Real-time dynamics in strongly correlated mesoscopic systems – DYMESYS

We have extended the functional renormalization group that we developed in our previous work to study the effect of a magnetic field on non-equilibrium transport through a quantum dot. . We have shown that the equations of renormalization constant exchange became highly anisotropic . This leads to singularities in the finite frequency noise when the frequency takes the following four values ??| eV + B | ,
| eV -B | - | eV + B | - | B - eV | where V is voltage and B the magnetic field .
In addition, we have shown that the presence of the field leads to an anisotropy in the spin decoherence of the transverse and longitudinal directions and a strong source of additional phase shift for the spin of the quantum dot . One of the measurable consequences is a sharp decrease in narrow resonances predicted for the ac conductance off-equilibrium
As part of the experimental thesis of Julien Basset we made ??the first measurement of fluctuations of high frequency current in the Kondo regime carried out with a carbon nanotube quantum dot. This was achieved by coupling the quantum noise detector ( a superconductor / superconductor junction insulation / ( SIS ) ) via a resonant superconducting circuit. A singularity in the noise coupled to the resonance Kondo had been highlighted in the voltage V = hf / e when the measurement frequency f is of the order of kBTK / h . This singularity is greatly reduced when the frequency is of the order of 3kBTK / h . This behavior is in good agreement with the theoretical predictions considering a higher rate of spin dephasing induced by the applied voltage.

We showed singularities in the finite frequency noise when the frequency takes the following four values ??| eV + B | , | eV -B |,
- | eV + B |, - | B - eV | where V denotes the voltage and the magnetic field B .
In addition, we have shown that the presence of the field leads to an anisotropy in the spin decoherence of the transverse and longitudinal directions and a strong source of additional phase shift for the spin of the quantum dot . One of the measurable consequences experimentally a sharp decrease in narrow resonances predicted for the conductance ac off- balance.
Experience: As part of the thesis of Julien Basset we made ??the first measurement of fluctuations of high frequency current in the Kondo regime carried out with a carbon nanotube quantum dot. This was achieved by coupling the quantum noise detector ( a superconductor / superconductor junction insulation / ( SIS ) ) via a resonant superconducting circuit. A singularity in the noise coupled to the resonance Kondo had been highlighted in the voltage V = hf / e when the measurement frequency f is of the order of kBTK / h . This singularity is greatly reduced when the frequency is of the order of 3kBTK / h . This behavior is in good agreement with the theoretical predictions considering a higher rate of spin dephasing induced by the applied voltage.

We started with the Romanian partner extending the functional renormalization group to a Kondo quantum dot coupled to a plurality of terminals conduction.
Experimentally , these measures were first carried out in this plan and need to be refined . In particular maximizing the detected signal could provide access to the finer details of this singularity and test more precisely the agreement with theoretical predictions. The last place particular emphasis on the interests of the behavior of sound in the presence of magnetic field. To improve the sensitivity of the detection system we seek to change the coupling circuit to obtain a spectral finesse larger detection while maintaining or increasing the strength of coupling between the noise source and the detector. Different types of circuits are being studied and are currently being tested at low temperatures.

[1] J. Basset, A. Kasumov, C. P. Moca, G. Zarand, P. Simon, H. Bouchiat, R. Deblock, Phys. Rev. Lett. 108, 046802 (2012).
[2] D.-J. Choi, M. V. Rastei, P. Simon, L. Limot, Phys. Rev. Lett. 108, 266803 (2012).
[3] J.-S. Lim, R. Lopez, L. Limot, P. Simon, L. Limot, Phys. Rev. B 88, 165403 (2013)
[4] Julien Basset, Hélène Bouchiat, Richard Deblock, Phys.Rev.B 85, 085435 (2012).
[5] C. P. Moca, P. Simon, C.-H. Chung, G. Zarand, arXiv:1312.4686

The recent development in fabrication, and operating in a controlled manner of nanoelectronic devices with a few hundred angstroms scale or below, are likely to provide our future technology and serve as basic tools for storing information, quantum computation or spin manipulation. Understanding how these nanometric devices work represents a major challenge for today’s theoretical and experimental physics. To reach this goal we need first to figure out the fundamental issues that govern their behavior and therefore to provide a detailed theory of correlations and transport in atomic scale and mesoscopic structures. More specifically, in order to find efficient ways of manipulating and controlling the spin currents, we must understand the microscopic processes that lead to spin relaxation and dephasing, and eventually their interplay with interactions.
In the present project, our main purpose is to understand non-equilibrium transport through strongly correlated mesoscopic systems. To achieve this goal we shall exploit the expertise of two research partners. The first one, «Laboratoire de Physique des Solides» in Orsay, involves two complementary teams, a theoretical one (led by Pascal Simon) and an experimental one (led by Richard Deblock). The second partner, «University of Oradea» in Romania involves a theoretical team (led by Catalin Pascu Moca). We plan to study non-equilibrium transport in mesoscopic systems subject to strong interactions, by combining the power of quantum field theoretical methods and numerical analysis with the experimental probation of the theoretical findings. Our research groups will develop new theoretical methods that will enable us to study and understand out of equilibrium, charge and spin transport through these nanometer scale devices. We plan to construct a real time renormalization group scheme, and then use it to investigate frequency dependent quantities that characterize transport under non-equilibrium conditions. Here we have in mind a thorough analysis of charge/spin ac-conductance and noise, scattering rates, relaxation effects and many other transport properties under non-equilibrium conditions. We shall also address the «quantum quench» problem in Kondo correlated systems, i.e. the implications on transport and the modifications in the response functions of a sudden change in the system Hamiltonian. To fulfill this goal, we first plan to carry an ambitious project by developing and extending the currently available numerical renormalization group code, the «Flexible-DMNRG», by incorporating time dependent processes.
On the experimental side we are planning to engineer high frequency quantum detectors for noise measurements in quantum dots in the Kondo regime made of carbon nanotubes. Quantum quench problem shall be investigated experimentally also, by using pulsed voltage probes. It will allow us to monitor different quantities, such as the conductance or noise subject to a quench in one of the system parameters.
We strongly believe that our approach is an optimal one for attacking real-time dynamics problems and that the interplay between the experiment and theory is the strength of the present proposal. More than that, our experimental findings will motivate and guide our theoretical developments along the way.