arXiv subjects

Rafael Sánchez

Publications and source records attributed to Rafael Sánchez.

63 records · Page 4Linked to original sources

arXiv2011

Optimal energy quanta to current conversion

We present a microscopic discussion of a nano-sized structure which uses the quantization of energy levels and the physics of single charge Coulomb interaction to achieve an optimal conversion of heat flow to directed current. In our structure the quantization of energy levels and the Coulomb blockade lead to the transfer of quantized packets of energy from a hot source into an electric conductor to which it is capacitively coupled. The fluctuation generated transfer of a single energy quantum translates into the directed motion of a single electron. Thus in our structure the ratio of the charge current to the heat current is determined by the ratio of the charge quantum to the energy quantum. An important novel aspect of our approach is that the direction of energy flow and the direction of electron motion are decoupled.

cond-mat.mes-hall↗

arXiv2010

Control of Spin Blockade by AC Magnetic Fields in Triple Quantum Dots

We analyze coherent spin phenomena in triple quantum dots in triangular configuration under crossed DC and AC magnetic fields. In particular, we discuss the interplay between Aharonov-Bohm current oscillations, coherent electron trapping and spin blockade under electron spin resonance conditions. We demonstrate that, for certain field frequencies, AC magnetic fields induce an antiresonant behavior in the current, allowing for both removal and restoration of entangled spin blockaded states by tuning the AC field frequency. Our theoretical predictions indicate how to manipulate spin qubits in a triangular quantum dot array.

cond-mat.mes-hall↗

arXiv2010

Mesoscopic Coulomb drag, broken detailed balance and fluctuation relations

When a biased conductor is put in proximity with an unbiased conductor a drag current can be induced in the absence of detailed balance. This is known as the Coulomb drag effect. However, even in this situation far away from equilibrium where detailed balance is explicitly broken, theory predicts that fluctuation relations are satisfied. This surprising effect has, to date, not been confirmed experimentally. Here we propose a system consisting of a capacitively coupled double quantum dot where the nonlinear fluctuation relations are verified in the absence of detailed balance.

cond-mat.mes-hall↗

arXiv2008

Spin correlations in spin blockade

We investigate spin currents and spin-current correlations for double quantum dots in the spin blockade regime. By analysing the time evolution of the density matrix, we obtain the spin resolved currents and derive from a generating function an expression for the fluctuations and correlations. Both the charge current and the spin current turn out to be generally super-Poissonian. Moreover, we study the influence of ac fields acting upon the transported electrons. In particular, we focus on fields that cause spin rotation or photon-assisted tunnelling.

cond-mat.mes-hall↗

arXiv2008

Resonance fluorescence in driven quantum dots: electron and photon correlations

We study the counting statistics for electrons and photons being emitted from a driven two level quantum dot. Our technique allows us to calculate their mutual correlations as well. We study different transport configurations by tuning the chemical potential of one of the leads to find that the electronic and photonic fluctuations can be externally manipulated by tuning the AC and transport parameters. We also propose special configurations where electron-photon correlation is maximal meaning that spontaneous emission of photons with a well defined energy is regulated by single electron tunneling. Interesting features are also obtained for energy dependent tunneling.

cond-mat.mes-hall↗

arXiv2008

Coherent spin rotations in open driven double quantum dots

We analyze the charge and spin dynamics in a DC biased double quantum dot driven by crossed DC and AC magnetic fields. In this configuration, spatial delocalization due to inter-dot tunnel competes with intra-dot spin rotations induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunnelling current. When the Zeeman splitting has the same value in both dots and spin flip is negligible, the electrons remain in the triplet subspace (dark subspace) performing coherent spin rotations and the current does not flow. This electronic trapping is removed either by finite spin relaxation or when the Zeeman splitting is different in each quantum dot. In the first case, our results show that measuring the current will allow to get information on the spin relaxation time. In the last case, we will show that applying a resonant bichromatic magnetic field, the electrons become trapped in a coherent superposition of states and electronic transport is blocked. Then, manipulating AC magnetic fields, electrons are driven to perform coherent spin rotations which can be unambiguously detected by direct measurement of the tunneling current.

cond-mat.mes-hall↗

arXiv2008

Electron Bunching in Stacks of Coupled Quantum Dots

We study the transport properties of two double quantum dots in a parallel arrangement at temperatures of a few Kelvin. Thereby, we show that decoherence entailed by the substrate phonons affects the shot noise. For asymmetric coupling between the dots and the respective lead, the current noise is sub-Poissonian for resonant tunneling, but super-Poissonian in the vicinity of the resonances. Our results indicate that phonon emission and absorption is responsible for the shot noise enhancement. The observed asymmetry of the peaks at low temperatures stems from spontaneous emission.

cond-mat.mes-hall↗

arXiv2007

Resonance Fluorescence in Transport through Quantum Dots: Noise Properties

We study a two-level quantum dot embedded in a phonon bath and irradiated by a time-dependent ac field and develope a method that allows us to extract simultaneously the full counting statistics of the electronic tunneling and relaxation (by phononic emission) events as well as their correlation. We find that the quantum noise of both the transmitted electrons and the emitted phonons can be controlled by the manipulation of external parameters such as the driving field intensity or the bias voltage.

cond-mat.mes-hall↗

arXiv2006

Spin filtering through excited states in double quantum dot pumps

Recently it has been shown that ac-driven double quantum dots can act as spin pumps and spin filters. By calculating the current through the system for each spin polarization, by means of the time evolution of the reduced density matrix in the sequential tunneling regime (Born-Markov approximation), we demonstrate that the spin polarization of the current can be controlled by tuning the parameters (amplitude and frequency) of the ac field. Importantly, the pumped current as a function of the applied frequency presents a series of peaks which are uniquely associated with a definite spin polarization. We discuss how excited states participating in the current allow the system to behave as a bipolar spin filter by tuning the ac frequency and intensity. We also discuss spin relaxation and decoherence effects in the pumped current and show that measuring the width of the current vs frequency peaks allows to determine the spin decoherence time $T_{2}$.

cond-mat.mes-hall↗