arXiv subjects

D. B. Karki

Publications and source records attributed to D. B. Karki.

18 recordsLinked to original sources

arXiv2026

Tunneling in multi-site mesoscopic quantum Hall circuits

Transport properties of single- and two-site mesoscopic quantum Hall (QH) circuits at high transparencies can be described in terms of the lowest-order backscattering processes, enabling a mapping to the boundary sine-Gordon model. We show that this description breaks down in circuits with four or more sites, where higher-order backscattering processes become relevant and qualitatively modify the low-energy physics, while remaining exactly marginal in three-site geometries. Focusing on the four-site circuit, we derive an effective low-energy theory that captures the resulting interaction-driven physics and reveal the emergence of unique quantum-critical points. In the vicinity of these critical points, we obtain universal conductance and scaling behavior and establish the robustness of the associated non-Fermi liquid physics. We further introduce tunneling in multichannel multi-site QH circuits and propose a promising route for realizing diverse quantum-critical phenomena. We show that a boundary sine-Gordon description can be restored in multichannel multi-site QH circuits by appropriately looping selected edge channels, a procedure that is experimentally feasible. Finally, we analyze the non-equilibrium heating effects relevant to transport measurements in QH circuits. Altogether, our results establish multi-site QH circuits as a versatile and highly controllable platform for simulating interaction-driven quantum critical phenomena.

cond-mat.mes-hall↗

arXiv2024

Multi-mode Coulomb blockade oscillations

We develop a theory of Coulomb blockade oscillations in transport and thermodynamic properties of a mesoscopic device having multiple charging energy modes. This setup can be realized using a nanoelectronic circuit comprising coupled hybrid metal-semiconductor islands. We show that this device can have various distinctive operational regimes depending on the strength of charging modes and base temperature. We focus on three different regimes; quantum regime, thermal regime and quantum-thermal mixed regime, in which the shape of the Coulomb blockade oscillations manifests well-defined features that can be accessed via conductance measurements. Our theory covers full crossover among these regimes, and also accounts for an accidental vanishing of one of the charging modes.

cond-mat.mes-hall↗

arXiv2024

Quantum criticality in coupled hybrid metal-semiconductor islands

We show that the combined effects of dynamical Coulomb blockade and integer quantum Hall effect in a coupled hybrid metal-semiconductor setup provide a pathway for realizing resonant tunneling in Luttinger liquids. This hybrid setup can be brought to the quantum critical regime by varying gate voltages and contact resistances. We explore the nature of quantum criticality, Kondo effect, charge fractionalization and transport in such a hybrid setup, and verify their robust non-Fermi liquid behaviors. Our work opens a promising route for quantum simulating exotic zero temperature quantum critical phenomena associated with Luttinger liquid physics in a nanoengineered electronic circuit with well-defined quantum Hall channels.

cond-mat.mes-hall↗

arXiv2023

Tunneling between two systems of interacting chiral fermions

We develop a theory of tunneling between two systems of spinless chiral fermions. This setup can be realized at the edge of a quantum Hall bilayer structure. We find that the differential conductance of such a device in the absence of interactions has an infinitely sharp peak as a function of applied voltage. Interaction between fermions results in broadening of the conductance peak. We focus on the regime of strong interactions, in which the shape of the peak manifests well defined features associated with the elementary excitations of the system.

cond-mat.mes-hall↗

arXiv2023

Physics of the Majorana-superconducting qubit hybrids

Manipulation of decoupled Majorana zero modes (MZMs) could enable topologically-protected quantum computing. However, the practical realization of a large number of perfectly decoupled MZMs needed to perform nontrivial quantum computation has proven to be challenging so far. Fortunately, even a small number of imperfect MZMs can be used to qualitatively extend the behavior of standard superconducting qubits, allowing for new approaches for noise suppression, qubit manipulation and read-out. Such hybrid devices take advantage of interplay of Cooper pair tunneling, coherent single electron tunneling, and Majorana hybridization. Here we provide a qualitative understanding of this system, give analytical results for its ground state energy spanning full parameter range, and describe potential sensing applications enabled by the interplay between Majorana and Cooper pair tunneling.

cond-mat.mes-hall↗

arXiv2023

$\mathbb{Z}_3$ parafermion in the double charge-Kondo model

Quantum impurity models with frustrated Kondo interactions can support quantum critical points with fractionalized excitations. Recent experiments [arXiv:2108.12691] on a circuit containing two coupled metal-semiconductor islands exhibit transport signatures of such a critical point. Here we show using bosonization that the double charge-Kondo model describing the device can be mapped in the Toulouse limit to a sine-Gordon model. Its Bethe-ansatz solution shows that a $\mathbb{Z}_3$ parafermion emerges at the critical point, characterized by a fractional $\tfrac{1}{2}\ln(3)$ residual entropy, and scattering fractional charges $e/3$. We also present full numerical renormalization group calculations for the model and show that the predicted behavior of conductance is consistent with experimental results.

cond-mat.mes-hall↗

arXiv2022

Local bistability under microwave heating for spatially mapping disordered superconductors

We theoretically study a strongly disordered superconducting layer heated by near-field microwave radiation from a nanometric metallic tip. The microwaves heat up the quasiparticles, which cool by phonon emission and conduction away from the heated area. Due to a bistability with two stable states of the electron temperature under the tip, the heating can be tuned to induce a submicrometer-sized normal region bounded by a sharp domain wall between high- and low-temperature states. We propose this as a local probe to access different physics from existing methods, for example, to map out inhomogeneous superfluid flow in the layer. The bistability-induced domain wall can significantly improve its spatial resolution.

cond-mat.mes-hall↗

arXiv2022

Multistage Kondo effect in a multiterminal geometry: A modular quantum interferometer

Quantum systems characterized by an interplay between several resonance scattering channels demonstrate very rich physics. To illustrate it we consider a multistage Kondo effect in nanodevices as a paradigmatic model for a multimode resonance scattering. We show that the channel crosstalk results in a destructive interference between the modes. This interplay can be controlled by manipulating the tunneling junctions in the multilevel and multiterminal geometry. We present a full-fledged theory of the multistage Kondo effect at the strong-coupling Fermi-liquid fixed point and discuss the influence of quantum interference effects to the quantum transport observables.

cond-mat.mes-hall↗

arXiv2021

Transmission phase evolution in fully screened and overscreened Kondo impurities

We study the coherent properties of fully screened and overscreened Kondo effects based on the Nozieres local Fermi-liquid theory and Affleck-Ludwig boundary conformal-field-theory approach, respectively. Coherent transports through an SU($N)$ generalization of fully screened and the multi-$\mathcal{K}$-channel overscreened Kondo impurities at and beyond the particle-hole (PH) symmetric point are thoroughly investigated. We report distinctive Fermi-liquid coefficients characterizing the finite temperature correction to the transmission phase shift and normalized visibility in fully screened Kondo regime, which can be measured with the existing experimental setups. Our work equally uncovers the significance of temperature correction to the transmission phase shift and visibility in non Fermi-liquid regime associated with the PH asymmetric overscreened Kondo effects with an arbitrary number of conduction channels $\mathcal{K}$. We propose viable roots of verifying our predictions in connection to the recent experiments, in particular with the experiments studying highly symmetric forms of fully screened Kondo effects and two-channel overscreened Kondo effects realized in quantum dot nano-structures.

cond-mat.str-el↗

arXiv2021

Overscreened Kondo problem with large spin and large number of orbital channels: Two distinct semiclassical limits in quantum transport observables

We investigate the quantum transport through Kondo impurity assuming both a large number of orbital channels $\mathcal K$$\gg $$1$ for the itinerant electrons and a semi-classical spin ${\cal S}$ $\gg $ $1$ for the impurity. The non-Fermi liquid regime of the Kondo problem is achieved in the overscreened sector $\mathcal K>2\mathcal{S}$. We show that there exist two distinct semiclassical regimes for the quantum transport through impurity: i) $\mathcal K$ $\gg$ $\mathcal S$ $\gg$ $1$, differential conductance vanishes and ii) $\mathcal S$$/$$\mathcal K{=}\mathcal C$ with $ 0$$<$$\mathcal C$$<$$1/2$, differential conductance reaches some non-vanishing fraction of its unitary value. Using conformal field theory approach we analyze behavior of the quantum transport observables and residual entropy in both semiclassical regimes. We show that the semiclassical limit ii) preserves the key features of resonance scattering and the most essential fingerprints of the non-Fermi liquid behavior. We discuss possible realization of two semiclassical regimes in semiconductor quantum transport experiments.

cond-mat.str-el↗

arXiv2020

Coulomb blockade oscillations of heat conductance in the charge Kondo regime

We develop a method of theoretically investigating the charge, energy and heat transport in the presence of the charge Kondo correlations. The Coulomb blockade oscillations of heat conductance in the single electron transistor exhibiting the charge Kondo effects are investigated. We explore the Wiedemann Franz ratio in both charge-single channel and charge-two channel Kondo regime. The close connections of our findings with the recent experiments on multi-channel charge Kondo effects are discussed.

cond-mat.mes-hall↗

arXiv2020

Quantum thermoelectric and heat transport in the overscreened Kondo regime: Exact conformal field theory results

We develop a conformal-field theory approach for investigation of the quantum charge-, heat- and thermoelectric- transport through a quantum impurity fine tuned to a non-Fermi liquid regime. The non-Fermi-liquid operational mode is associated with the overscreened spin Kondo effect and controlled by the number of orbital channels. The universal low-temperature scaling and critical exponents for Seebeck and Peltier coefficients are investigated for the multichannel geometry. We derive and analyze the universal dependence of the thermoelectric coefficients on the number of orbital channels. We discuss the universality of Lorenz ratio and power factor beyond the Fermi Liquid paradigm. Different methods of verifying our findings based on the recent experiments are proposed.

cond-mat.mes-hall↗

arXiv2020

Wiedemann-Franz law in scattering theory revisited

The violation of Wiedemann-Franz (WF) law has been widely discussed in quantum transport experiments as an indication of deviation from Fermi-liquid behavior. The conventional form of WF law is only concerned with the transmission function at Fermi-level which, however, vanishes in many practical situations. We reinvestigate the WF law in noninteracting quantum systems with vanishing zero energy transmission and report a universal number $21/5$ as an upper bound of Lorenz ratio $\mathscr{R}$ in weakly energy-dependent scattering theory. We provide different experimental realizations for the observation of $\mathscr{R}=21/5$ namely the transport setups with graphene, the multi-level quantum dot and double quantum dot. The reported universal Lorenz ratio paves an efficient way of experimentally obtaining the information about the associated quantum interferences in the system. Our work also provides enough evidence which concludes that the violation of WF law does not necessarily imply the non-Fermi-liquid nature of underlying transport processes; equally, the Fermi-liquid transport characteristics cannot be concluded by an observed validation of WF law.

cond-mat.mes-hall↗

arXiv2019

Effects of strong electron interactions and resonance scattering on power output of nano-devices

We develop a Fermi-liquid based approach to investigate the power output of nano devices in the presence of strong interactions and resonance scattering. The developed scheme is then employed to study the power output of a SU($N$) Kondo impurity at the strong-coupling regime. The interplay between Kondo resonance and the filling-factors in the SU($N$) quantum systems is found to be a key to enhance output power. Such enhancement results an output power corresponding to $50\%$ of the quantum upper bound. We demonstrate that given a proper tuning of the electron occupancy, the investigated power grows linearly with degeneracy of Kondo state ($N$). This relation can hence be exploited to obtain output power that is larger than the one in existing non interacting setups.

cond-mat.mes-hall↗

arXiv2019

Nonlinear Seebeck effect of SU($N$) Kondo impurity

We develop a theoretical framework to study the influences of coupling asymmetry on the thermoelectrics of a strongly coupled SU($N$) Kondo impurity based on a local Fermi liquid theory. Applying non-equilibrium Keldysh formalism, we investigate charge current driven by the voltage bias and temperature gradient in the strong coupling regime of an asymmetrically coupled SU($N$) quantum impurity. The thermoelectric characterizations are made via non-linear Seebeck effects. We demonstrate that the beyond particle-hole (PH) symmetric SU($N$) Kondo variants are highly desirable with respect to the corresponding PH symmetric setups in order to have significantly improved thermoelectric performance. The greatly enhanced Seebeck coefficients by tailoring the coupling asymmetry of beyond PH symmetric SU($N$) Kondo effects are explored. Apart from presenting the analytical expressions of asymmetry dependent transport coefficients for general SU($N$) Kondo effects, we make a close connection of our findings with the experimentally studied SU(2) and SU(4) Kondo effects in quantum dot nano structures. Seebeck effects associated with the theoretically proposed SU(3) Kondo effects are discussed in detail.

cond-mat.mes-hall↗

arXiv2018

Full counting statistics of the two-stage Kondo effect

We developed a theoretical framework which extends the method of \textit{full counting statistics} (FCS) from conventional single channel Kondo screening schemes to multi-channel Kondo paradigm. The developed idea of FCS has been demonstrated considering an example of two-stage Kondo (2SK) model. We analyzed the charge transferred statistics in the strong-coupling regime of a 2SK model using non-equilibrium Keldysh formulation. A bounded value of Fano factor, $1\leq F\leq 5/3$, confirmed the cross-over regimes of charge transfered statistics in 2SK effect, from Poissonian to super-Poissonian. An innovative way of measuring transport properties of 2SK effect, by the independent measurements of charge current and noise, has been proposed

cond-mat.mes-hall↗

arXiv2018

Two-color Fermi liquid theory for transport through a multilevel Kondo impurity

We consider a quantum dot with ${\cal K}{\geq} 2$ orbital levels occupied by two electrons connected to two electric terminals. The generic model is given by a multi-level Anderson Hamiltonian. The weak-coupling theory at the particle-hole symmetric point is governed by a two-channel $S{=}1$ Kondo model characterized by intrinsic channels asymmetry. Based on a conformal field theory approach we derived an effective Hamiltonian at a strong-coupling fixed point. The Hamiltonian capturing the low-energy physics of a two-stage Kondo screening represents the quantum impurity by a two-color local Fermi-liquid. Using non-equilibrium (Keldysh) perturbation theory around the strong-coupling fixed point we analyze the transport properties of the model at finite temperature, Zeeman magnetic field and source-drain voltage applied across the quantum dot. We compute the Fermi-liquid transport constants and discuss different universality classes associated with emergent symmetries.

cond-mat.mes-hall↗

arXiv2017

Thermoelectric Transport through SU(N) Kondo Impurity

We investigate thermoelectric transport through a SU(N) quantum impurity in the Kondo regime. The strong coupling fixed point theory is described by the local Fermi-liquid paradigm. Using Keldysh technique we analyse the electric current through the quantum impurity at both finite bias voltage and finite temperature drop across it. The theory of a steady state at zero-current provides a complete description of the Seebeck effect. We find pronounced non-linear effects in temperature drop at low temperatures. We illustrate the significance of the non-linearities for enhancement of thermopower by two examples of SU(4) symmetric regimes characterized by a filling factor m: i) particle-hole symmetric at m=2 and ii) particle-hole non-symmetric at m=1. We analyse the effects of potential scattering and coupling asymmetry on the transport coefficients. We discuss connections between the theory and transport experiments with coupled quantum dots and carbon nanotubes.

cond-mat.mes-hall↗