arXiv subjects

Fabio Taddei

Publications and source records attributed to Fabio Taddei.

At least 55 records · Page 3Linked to original sources

arXiv2018

Non-Linear Thermovoltage in a Single-Electron Transistor

We perform direct thermovoltage measurements in a single-electron transistor, using on-chip local thermometers, both in the linear and non-linear regimes. Using a model which accounts for co-tunneling, we find excellent agreement with the experimental data with no free parameters even when the temperature difference is larger than the average temperature (far-from-linear regime). This allows us to confirm the sensitivity of the thermovoltage on co-tunneling and to find that in the non-linear regime the temperature of the metallic island is a crucial parameter. Surprisingly, the metallic island tends to overheat even at zero net charge current, resulting in a reduction of the thermovoltage.

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arXiv2018

Absorption refrigerators based on Coulomb-coupled single-electron systems

We analyze a simple implementation of an absorption refrigerator, a system that requires heat and not work to achieve refrigeration, based on two Coulomb coupled single-electron systems. We analytically determine the general condition to achieve cooling-by-heating, and we determine the system parameters that simultaneously maximize the cooling power and cooling coefficient of performance (COP) finding that the system displays a particularly simple COP that can reach Carnot's upper limit. We also find that the cooling power can be indirectly determined by measuring a charge current. Analyzing the system as an autonomous Maxwell demon, we find that the highest efficiencies for information creation and consumption can be achieved, and we relate the COP to these efficiencies. Finally, we propose two possible experimental setups based on quantum dots or metallic islands that implement the non-trivial cooling condition. Using realistic parameters, we show that these systems, which resemble existing experimental setups, can develop an observable cooling power.

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arXiv2018

Thermal drag in electronic conductors

We study the electronic thermal drag in two different Coulomb-coupled systems, the first one composed of two Coulomb blockaded metallic islands and the second one consisting of two parallel quantum wires. The two conductors of each system are electrically isolated and placed in the two circuits (the drive and the drag) of a four-electrode setup. The systems are biased, either by a temperature $ΔT$ or a voltage $V$ difference, on the drive circuit, while no biases are present on the drag circuit. In the case of a pair of metallic islands we use a master equation approach to determine the general properties of the dragged heat current $I^{\rm (h)}_{\rm drag}$, accounting also for co-tunneling contributions and the presence of large biases. Analytic results are obtained in the sequential tunneling regime for small biases, finding, in particular, that $I^{\rm (h)}_{\rm drag}$ is quadratic in $ΔT$ or $V$ and non-monotonous as a function of the inter-island coupling. Finally, by replacing one of the electrodes in the drag circuit with a superconductor, we find that heat can be extracted from the other normal electrode. In the case of the two interacting quantum wires, using the Luttinger liquid theory and the bosonization technique, we derive an analytic expression for the thermal trans-resistivity $ρ^{\rm (h)}_{12}$, in the weak-coupling limit and at low temperatures. $ρ^{\rm (h)}_{12}$ turns out to be proportional to the electric trans-resistivity $ρ^{\rm (c)}_{12}$, in such a way that their ratio (a kind of Wiedemann-Franz law) is proportional to $T^3$. We find that the thermal trans-resistivity is proportional to $T$ for low temperatures and decreases like $1/T$ for intermediate temperatures or like $1/T^3$ for high temperatures. We complete our analyses by performing numerical simulations that confirm the above results and allow to access the strong coupling regime.

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arXiv2018

Nonlocal superconducting correlations in graphene in the quantum Hall regime

We study Andreev processes and nonlocal transport in a three-terminal graphene-superconductor hybrid system under a quantizing perpendicular magnetic field [G.-H. Lee et al., Nature Phys. 13, 693 (2017)]. We find that the amplitude of the crossed Andreev reflection (CAR) processes crucially depends on the orientation of the lattice. By employing Landauer-Büttiker scattering theory, we find that CAR is generally very small for a zigzag edge, while for an armchair edge it can be larger than the normal transmission, thereby resulting in a negative nonlocal resistance. In the case of an armchair edge and with a wide superconducting region (as compared to the superconducting coherence length), CAR exhibits large oscillations as a function of the magnetic field due to interference effects. This results in sign changes of the nonlocal resistance.

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arXiv2017

Thermoelectric properties of an interacting quantum dot-based heat engine

We study the thermoelectric properties and heat-to-work conversion performance of an interacting, multi-level quantum dot (QD) weakly coupled to electronic reservoirs. We focus on the sequential tunneling regime. The dynamics of the charge in the QD is studied by means of master equations for the probabilities of occupation. From here we compute the charge and heat currents in the linear response regime. Assuming a generic multi-terminal setup, and for low temperatures (quantum limit), we obtain analytical expressions for the transport coefficients which account for the interplay between interactions (charging energy) and level quantization. In the case of systems with two and three terminals we derive formulas for the power factor Q and the figure of merit ZT for a QD-based heat engine, identifying optimal working conditions which maximize output power and efficiency of heat-to-work conversion. Beyond the linear response we concentrate on the two-terminal setup. We first study the thermoelectric non-linear coefficients assessing the consequences of large temperature and voltage biases, focusing on the breakdown of the Onsager reciprocal relation between thermopower and Peltier coefficient. We then investigate the conditions which optimize the performance of a heat engine, finding that in the quantum limit output power and efficiency at maximum power can almost be simultaneously maximized by choosing appropriate values of electrochemical potential and bias voltage. At last we study how energy level degeneracy can increase the output power.

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arXiv2016

Nonlocal topological valley transport at large valley Hall angles

Berry curvature hot spots in two-dimensional materials with broken inversion symmetry are responsible for the existence of transverse valley currents, which give rise to giant nonlocal dc voltages. Recent experiments in high-quality gapped graphene have highlighted a saturation of the nonlocal resistance as a function of the longitudinal charge resistivity $ρ_{{\rm c}, xx}$, when the system is driven deep into the insulating phase. The origin of this saturation is, to date, unclear. In this work we show that this behavior is fully compatible with bulk topological transport in the regime of large valley Hall angles (VHAs). We demonstrate that, for a fixed value of the valley diffusion length, the dependence of the nonlocal resistance on $ρ_{{\rm c}, xx}$ weakens for increasing VHAs, transitioning from the standard $ρ^3_{{\rm c}, xx}$ power-law to a result that is independent of $ρ_{{\rm c}, xx}$.

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arXiv2015

Nanoscale Mach-Zehnder interferometer with spin-resolved quantum Hall edge states

We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data.

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arXiv2015

Finite-frequency noise in a topological superconducting wire

In this paper we study the finite-frequency current cross-correlations for a topological superconducting nanowire attached to two terminals at one of its ends. Using an analytic 1D model we show that the presence of a Majorana bound state yields vanishing cross-correlations for frequencies larger than twice the applied transport voltage, in contrast to what is found for a zero-energy ordinary Andreev bound state. Zero cross-correlations at high frequency have been confirmed using a more realistic tight-binding model for finite-width topological superconducting nanowires. Finite-temperature effects have also been investigated.

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arXiv2015

Separation of heat and charge currents for boosted thermoelectric conversion

In a multi-terminal device the (electronic) heat and charge currents can follow different paths. In this paper we introduce and analyse a class of multi-terminal devices where this property is pushed to its extreme limits, with charge $and$ heat currents flowing in different reservoirs. After introducing the main characteristics of such $heat-charge$ $current$ $separation$ regime we show how to realise it in a multi-terminal device with normal and superconducting leads. We demonstrate that this regime allows to control independently heat and charge flows and to greatly enhance thermoelectric performances at low temperatures. We analyse in details a three-terminal setup involving a superconducting lead, a normal lead and a voltage probe. For a generic scattering region we show that in the regime of heat-charge current separation both the power factor and the figure of merit $ZT$ are highly increased with respect to a standard two-terminal system. These results are confirmed for the specific case of a system consisting of three coupled quantum dots.

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arXiv2015

A magnetic thermal switch for heat management at the nanoscale

In a multi-terminal setup, when time-reversal symmetry is broken by a magnetic field, the heat flows can be managed by designing a device with programmable Boolean behavior. We show that such device can be used to implement operations like on/off switching, reversal, selected splitting and swap of the heat currents. For each feature, the switching from one working condition to the other is obtained by inverting the magnetic field. This offers interesting opportunities of conceiving a programmable setup, whose operation is controlled by an external parameter (the magnetic field) without need to alter voltage and thermal biases applied to the system. Our results, generic within the framework of linear response, are illustrated by means of a three-terminal electronic interferometer model.

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arXiv2015

Thermopower of three-terminal topological superconducting systems

We study the thermopower of a three-terminal setup composed of a quantum dot attached to three electrodes, one of which is a topological superconductor. In the model, superconductivity is explicitly taken into account. We compare the results for s-wave (trivial) and p-wave (topological) superconductors and observe that for small temperatures the thermopower has different sign in the two cases. This behavior is strongly dependent on temperature and we estimate an energy scale that controls the sign in the p-wave case, which results proportional to the square root of the gap and the coupling to superconductor. The analytical results obtained with a simple 1D model are confirmed by a more realistic tight-binding model.

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arXiv2014

Thermoelectric efficiency of three-terminal quantum thermal machines

The efficiency of a thermal engine working in linear response regime in a multi-terminals configuration is discussed. For the generic three-terminal case, we provide a general definition of local and non-local transport coefficients: electrical and thermal conductances, and thermoelectric powers. Within the Onsager formalism, we derive analytical expressions for the efficiency at maximum power, which can be written in terms of generalized figures of merit. Also, using two examples, we investigate numerically how a third terminal could improve the performance of a quantum system, and under which conditions non-local thermoelectric effects can be observed.

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arXiv2014

Andreev levels spectroscopy of topological three-terminal junctions

We calculate the differential conductance at a probe inserted in the weak link of a topological Josephson junction, consisting of a semiconducting nanowire deposited on top of two separated superconductors. Our aim is to understand how the peculiar features in the spectrum of Andreev bound states, arising due to the presence of Majorana bound states at the ends of the two topological superconducting wires defining the junction, can be determined through a measurement of the differential conductance. We find that when the probe allows a single propagating mode, the differential conductance presents a dip at zero voltage of zero conductance close to the position where the spectrum exhibits the topologically protected crossing. This can be viewed as a signature of the presence of Majorana states, which does not require fermion parity conservation and is robust against parameters' changes, as well as disorder. On the contrary, when the probe allows two or more propagating modes the differential conductance resembles the spectrum of Andreev bound states. This has been established making use of both numerical and analytical methods.

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arXiv2013

Topological pumping in class-D superconducting wires

We study adiabatic pumping at a normal metal/class-D superconductor hybrid interface when superconductivity is induced through the proximity effect in a spin-orbit coupled nanowire in the presence of a tilted Zeeman field. When the induced order parameter in the nanowire is non-uniform, the phase diagram has isolated trivial regions surrounded by topological ones. We show that in this case the pumped charge is quantized in units of the elementary charge $e$ and has a topological nature.

cond-mat.supr-con↗

arXiv2013

Parity dependent Josephson current through a helical Luttinger liquid

We consider a superconductor-two dimensional topological insulator- superconductor junction (S-2DTI-S) and study how the 2π- and 4π-periodic Josephson currents are affected by the electron-electron interaction. In the long-junction limit the supercurrent can by evaluated by modeling the system as a helical Luttinger liquid coupled to superconducting reservoirs. After having introduced bosonization in the presence of the parity constraint we turn to consider the limit of perfect and poor interfaces. For transparent interfaces, where perfect Andreev reflections occur at the boundaries, the Josephson current is marginally affected by the interaction. On the contrary, if strong magnetic scatterers are present in the weak link, the situation changes dramatically. Here Coulomb interaction plays a crucial role both in low and high temperature regimes. Furthermore, a phase-shift of Josephson current can be induced by changing the direction of the magnetization of the impurity.

cond-mat.supr-con↗

arXiv2013

Interactions in Electronic Mach-Zehnder Interferometers with Copropagating Edge Channels

We study Coulomb interactions in the finite bias response of Mach-Zehnder interferometers, which exploit copropagating edge states in the integer quantum Hall effect. Here, interactions are particularly important since the coherent coupling of edge channels is due to a resonant mechanism that is spoiled by inelastic processes. We find that interactions yield a saturation, as a function of bias voltage, of the period-averaged interferometer current, which gives rise to unusual features, such as negative differential conductance, enhancement of the visibility of the current, and nonbounded or even diverging visibility of the differential conductance.

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arXiv2012

Coherent edge mixing and interferometry in quantum Hall bilayers

We discuss the implementation of a beam splitter for electron waves in a quantum Hall bilayer. Our architecture exploits inter-layer tunneling to mix edge states belonging to different layers. We discuss the basic working principle of the proposed coherent edge mixer, possible interferometric implementations based on existing semiconductor-heterojunction technologies, and advantages with respect to canonical quantum Hall interferometers based on quantum point contacts.

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arXiv2012

Imaging backscattering through impurity-induced antidots in quantum Hall constrictions

We exploit the biased tip of a scanning gate microscope (SGM) to induce a controlled backscattering between counter-propagating edge channels in a wide constriction in the quantum Hall regime. We compare our detailed conductance maps with a numerical percolation model and demonstrate that conductance fluctuations observed in these devices as a function of the gate voltage originate from backscattering events mediated by localized states pinned by potential fluctuations. Our imaging technique allows us to identify the necessary conditions for the activation of these backscattering processes and also to reconstruct the constriction confinement potential profile and the underlying disorder.

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