in the density of states in cuprates

In the framework of the t-J model for cuprates we analyze the development of a pseudo gap in the density of states (DOS), which at low doping starts to emerge for temperatures T<J and persists up to the optimum doping. The analysis is based on numerical results for spectral functions obtained with the finite-temperature Lanczos method for finite two-dimensional clusters. We find that the pseudo gap scales with J and is robust also in the presence of nearest neighbor repulsive interaction. Numerical results are additionally compared with the self consistent Born approximation (SCBA) results for hole-like (photoemission) and electron-like (inverse photoemission) spectra at T=0. The analysis is suggesting that the origin of the pseudo gap is in short-range antiferromagnetic (AFM) spin correlations and strong asymmetry between the hole and electron spectra in the underdoped regime.

cond-mat.str-el↗