arXiv subjects

Karyn Le Hur

Publications and source records attributed to Karyn Le Hur.

At least 127 records · Page 7Linked to original sources

arXiv2006

Heavy electrons from Hund's rule and short-range Antiferromagnetism

We investigate the one-dimensional ferromagnetic Kondo lattice in the hole-rich region. Of interest to us is the intermediate situation where the ferromagnetic Kondo coupling (Hund's coupling) is comparable to the electron bandwidth. The forced alignment favors triplet states whereas singlet states enter in a quite low-density regime. The direct antiferromagnetic exchange between the core spins is assumed to still prevail over the double exchange. We discuss a solvable limit showing that short-range antiferromagnetism in the spin array will affect the coherent propagation of triplets, i.e., turns a light triplet onto a heavy singlet, resulting in a heavy electron ground state.

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arXiv2006

The electron lifetime in Luttinger liquids

We investigate the decoherence of the electron wavepacket in purely ballistic one-dimensional systems described through the Luttinger liquid (LL). At a finite temperature $T$ and long times $t$, we show that the electron Green's function for a fixed wavevector close to one Fermi point decays as $\exp(-t/τ_F)$, as opposed to the power-law behavior occurring at short times, and the emerging electron lifetime obeys $τ_F^{-1}\propto T$ for spinful as well as spinless electrons. For strong interactions, $(Tτ_F)\ll 1$, reflecting that the electron is not a good Landau quasiparticle in LLs. We justify that fractionalization is the main source of electron decoherence for spinful as well as spinless electrons clarifying the peculiar electron mass renormalization close to the Fermi points. For spinless electrons and weak interactions, our intuition can be enriched through a diagrammatic approach or Fermi Golden rule and through a Johnson-Nyquist noise picture. We stress that the electron lifetime (and the fractional quasiparticles) can be revealed from Aharonov-Bohm experiments or momentum resolved tunneling. We aim to compare the results with those of spin-incoherent and chiral LLs.

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arXiv2006

Magnetically induced oscillations of the spin polarization in the Datta-Das geometry

The control of intrinsic magnetic degrees of freedom is very important as it offers a practical means to manipulate and probe electron spin transport. Tunable spin-orbit effect in quantum wires can in principle serve as a means to achieve this goal. Here, we investigate within the scattering matrix approach the effect of an in-plane magnetic field on the conductance of the quantum wire in the Datta-Das geometry and show that the interplay of the spin-orbit interaction with the magnetic field provides enhanced control over the electron spin polarization. In particular, we predict a novel effect of magnetically induced oscillations of the electron spin in a certain range of magnetic field.

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arXiv2006

A Mesoscopic Resonating Valence Bond system on a triple dot

We introduce a mesoscopic pendulum from a triple dot. The pendulum is fastened through a singly-occupied dot (spin qubit). Two other strongly capacitively islands form a double-dot charge qubit with one electron in excess oscillating between the two low-energy charge states (1,0) and (0,1); this embodies the weight of the pendulum. The triple dot is placed between two superconducting leads as shown in Fig. 1. Under well-defined conditions, the main proximity effect stems from the injection of resonating singlet (valence) bonds on the triple dot. This gives rise to a Josephson current that is charge- and spin-dependent. Consequences in a SQUID-geometry are carefully investigated.

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arXiv2005

Decoherence of Einstein-Podolsky-Rosen pairs in a noisy Andreev entangler

We investigate quantum noise effect on the transportation of nonlocal Cooper pairs accross the realistic Andreev entangler which consists of an s-wave superconductor coupled to two small quantum dots at resonance which themselves are coupled to normal leads. The noise emerges due to voltage fluctuations felt by the electrons residing on the two dots as a result of the finite resistances in the gate leads or of any resistive lead capacitively coupled to the dots. In the ideal noiseless case, the setup provides a trustable source of mobile and nonlocal spin-entangled electrons and the transport is dominated by a two-particle Breit-Wigner resonance that allows the injection of two spin-entangled electrons into different leads at the same energy [P. Recher, E. V. Sukhorukov, and D. Loss, Phys. Rev. B 63, 165314 (2001)]. We seek to revisit the transport of those nonlocal Cooper pairs as well as the efficiency of such an Andreev entangler when including the quantum noise (decoherence).

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arXiv2005

Coulomb drag between two spin incoherent Luttinger liquids

In a one dimensional electron gas at low enough density, the magnetic (spin) exchange energy $J$ between neighboring electrons is exponentially suppressed relative to the characteristic charge energy, the Fermi energy $E_F$. At non-zero temperature $T$, the energy hierarchy $J \ll T \ll E_F$ can be reached, and we refer to this as the spin incoherent Lutinger liquid state. We discuss the Coulomb drag between two parallel quantum wires in the spin incoherent regime, as well as the crossover to this state from the low temperature regime by using a model of a fluctuating Wigner solid. As the temperature increases from zero to above $J$ for a fixed electron density, the $2k_F$ oscillations in the density-density correlations are lost. As a result, the temperature dependence of the Coulomb drag is dramatically altered and non-monotonic dependence may result. Drag between wires of equal and unequal density are discussed, as well as the effects of weak disorder in the wires. We speculate that weak disorder may play an important role in extracting information about quantum wires in real drag experiments.

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arXiv2005

Unification of electromagnetic noise and Luttinger liquid via a quantum dot

We investigate the effect of dissipation on a small quantum dot (resonant level) tunnel-coupled to a chiral Luttinger liquid (LL) with the LL parameter $K$. The dissipation stems from the coupling of the dot to an electric environment, being characterized by the resistance $R$, via Coulomb interactions. We show that this problem can be mapped onto a Caldeira-Leggett model where the (ohmic) bath of harmonic oscillators is governed by the effective dissipation strength $α=(2\tilde{K})^{-1}$ with $\tilde{K}^{-1}=K^{-1}+2R/R_K$ and $R_K=h/e^2$ the quantum of resistance. Experimental consequences are discussed and the limit $K=1/2^+$ is thoroughly studied at small $R/R_K$ through the spin-boson-fermion model.

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arXiv2005

Hidden Caldeira-Leggett dissipation in a Bose-Fermi Kondo model

We show that the Bose-Fermi Kondo model (BFKM), which may find applicability both to certain dissipative mesoscopic qubit devices and to heavy fermion systems described by the Kondo lattice model, can be mapped exactly onto the Caldeira-Leggett model. This mapping requires an ohmic bosonic bath and an Ising-type coupling between the latter and the impurity spin. This allows us to conclude unambiguously that there is an emergent Kosterlitz-Thouless quantum phase transition in the BFKM with an ohmic bosonic bath. By applying a bosonic numerical renormalization group approach, we thoroughly probe physical quantities close to the quantum phase transition.

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arXiv2005

Transport in a spin incoherent Luttinger liquid

We theoretically investigate transport in a spin incoherent one dimensional electron system, which may be realized in quantum wires at low electron density and finite temperature. Both the pure and disordered cases are considered, both in finite wires and in the thermodynamic limit. The effect of Fermi liquid leads attached to the finite length system is also addressed. In the infinite system, we find a phase diagram identical to that obtained for a spinless Luttinger Liquid, provided we make the identification $g=2g_c$, where $g$ is the interaction parameter in a spinless Luttinger Liquid and $g_c$ is the interaction parameter of the charge sector of a Luttinger Liquid theory for electrons with spin. For a finite length wire attached to Fermi liquid leads the transport depends on details of the disorder in the wire. A simple picture for the cross-over from the spin incoherent regime to the spin coherent regime as the temperature is varied is also discussed, as well as some physical implications.

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arXiv2005

Dephasing from electron fractionalization

We investigate the dephasing of mesoscopic interferences by electron-electron interactions in a strictly one-dimensional geometry composed of two weakly-coupled (clean and very long) Luttinger liquids. The main goal of this paper is to demonstrate that interactions in one dimension can produce a visible attenuation of Aharonov-Bohm oscillations. Through a Nyquist noise type description of the interactions and a direct (exact) calculation based on the Luttinger theory, in our setup we firmly emphasize that the dephasing time results from the electron fractionalization time.

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arXiv2005

Probing spin and orbital Kondo effects with a mesoscopic interferometer

We investigate theoretically the transport properties of a closed Aharonov-Bohm interferometer containing two quantum dots in the strong coupling regime. We find two distinct physical scenarios depending on the strength of the interdot Coulomb interaction. When the interdot Coulomb interaction is negligible only spin fluctuations are important and each dot develops a Kondo resonance at the Fermi level independently of the applied magnetic flux. The transport is characterized by the interference of these two independent Kondo resonances. On the contrary, for large interdot interaction, only one electron can be accommodated onto the double dot system. In this situation, not only the spin can fluctuate but also the orbital degree of freedom (the pseudo-spin). As a result, we find different ground states depending on the value of the applied flux. When $ϕ=π$ (mod $2π$) ($ϕ=2πΦ/Φ_0$, where $Φ$ is applied flux, and $Φ_0=h/e$ the flux quantum) the electronic transport can take place via simultaneous correlations in the spin and pseudo-spin sectors, leading to the highly symmetric SU(4) Kondo state. Nevertheless, we find situations with $ϕ>0$ (mod $2π$) where the pseudo-spin quantum number is not conserved during tunneling events, giving rise to the common SU(2) Kondo state with an enhanced Kondo temperature. We investigate the crossover between both ground states and discuss possible experimental signatures of this physics as a function of the applied magnetic flux.

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arXiv2004

Double-dot charge qubit and transport via dissipative cotunneling

We investigate transport through an exotic ``charge'' qubit composed of two strongly capacitively coupled quantum dots (QDs), each being independently connected to a side gate which in general exhibits a fluctuating electrostatic field ({\em i.e.}, Nyquist/Johnson noise). Two quantum phases are found: the ``Kondo'' phase where an orbital-Kondo entanglement emerges and a ``local moment'' phase in which the noise destroys the Kondo effect leaving the orbital spin unscreened and resulting in a clear suppression of the conductance. In the Kondo realm, the transfer of charge across the setting is accompanied by zero-point charge fluctuations in the two dissipative environments and then the I-V characteristics are governed by what we call ``dissipative cotunneling''.

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arXiv2004

Do quantum dots allow one access to pseudogap Kondo physics?

For the last decade, tunable quantum dot systems have allowed the investigation of Kondo physics wherein the quenching of a single spin on an artificial atom affects the conductance. The pseudogap Kondo model featuring a density of states $ρ(ε)$ = C$|ε|^{r}$, introduced by Withoff and Fradkin in 1990 was predicted to exhibit Kondo-like physics above a critical value of the Kondo coupling, J$_c$, which several groups have shown by numerical renormalization group (RG) is finite for r$< {1/2}$. Gonzalez-Buxton {\it{et al}} showed that the strong coupling limit of the particle-hole symmetric model leads to a non-trivial $\frac{π(1-r)}{2}$ phase shift at low temperatures indicating incomplete screening of the local moment, while away from particle-hole symmetry one generically flows towards a ground state with $δ\sim π$. We examine the implications of this model for quantum dots whose leads are Fermi-liquid-like, yet possess a tunneling density of states which is suppressed at the Fermi energy as a power law.

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arXiv2004

Coulomb blockade of a noisy metallic box: A realization of Bose-Fermi Kondo models

We focus on a metallic quantum dot coupled to a reservoir of electrons through a single-mode quantum point contact and capacitively connected to a back-gate, by taking into account that the gate voltage can exhibit noise; This will occur when connecting the gate lead to an external transmission line with a finite Ohmic resistance. The voltage (charge) fluctuations at the back gate can be described through a Caldeira-Leggett model of harmonic oscillators. For weak tunneling between the lead qnd the dot, exploiting the anisotropic Bose-Fermi Kondo model, we show that zero-point fluctuations of the environment can markedly alter the previous Matveev Kondo fixed point leading to an amplification of the Coulomb blockade phenomenon.

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arXiv2003

Maximized Orbital and Spin Kondo effects in a single-electron transistor

We investigate the charge fluctuations of a single-electron box (metallic grain) coupled to a lead via a smaller quantum dot in the Kondo regime. The most interesting aspect of this problem resides in the interplay between spin Kondo physics stemming from the screening of the spin of the small dot and orbital Kondo physics emerging when charging states of the grain with (charge) Q=0 and Q=e are almost degenerate. Combining Wilson's numerical renormalization-group method with perturbative scaling approaches we push forward our previous work [K. Le Hur and P. Simon, Phys. Rev. B 67, 201308R (2003)]. We emphasize that for symmetric and slightly asymmetric barriers, the strong entanglement of charge and spin flip events in this setup inevitably results in a non trivial stable SU(4) Kondo fixed point near the degeneracy points of the grain. By analogy with a small dot sandwiched between two leads, the ground state is Fermi-liquid like which considerably smears out the Coulomb staircase behavior and hampers the Matveev logarithmic singularity to arise. Most notably, the associated Kondo temperature $T_K^{SU(4)}$ might be raised compared to that in the conductance experiments through a small quantum dot $(\sim 1K)$ which makes the observation of our predictions a priori accessible. We discuss the robustness of the SU(4) correlated state against the inclusion of an external magnetic field, a deviation from the degeneracy points, particle-hole symmetry in the small dot, asymmetric tunnel junctions and comment on the different crossovers.

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arXiv2003

Fractional plateaus in the Coulomb blockade of coupled quantum dots

Ground-state properties of a double-large-dot sample connected to a reservoir via a single-mode point contact are investigated. When the interdot transmission is perfect and the dots controlled by the same dimensionless gate voltage, we find that for any finite backscattering from the barrier between the lead and the left dot, the average dot charge exhibits a Coulomb-staircase behavior with steps of size e/2 and the capacitance peak period is halved. The interdot electrostatic coupling here is weak. For strong tunneling between the left dot and the lead, we report a conspicuous intermediate phase in which the fractional plateaus get substantially altered by an increasing slope.

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arXiv2003

Smearing of charge fluctuations in a grain by spin-flip assisted tunneling

We investigate the charge fluctuations of a grain (large dot) coupled to a lead via a small quantum dot in the Kondo regime. We show that the strong entanglement of charge and spin flips in this setup can result in a stable SU(4) Kondo fixed point, which considerably smears out the Coulomb staircase behavior already in the weak tunneling limit. This behavior is robust enough to be experimentally observable.

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arXiv2002

Electron fractionalization induced dephasing in Luttinger liquids

Using the appropriate fractionalization mechanism, we correctly derive the temperature (T) and interaction dependence of the electron lifetime $τ_F$ in Luttinger liquids. For strong enough interactions, we report that $(Tτ_F)\propto g$, with $g\ll 1$ being the standard Luttinger exponent; This reinforces that electrons are {\it not} good quasiparticles. We immediately emphasize that this is of importance for the detection of electronic interferences in ballistic 1D rings and carbon nanotubes, inducing ``dephasing'' (strong reduction of Aharonov-Bohm oscillations).

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