Robert S. Whitney

Robert S. Whitney
French National Centre for Scientific Research | CNRS · Laboratoire de Physique et Modélisation des Milieux Condensés

PhD

About

68
Publications
5,160
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2,232
Citations
Introduction
I am a theoretical physicist studying quantum mechanics. I apply this theory to the motion of electrons in nanometre-sized circuits. KEYWORDS : + thermoelectricity in nanostructures + irreversibility in open quantum systems + quantum thermodynamics + quantum chaos
Additional affiliations
February 2011 - present
French National Centre for Scientific Research
Position
  • Researcher
February 2006 - January 2011
Institut Laue-Langevin
Position
  • PostDoc Position
December 2003 - January 2006
University of Geneva
Position
  • PostDoc Position

Publications

Publications (68)
Article
Full-text available
We investigate a multiterminal mesoscopic conductor in the quantum Hall regime, subject to temperature and voltage biases. The device can be considered as a nonequilibrium resource acting on a working substance. We previously showed that cooling and power production can occur in the absence of energy and particle currents from a nonequilibrium reso...
Preprint
We investigate a multi-terminal mesoscopic conductor in the quantum Hall regime, subject to temperature and voltage biases. The device can be considered as a nonequilibrium resource acting on a working substance. We previously showed that cooling and power production can occur in the absence of energy and particle currents from a nonequilibrium res...
Article
We study transport through a single-level system placed between two reservoirs with band-structure, taking strong level-reservoir coupling of the order of the energy-scales of these band-structures. An exact solution in the absence of interactions gives the nonlinear Lamb shift. As expected, this moves the perfectly-transmitting state (the reservoi...
Preprint
We study transport through a single-level system placed between two reservoirs with band-structure, taking strong level-reservoir coupling of the order of the energy-scales of these band-structures. An exact solution in the absence of interactions gives the nonlinear Lamb shift. As expected, this moves the perfectly-transmitting state (the reservoi...
Article
Maxwell demons are creatures that are imagined to be able to reduce the entropy of a system without performing any work on it. Conventionally, such a Maxwell demon’s intricate action consists of measuring individual particles and subsequently performing feedback. We show that much simpler setups can still act as demons: we demonstrate that it is su...
Article
We use exact techniques to demonstrate theoretically the pumping of fractional charges in a single-level noninteracting quantum dot, when the dot-reservoir coupling is adiabatically driven from weak to strong coupling. The pumped charge averaged over many cycles is quantized at a fraction of an electron per cycle, determined by the ratio of Lamb sh...
Article
We study a quantum dot coupled to two semiconducting reservoirs, when the dot level and the electrochemical potential are both close to a band edge in the reservoirs. This is modeled with an exactly solvable Hamiltonian without interactions (the Fano-Anderson model). The model is known to show an abrupt transition as the dot-reservoir coupling is i...
Preprint
Topological fractional charge pumping is seen in models of strongly-correlated or topological systems. We show that similar pumping occurs in a much less exotic system; a single-level non-interacting quantum dot, when driving the dot-reservoir coupling from weak to strong coupling. The pumped charge averaged over many cycles is quantized at a fract...
Article
Full-text available
We consider the thermoelectric response of chaotic or disordered quantum dots in the limit of phase-coherent transport, statistically described by random matrix theory. We calculate the full distribution of the thermoelectric coefficients (Seebeck S and Peltier \PiΠ ), and the thermoelectric figure of merit ZT, for large open dots at arbitrary temp...
Preprint
Full-text available
We study a quantum dot coupled to two semiconducting reservoirs, when the dot level and the electrochemical potential are both close to a band edge in the reservoirs. This is modelled with an exactly solvable Hamiltonian without interactions (the Fano-Anderson model). The model shows an abrupt transition in its physics as the dot-reservoir coupling...
Preprint
Maxwell demons are creatures that are imagined to be able to reduce the entropy of a system without performing any work on it. Conventionally, such a Maxwell demon's intricate action consists in measuring individual particles and subsequently performing feedback. Here we show that much simpler setups can still act as demons: we demonstrate that it...
Article
Full-text available
Reflets de la physique constitue avec le site web l'outil de communication privilégié de la SFP auprès de ses membres.
Preprint
We consider the thermoelectric response of chaotic or disordered quantum dots in the limit of phase-coherent transport, statistically described by random matrix theory. We calculate the full distribution of the thermoelectric coefficients (Seebeck $S$ and Peltier $\Pi$), and the thermoelectric figure of merit $ZT$, for large open dots at arbitrary...
Preprint
This mini-review is intended as a short introduction to electron flow in nanostructures. Its aim is to provide a brief overview of this topic for people who are interested in the thermodynamics of quantum systems but know little about nanostructures. We particularly emphasize devices that work in the steady-state, such as simple thermoelectrics, bu...
Chapter
This chapter is intended as a short introduction to electron flow in nanostructures. Its aim is to provide a brief overview of this topic for people who are interested in the thermodynamics of quantum systems, but know little about nanostructures. We particularly emphasize devices that work in the steady-state, such as simple thermoelectrics, but a...
Article
Full-text available
We bring together Keldysh theory and quantum thermodynamics, by showing that a real-time diagramatic technique can provide a quantum equivalent of stochastic thermodynamics for non-Markovian quantum machines (heat engines, refrigerators, etc). Taking any interacting quantum system with arbitrary coupling to ideal reservoirs of electrons and bosons...
Article
In reply to the news articles "UK physics faces EU choice" (June 2016) and "UK physics grapples with Brexit" (August 2016).
Article
In recent years, the study of heat to work conversion has been re-invigorated by nanotechnology. Steady-state devices do this conversion without any macroscopic moving parts, through steady-state flows of microscopic particles such as electrons, photons, phonons, etc. This review aims to introduce some of the theories used to describe these steady-...
Article
We analyze the power output of a quantum dot machine coupled to two electronic reservoirs via thermoelectric contacts, and to two thermal reservoirs-one hot and one cold. This machine is a nanoscale analogue of a conventional thermocouple heat-engine, in which the active region being heated is unavoidably also exchanging heat with its cold environm...
Article
Full-text available
We consider the nonlinear scattering theory for three-terminal thermoelectric devices, used for power generation or refrigeration. Such a system is a quantum phase-coherent version of a thermocouple, and the theory applies to systems in which interactions can be treated at a mean-field level. We consider an arbitrary three-terminal system in any ex...
Article
We analyze the power output of a quantum dot machine coupled to two electronic reservoirs via thermoelectric contacts, and to two thermal reservoirs - one hot and one cold. This machine is a nanoscale analogue of a conventional thermocouple heat-engine, in which the active region being heated is unavoidably also exchanging heat with its cold enviro...
Conference Paper
Full-text available
We investigate how energy filtering manifests and operates inside a quantum dot-based photovoltaic device means of the Green's function formalism. We find that the filtering process increases both charge and heat produced in the device. However, currents surprisingly minima when the filter energy is equal to the energy due to a reduced carrier phot...
Article
Full-text available
We investigate the nonlinear Landauer-Buttiker scattering theory for quantum systems with strong Seebeck and Peltier effects, and their use as heat-engines and refrigerators with finite power outputs. This article gives detailed derivations of the results summarized in Phys. Rev. Lett. 112, 130601 (2014). It shows how to use the scattering theory t...
Article
Full-text available
Machines are only Carnot efficient if they are reversible, but then their power output is vanishingly small. Here we ask, what is the maximum efficiency of an irreversible device with finite power output? We use a nonlinear scattering theory to answer this question for thermoelectric quantum systems, heat engines or refrigerators consisting of nano...
Article
Full-text available
A bounded random walk exhibits strong correlations between collisions with a boundary. For a one-dimensional walk, we obtain the full statistical distribution of the number of such collisions in a time t. In the large t limit, the fluctuations in the number of collisions are found to be size independent (independent of the distance between boundari...
Article
Full-text available
I consider the non-equilibrium DC transport of electrons through a quantum system with a thermoelectric response. This system may be any nanostructure or molecule modeled by the nonlinear scattering theory which includes Hartree-like electrostatic interactions exactly, and certain dynamic interaction effects (decoherence and relaxation) phenomenolo...
Article
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We consider refrigeration and heat engine circuits based on the nonlinear thermoelectric response of point-contacts at pinch-off, allowing for electrostatic interaction effects. We show that a refrigerator can cool to much lower temperatures than predicted by the thermoelectric figure-of-merit ZT (which is based on linear-response arguments). The l...
Article
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Hamiltonian systems can be classified into ten classes, in terms of the presence or absence of time-reversal symmetry, particle-hole symmetry and sublattice/chiral symmetry. We construct a quantum coherent scattering theory of linear transport for coupled electric, heat and spin transport; including the effect of Andreev reflection from superconduc...
Article
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We consider sweeping a system through a Landau-Zener avoided crossing, when that system is also coupled to an environment or noise. Unsurprisingly, we find that decoherence suppresses the coherent oscillations of quantum superpositions of system states, as superpositions decohere into mixed states. However, we also find an effect we call "Lamb-assi...
Article
Full-text available
The conductance of a pair of quantum dots, coupled through a tunnel barrier and connected to two external leads, exceeds the conductance of the tunnel barrier alone (tunneling enhancement effect) if the device is symmetrical, while it strongly decreases if the symmetry is destroyed. This device could then be used to implement a sensitive detector o...
Article
We construct a theory of coherent transport through a ballistic quantum dot coupled to a superconductor. We show that the leading-order quantum correction to the two-terminal conductance of these Andreev quantum dots may change sign depending on (i) the number of channels carried by the normal leads or (ii) the magnetic flux threading the dot. In c...
Article
We apply the semiclassical theory of transport to quantum dots with exact and approximate spatial symmetries; left-right mirror symmetry, up-down mirror symmetry, inversion symmetry, or fourfold symmetry. In this work-the first of a pair of articles-we consider (a) perfectly symmetric dots and (b) nearly symmetric dots in which the symmetry is brok...
Article
In this work-the second of a pair of articles-we consider transport through spatially symmetric quantum dots with leads whose widths or positions do not obey the spatial symmetry. We use the semiclassical theory of transport to find the symmetry-induced contributions to weak localization corrections and universal conductance fluctuations for dots w...
Article
We investigate thermoelectric transport through Andreev interferometers. We show that the ratio of the thermal and the charge conductance exhibits large oscillations with the phase difference $\phi$ between the two superconducting contacts, and that the Wiedemann-Franz law holds only when $\phi=\pi$. A large average thermopower furthermore emerges...
Conference Paper
Full-text available
The conductance of a tunnel barrier can exhibit a very large enhancement, as a result of constructive interference, if two constrictions are symmetrically placed around the barrier itself, thus defining a cavity. This phenomenon could be exploited for a sensitive detector of electric or magnetic fields, due to the strong dependence of the enhanceme...
Article
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We briefly review three ways that environmental noise can slow-down (or speed-up) quantum transitions; (i) Lamb shifts, (ii) over-damping and (iii) orthogonality catastrophe. We compare them with the quantum Zeno effect induced by observing the system. These effects are relevant to poor qubits (those strongly coupled to noise). We discuss Berry pha...
Article
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We predict a huge interference effect contributing to the conductance through large ultraclean quantum dots of chaotic shape. When a double-dot structure is made such that the dots are the mirror image of each other, constructive interference can make a tunnel barrier located on the symmetry axis effectively transparent. We show (via theoretical an...
Article
We consider a two-level system coupled to a highly non-Markovian environment when the coupling axis rotates with time. The environment may be quantum (for example a bosonic bath or a spin bath) or classical (such as classical noise). We show that an Anderson orthogonality catastrophe suppresses transitions, so that the system's instantaneous eigens...
Article
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We study the effect of a noisy environment on spin and charge transport in ballistic quantum wires with spin-orbit coupling (Rashba coupling). We find that the wire then acts as a dephasing diode, inducing very different dephasing of the spins of right and left movers. We also show how Berry phase (geometric phase) in a curved wire can induce such...
Article
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We investigate the effect of dephasing/decoherence on quantum transport through open chaotic ballistic conductors in the semiclassical limit of small Fermi wavelength to system size ratio, $\\lambda_F/L $\lt$$\lt$ 1$. We use the trajectory-based semiclassical theory to study a two-terminal chaotic dot with decoherence originating from: (i) an exter...
Article
The perturbative master equation (Bloch-Redfield) is extensively used to study dissipative quantum mechanics - particularly for qubits - despite the 25 year old criticism that it violates positivity (generating negative probabilities). We take an arbitrary system coupled to an environment containing many degrees-of-freedom, and cast its perturbativ...
Article
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Dephasing in open quantum chaotic systems has been investigated in the limit of large system sizes to the Fermi wavelength ratio, L/λF 〉 1. The weak localization correction g wl to the conductance for a quantum dot coupled to (i) an external closed dot and (ii) a dephasing voltage probe is calculated in the semiclassical approximation. In addition...
Article
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We investigate the effect of dephasing/decoherence on quantum transport through open chaotic ballistic conductors in the semiclassical limit of small Fermi wavelength to system size ratio, $\lambda_F/L << 1$. We use the trajectory-based semiclassical theory to study a two-terminal chaotic dot with decoherence originating from: (i) an external close...
Article
Semiclassical methods can now explain many mesoscopic effects (shot-noise, conductance fluctuations, etc) in clean chaotic systems, such as chaotic quantum dots. In the deep classical limit (wavelength much less than system size) the Ehrenfest time (the time for a wavepacket to spread to a classical size) plays a crucial role, and random matrix the...
Article
We add simple tunnelling effects and ray-splitting into the recent trajectory-based semiclassical theory of quantum chaotic transport. We use this to derive the weak-localization correction to conductance and the shot-noise for a quantum chaotic cavity (billiard) coupled to $n$ leads via tunnel-barriers. We derive results for arbitrary tunnelling r...
Article
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We investigate dephasing in open quantum chaotic systems in the limit of large system size to Fermi wavelength ratio, $L/\lambda_F >> 1$. We semiclassically calculate the weak localization correction $g^{wl}$ to the conductance for a quantum dot coupled to (i) an external closed dot and (ii) a dephasing voltage probe. In addition to the universal a...
Article
Full-text available
We construct a trajectory-based semiclassical theory of shot noise in clean chaotic cavities. In the universal regime of vanishing Ehrenfest time tau(E), we reproduce the random matrix theory result and show that the Fano factor is exponentially suppressed as tau(E) increases. We demonstrate how our theory preserves the unitarity of the scattering...
Article
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We investigate transport properties of quantized chaotic systems in the short wavelength limit. We focus on non-coherent quantities such as the Drude conductance, its sample-to-sample fluctuations, shot-noise and the transmission spectrum, as well as coherent effects such as weak localization. We show how these properties are influenced by the emer...
Article
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Here we summarise and place in context two of our earlier works. There we investigate the geometric phase or Berry phase (BP) acquired by a spin‐half which is subject to external noise in addition to a slowly varying magnetic field (which generates the Berry phase). The noise may be due to the fluctuations of either quantum or classical degrees‐of‐...
Article
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This paper is withdrawn. While almost all findings reported here remain valid, some of our comments on the fate of weak localization corrections to the conductance in the deep semiclassical limit are now obsolete. Contrarily to what was suggested here, we find an exponential suppression of weak localization at finite Ehrenfest time. For more detail...
Article
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We present a semiclassical theory for the scattering matrix S of a chaotic ballistic cavity at finite Ehrenfest time. Using a phase‐space representation we show that the Liouville conservation of phase‐space volume decomposes S as S = S cl ⊕ S qm. The short‐time, classical contribution S cl generates deterministic transmission eigenvalues T = 0 o...
Article
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We investigate the behavior of the shot‐noise power through quantum mechanical cavities in the semiclassical limit of small electronic wavelength. In the absence of impurity scattering, the Fano factor F, giving the noise to current ratio, was previously found to disappear as more and more classical, hence deterministic and noiseless transmission c...
Article
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We present a semiclassical theory for the scattering matrix S of a chaotic ballistic cavity at finite Ehrenfest time. Using a phase-space representation coupled with a multibounce expansion, we show how the Liouville conservation of phase-space volume decomposes S as S=S(cl) plus sign in circle S(qm). The short-time, classical contribution S(cl) ge...
Article
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We investigate the geometric phase or Berry phase acquired by a spin half which is both subject to a slowly varying magnetic field and weakly coupled to a dissipative environment (either quantum or classical). We study how this phase is modified by the environment and find that the modification is of a geometric nature. While the original Berry pha...
Article
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We discuss the concept of the Berry phase in a dissipative system. We show that one can identify a Berry phase in a weakly-dissipative system and find the respective correction to this quantity, induced by the environment. This correction is expressed in terms of the symmetrized noise power and is therefore insensitive to the nature of the noise re...
Article
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We investigate the effect of the environment on a Berry phase measurement involving a spin-half. We model the spin + environment using a biased spin-boson Hamiltonian with a time-dependent magnetic field. We find that, contrary to naive expectations, the Berry phase acquired by the spin can be observed, but only on time scales which are neither too...
Article
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For certain types of quantum graphs we show that the random-matrix form factor can be recovered to at least third order in the scaled time $\tau$ from periodic-orbit theory. We consider the contributions from pairs of periodic orbits represented by diagrams with up to two self-intersections connected by up to four arcs and explain why all other dia...
Article
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Using periodic-orbit theory beyond the diagonal approximation we investigate the form factor, K(tau), of a generic quantum graph with mixing classical dynamics and time-reversal symmetry. We calculate the contribution from pairs of self-intersecting orbits that differ from each other only in the orientation of a single loop. In the limit of large g...
Article
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Berry phase was originally defined for systems whose states are separated by finite energy gaps. One might naively expect that a system without a gap cannot have a Berry phase. Despite this we ask whether a Berry phase can be observed in a system which has a continuous spectrum because its coupling to the environment has broadened its energy levels...
Article
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We attempt to systematically derive perturbative quantum corrections to the Berry diagonal approximation of the two-level correlation function (TLCF) for chaotic systems. To this end, we develop a `weak diagonal approximation' based on a recent description of the first weak localization correction to conductance in terms of the Gutzwiller trace for...

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