arXiv subjects

Andrew G. Semenov

Publications and source records attributed to Andrew G. Semenov.

30 records · Page 2Linked to original sources

arXiv2014

Decoherence of Cooper pairs and subgap magnetoconductance of superconducting hybrids

We demonstrate that electron-electron interactions fundamentally restrict the penetration length of superconducting correlations into a diffusive normal metal (N) attached to a superconductor (S). We evaluate the subgap magnetoconductance $G$ of SN hybrids in the presence of electron-electron interactions and demonstrate that the effect of the magnetic field on $G$ is twofold: It includes ($i$) additional temperature independent dephasing of Cooper pairs and ($ii$) Zeeman splitting between the states with opposite spins. The dephasing length of Cooper pairs can be directly extracted from measurements of the subgap magnetoconductance in SN systems at low temperatures.

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arXiv2013

Persistent currents in quantum phase slip rings

We investigate the effect of interacting quantum phase slips on persistent current and its fluctuations in ultrathin superconducting nanowires and nanorings pierced by the external magnetic flux. We derive the effective action for these systems and map the original problem onto an effective sine-Gordon theory on torus. We evaluate both the flux dependent persistent current and the critical radius of the ring beyond which this current gets exponentially suppressed by quantum fluctuations. We also analyze fluctuations of persistent current caused by quantum phase slips. At low temperatures the supercurrent noise spectrum has the form of coherent peaks which can be tuned by the magnetic flux. Experimental observation of these peaks can directly demonstrate the existence of plasma modes in superconducting nanorings.

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arXiv2013

On the macroscopic quantization in mesoscopic rings and single-electron devices

In this letter the phenomenon of macroscopic quantization is investigated using the particle on the ring interacting with the dissipative environment as an example. It is shown that the phenomenon of macroscopic quantization has the clear physical origin in that case. It follows from the angular momentum conservation combined with momentum quantization for bare particle on the ring . The existence an observable which can take only integer values in the zero temperature limit is rigorously proved. With the aid of the mapping between particle on the ring and Ambegaokar-Eckern-Schon model, which can be used to describe single-electron devices, it is demonstrated that this observable is analogous to the "effective charge" introduced by Burmistrov and Pruisken for the single-electron box problem. Different consequences of the revealed physics are discussed, as well as a generalization of the obtained results to the case of more complicated systems.

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arXiv2012

Persistent current noise in normal and superconducting nanorings

We investigate fluctuations of persistent current (PC) in nanorings both with and without dissipation and decoherence. We demonstrate that such PC fluctuations may persist down to zero temperature provided there exists either interaction with an external environment or an external (periodic) potential produced, e.g., by quantum phase slips in superconducting nanorings. Provided quantum coherence is maintained in the system PC noise remains coherent and can be tuned by an external magnetic flux $Φ_x$ piercing the ring. If quantum coherence gets suppressed by interactions with a dissipative bath PC noise becomes incoherent and $Φ_x$-independent.

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arXiv2012

Dephasing of Cooper pairs and subgap electron transport in superconducting hybrids

We argue that electron-electron interactions fundamentally restrict the penetration length of Cooper pairs into a diffusive normal metal (N) from a superconductor (S). At low temperatures this Cooper pair dephasing length $L_φ$ remains finite and does not diverge at $T \to 0$. We evaluate the subgap conductance of NS hybrids in the presence of electron-electron interactions and demonstrate that this new length $L_φ$ can be directly extracted from conductance measurements in such structures.

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arXiv2011

Persistent current noise and electron-electron interactions

We analyze fluctuations of persistent current (PC) produced by a charged quantum particle moving in a ring and interacting with a dissipative environment formed by diffusive electron gas. We demonstrate that in the presence of interactions such PC fluctuations persist down to zero temperature. In the case of weak interactions and/or sufficiently small values of the ring radius $R$ PC noise remains coherent and can be tuned by external magnetic flux $Φ_x$ piercing the ring. In the opposite limit of strong interactions and/or large values of $R$ fluctuations in the electronic bath strongly suppress quantum coherence of the particle down to $T=0$ and induce incoherent $Φ_x$-independent current noise in the ring which persists even at $Φ_x=0$ when the average PC is absent.

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arXiv2010

Persistent current noise

We demonstrate that persistent current in meso- and nanorings may fluctuate down to zero temperature provided the current operator does not commute with the total Hamiltonian of the system. For a model of a quantum particle on a ring we explicitly evaluate PC noise power which has a form of sharp peaks which become broadened for multi-channel rings or in the presence of dissipation. PC noise can be tuned by an external magnetic flux which is a fundamental manifestation of quantum coherence in the system.

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arXiv2010

Weak localization, Aharonov-Bohm oscillations and decoherence in arrays of quantum dots

Combining scattering matrix theory with non-linear $σ$-model and Keldysh technique we develop a unified theoretical approach enabling one to non-perturbatively study the effect of electron-electron interactions on weak localization and Aharonov-Bohm oscillations in arbitrary arrays of quantum dots. Our model embraces (i) weakly disordered conductors (ii) strongly disordered conductors and (iii) metallic quantum dots. In all these cases at $T \to 0$ the electron decoherence time is found to saturate to a finite value determined by the universal formula which agrees quantitatively with numerous experimental results. Our analysis provides overwhelming evidence in favor of electron-electron interactions as a universal mechanism for zero temperature electron decoherence in disordered conductors.

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arXiv2010

Fluctuations of persistent current

We theoretically analyze equilibrium fluctuations of persistent current (PC) in nanorings. We demonstrate that these fluctuations persist down to zero temperature provided the current operator does not commute with the total Hamiltonian of the system. For a model of a quantum particle on a ring we explicitly evaluate PC noise power which has the form of sharp peaks at frequencies set by the corresponding interlevel distances. In rings with many conducting channels a much smoother and broader PC noise spectrum is expected. A specific feature of PC noise is that its spectrum can be tuned by an external magnetic flux indicating the presence of quantum coherence in the system.

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arXiv2009

Persistent currents in nanorings and quantum decoherence by Coulomb interaction

Employing instanton technique we evaluate equilibrium persistent current (PC) produced by a quantum particle moving in a periodic potential on a ring and interacting with a dissipative environment formed by diffusive electron gas. The model allows for detailed non-perturbative analysis of interaction effects and -- depending on the system parameters -- yields a rich structure of different regimes. We demonstrate that at low temperatures PC is exponentially suppressed at sufficiently large ring perimeters $2πR> L_φ$ where the dephasing length $L_φ$ is set by interactions and does not depend on temperature. This behavior represents a clear example of quantum decoherence by electron-electron interactions at $T\to 0$.

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arXiv2009

Aharonov-Bohm oscillations in disordered nanorings with quantum dots: Effect of electron-electron interactions

We investigate the effect of electron-electron interactions on Aharonov-Bohm (AB) current oscillations in nanorings formed by a chain of metallic quantum dots. We demonstrate that electron-electron interactions cause electron dephasing thereby suppressing the amplitude of AB oscillations at all temperatures down to T=0. The crossover between thermal and quantum dephasing is found to be controlled by the ring perimeter. Our predictions can be directly tested in future experiments.

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arXiv2009

Aharonov-Bohm oscillations in coupled quantum dots: Effect of electron-electron interactions

We theoretically analyze the effect of electron-electron interactions on Aharonov-Bohm (AB) current oscillations in ring-shaped systems with metallic quantum dots pierced by external magnetic field. We demonstrate that electron-electron interactions suppress the amplitude of AB oscillations $I_{AB}$ at all temperatures down to T=0 and formulate quantitative predictions which can be verified in future experiments. We argue that the main physical reason for such interaction-induced suppression of $I_{AB}$ is electron dephasing while Coulomb blockade effects remain insignificant in the case of metallic quantum dots considered here. We also emphasize a direct relation between our results and the so-called $P(E)$-theory describing tunneling of interacting electrons.

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