arXiv subjects

Tomaz Rejec

Publications and source records attributed to Tomaz Rejec.

11 recordsLinked to original sources

arXiv2021

Differential thermopower images as a probe of many-body effects in quantum point contacts

Mesoscopic circuit elements such as quantum dots and quantum point contacts (QPCs) offer a uniquely controllable platform for engineering complex quantum devices, whose tunability makes them ideal for generating and investigating interacting quantum systems. However, the conductance measurements commonly employed in mesoscopics experiments are poorly suited to discerning correlated phenomena from those of single-particle origin. Here, we introduce non-equilibrium thermopower measurements as a novel approach to probing the local density of states (LDOS), offering an energy-resolved readout of many-body effects. We combine differential thermopower measurements with non-equilibrium density functional theory (DFT) to both confirm the presence of a localized state at the saddle point of a QPC and reveal secondary states that emerge wherever the reservoir chemical potential intersects the gate-induced potential barrier. These experiments establish differential thermopower imaging as a robust and general approach to exploring quantum many-body effects in mesoscopic circuits.

cond-mat.mes-hall↗

arXiv2013

Spin thermopower in the overscreened Kondo model

We study the spin thermopower in the two-channel spin-1/2 Kondo model which exhibits the phenomenon of impurity spin overscreening and non-Fermi-liquid properties. While magnetic field lower than the Kondo temperature does not strongly polarize the impurity spin, we show that it nevertheless strongly affects the low-energy part of the spectral function. In turn, this leads to characteristic saturation of the spin Seebeck coefficient at the value of 0.388kB/|e| at T ~ T*, where T* ~ B^2/TK is the scale of the crossover between the intermediate-temperature non-Fermi-liquid regime and the low-temperature Fermi-liquid regime. We show that measuring the spin thermopower at low magnetic fields would provide a sensitive test for distinguishing regular Fermi liquid, singular Fermi liguid, and non-Fermi liquid behaviour in nanodevices.

cond-mat.str-el↗

arXiv2012

SU(3) Kondo effect in spinless triple quantum dots

We discuss a device --- a purely capacitively coupled interacting spinless triple quantum dot system --- for the observation of the SU(3) Kondo effect. Unlike more familiar SU(2) and SU(4) Kondo effects in quantum dot devices which lead to unitary linear conductance at low temperatures, the SU(3) Kondo scenario can be easily identified by the conductance pinned to a characteristic value of 3/4 of the unitary limit. This is associated with the interesting fact that the SU(3) Kondo effect does not occur at the particle-hole symmetric point, where the system is found instead in the valence-fluctuating regime with the total dot occupancy flipping between 1 and 2, but for gate voltages in the two Kondo plateaux where the dot occupancy is pinned to an integer value, either 1 or 2. From the thermodynamic analysis in the Kondo regime we find that the effective impurity orbital moment, defined through the impurity orbital susceptibility (chi_imp) multiplied by the temperature, is Tχ_imp=1 at high temperatures and then it increases to the characteristic value of Tχ_imp=4/3 corresponding to the three-fold degenerate local-moment fixed point where the impurity entropy is S_imp=ln 3. Then, at much lower temperatures, the system flows to the non-degenerate strong-coupling fixed point in which the SU(3) Kondo effect takes place. We also report results about the robustness of the SU(3) Kondo effect against various perturbations present in real experimental setups. Finally, we describe possible mechanisms to restore the SU(3) Kondo physics by properly tuning the on-site dot potentials. We briefly comment on the spinfull case which has very different behavior and shows Kondo plateaus in conductance for all integer values of the occupancy, including at the particle-hole symmetric point.

cond-mat.str-el↗

arXiv2012

Spin-thermopower in interacting quantum dots

Using analytical arguments and the numerical renormalization group method we investigate the spin-thermopower of a quantum dot in a magnetic field. In the particle-hole symmetric situation the temperature difference applied across the dot drives a pure spin current without accompanying charge current. For temperatures and fields at or above the Kondo temperature, but of the same order of magnitude, the spin-Seebeck coefficient is large, of the order of k_B/e. Via a mapping, we relate the spin-Seebeck coefficient to the charge-Seebeck coefficient of a negative-U quantum dot where the corresponding result was recently reported by Andergassen et al. in Phys. Rev. B 84, 241107 (2011). For several regimes we provide simplified analytical expressions. In the Kondo regime, the dependence of the spin-Seebeck coefficient on the temperature and the magnetic field is explained in terms of the shift of the Kondo resonance due to the field and its broadening with the temperature and the field. We also consider the influence of breaking the particle-hole symmetry and show that a pure spin current can still be realized provided a suitable electric voltage is applied across the dot. Then, except for large asymmetries, the behavior of the spin-Seebeck coefficient remains similar to that found in the particle-hole symmetric point.

cond-mat.mes-hall↗

arXiv2007

Charge rearrangement and screening in a quantum point contact

Compressibility measurements, sensitive to charge rearrangements, are performed on a quantum point contact (QPC). Screening due to mobile charges in the QPC is quantitatively measured, using a second point contact to detect the screened electrical potential. These measurements are performed from pinch-off through the opening of the first few modes in the QPC. While the measured signal closely matches a Thomas-Fermi-Poisson prediction, deviations from the classical behavior, in the form of additional dips, are apparent near the openings of the different modes, with the largest dip at the opening of the first mode. Density functional calculations attribute the first dip to exchange interactions. The other dips reflect the diverging density of states at the opening of each one-dimensional mode, which affects both kinetic and exchange contributions to the energy.

cond-mat.mes-hall↗

arXiv2007

SU(2) and SU(4) Kondo effect in double quantum dots

We investigate serial double quantum dot systems with on-site and inter-site interaction by means of Schönhammer-Gunnarsson projection-operator method. The ground state is established by the competition between extended Kondo phases and localized singlet phases in spi$ degrees of freedom. We present and discuss different phases, as discerned by characteristic correlation functions. We discuss also how different phases would be seen in linear transport measurements.

cond-mat.mes-hall↗

arXiv2006

Magnetic impurity formation in quantum point contacts

A quantum point contact (QPC), a narrow region separating two wider electron reservoirs, is the standard building block of sub-micron devices, such as quantum dots - small boxes of electrons, and qubits - the proposed basic elements of quantum computers. As a function of its width, the conductance through a QPC changes in integer steps of G0 = $2e^2/h$, signalling the quantization of its tranverse modes.1,2 Such measurements also reveal an additional shoulder at a value around 0.7, an observation which remains a puzzle even after more than a decade. Recently it has been suggested5,6 that this phenomenon can be explained if one invokes the existence of a magnetic impurity in the QPC at low densities. Here we present extensive numerical density-functional calculations that reveal the formation of a magnetic moment in the channel as the density increases above pinch-off, under very general conditions. In addition we show that such an impurity will also form at large magnetic fields, for a specific value of the field (corresponding to a degeneracy point between the upper spin state in the first mode and the lower spin state in the second mode), and sometimes even at the opening of the second mode in the QPC. Beyond explaining the source of the "0.7 anomaly", these results may have far reaching implications on spin filling of electronic states in quantum dots and on dephasing of quantum information stored in semiconductor qubits.

cond-mat.mes-hall↗

arXiv2006

Kondo effect in double quantum dots with inter-dot repulsion

We investigate a symmetrical double quantum dot system serially attached to the leads. The emphasis is on the numerical analysis of finite inter-dot tunneling in the presence of inter-dot repulsive capacitive coupling. The results reveal the competition between extended Kondo phases and local singlet phases in spin and charge degrees of freedom. The corresponding phase diagram is determined quantitatively.

cond-mat.mes-hall↗

arXiv2003

Conductance of nanosystems with interaction

The zero-temperature linear response conductance through an interacting mesoscopic region attached to noninteracting leads is investigated. We present a set of formulas expressing the conductance in terms of the ground-state energy of an auxiliary system, namely a ring threaded by a magnetic flux and containing the correlated electron region. We prove that the formalism is exact if the ground state of the system is a Fermi liquid. We show that in such systems the ground-state energy is a universal function of the magnetic flux, where the conductance is the relevant parameter. The method is illustrated with results for the transport through an interacting quantum dot and a simple Aharonov-Bohm ring with Kondo-Fano resonance physics.

cond-mat.mes-hall↗

arXiv2003

Formulae for zero-temperature conductance through a region with interaction

The zero-temperature linear response conductance through an interacting mesoscopic region attached to noninteracting leads is investigated. We present a set of formulae expressing the conductance in terms of the ground-state energy or persistent currents in an auxiliary system, namely a ring threaded by a magnetic flux and containing the correlated electron region. We first derive the conductance formulae for the noninteracting case and then give arguments why the formalism is also correct in the interacting case if the ground state of a system exhibits Fermi liquid properties. We prove that in such systems, the ground-state energy is a universal function of the magnetic flux, where the conductance is the only parameter. The method is tested by comparing its predictions with exact results and results of other methods for problems such as the transport through single and double quantum dots containing interacting electrons. The comparisons show an excellent quantitative agreement.

cond-mat.mes-hall↗

arXiv2003

Conductance of interacting Aharonov-Bohm systems

A simple formula for the zero-temperature linear response conductance of an interacting mesoscopic region, threaded by magnetic flux, and attached to noninteracting single-channel leads is presented. The formula is valid for a general interacting system exhibiting Fermi liquid properties. As an example of the efficiency of the formula the results for the conductance of a simple Aharonov-Bohm ring with Kondo-Fano resonance physics are presented and compared with numerical renormalization group results.

cond-mat.mes-hall↗