arXiv subjects

Pablo San-Jose

Publications and source records attributed to Pablo San-Jose.

68 records · Page 4Linked to original sources

arXiv2010

Zero Landau level in folded graphene nanoribbons

Graphene nanoribbons can be folded into a double layer system keeping the two layers decoupled. In the Quantum Hall regime folds behave as a new type of Hall bar edge. We show that the symmetry properties of the zero Landau level in metallic nanoribbons dictate that the zero energy edge states traversing a fold are perfectly transmitted onto the opposite layer. This result is valid irrespective of fold geometry, magnetic field strength and crystallographic orientation of the nanoribbon. Backscattering suppression on the N=0 Hall plateau is ultimately due to the orthogonality of forward and backward channels, much like in the Klein paradox.

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arXiv2010

Singular elastic strains and magnetoconductance of suspended graphene

Graphene membranes suspended off electric contacts or other rigid supports are prone to elastic strain, which is concentrated at the edges and corners of the samples. Such a strain leads to an algebraically varying effective magnetic field that can reach a few Tesla in sub-micron wide flakes. In the quantum Hall regime the interplay of the effective and the physical magnetic fields causes backscattering of the chiral edge channels, which can destroy the quantized conductance plateaus.

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arXiv2010

Low-energy instability of flexural phonons in graphene

We study the effect exerted by the electrons on the flexural phonons in graphene, accounting for the attractive interaction created by the exchange of electron-hole excitations. Combining the self-consistent computation of the phonon self-energy with renormalization group methods, we show that graphene has two different phases corresponding to soft and strong renormalization of the bending rigidity in the long-wavelength limit. In the first case, the system may have an intermediate scale in which the phonon dispersion is softened, but it manages finally to become increasingly rigid over large distance scales. The strongly renormalized phase is closer however to critical behavior, with an effective rigidity that becomes pinned in practice at very small values, implying a very large susceptibility for the development of a condensate of the flexural phonon field.

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arXiv2009

Quantum pumping in graphene

We show that graphene-based quantum pumps can tap into evanescent modes, which penetrate deeply into the device as a consequence of Klein tunneling. The evanescent modes dominate pumping at the Dirac point, and give rise to a universal response under weak driving for short and wide pumps, in close analogy to their role for the minimal conductivity in ballistic transport. In contrast, evanescent modes contribute negligibly to normal pumps. Our findings add a new incentive for the exploration of graphene-based nanoelectronic devices.

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arXiv2009

Resonant all-electric spin pumping with spin-orbit coupling

All-electric devices for the generation and filtering of spin currents are of crucial importance for spintronics experiments and applications. Here we consider a quantum dot between two metallic leads in the presence of spin-orbit coupling, and we analyze in the frame of a scattering matrix approach the conditions for generating spin currents in an adiabatically driven two-terminal device. We then focus on a dot with two resonant orbitals and show by specific examples that both spin filtering and pure spin current generation can be achieved. Finally, we discuss the effect of the Coulomb interaction.

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arXiv2009

Disorder-induced pseudodiffusive transport in graphene nanoribbons

We study the transition from ballistic to diffusive and localized transport in graphene nanoribbons in the presence of binary disorder, which can be generated by chemical adsorbates or substitutional doping. We show that the interplay between the induced average doping (arising from the non-zero average of the disorder) and impurity scattering modifies the traditional picture of phase-coherent transport. Close to the Dirac point, intrinsic evanescent modes produced by the impurities dominate transport at short lengths, giving rise to a new regime analogous to pseudodiffusive transport in clean graphene, but without the requirement of heavily doped contacts. This intrinsic pseudodiffusive regime precedes the traditional ballistic, diffusive and localized regimes. The last two regimes exhibit a strongly modified effective number of propagating modes, and a mean free path which becomes anomalously large close to the Dirac point.

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arXiv2009

Pseudospin valve in bilayer graphene: towards graphene-based pseudospintronics

We propose a non-magnetic, pseudospin-based version of a spin valve, in which the pseudospin polarization in neighboring regions of a graphene bilayer is controlled by external gates. Numerical calculations demonstrate a large on-off ratio of such a device. This finding holds promise for the realization of pseudospintronics: a form of electronics based upon the manipulation of pseudospin analogous to the control of physical spin in spintronics applications.

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arXiv2008

Geometric phases in semiconductor spin qubits: Manipulations and decoherence

We describe the effect of geometric phases induced by either classical or quantum electric fields acting on single electron spins in quantum dots in the presence of spin-orbit coupling. On one hand, applied electric fields can be used to control the geometric phases, which allows performing quantum coherent spin manipulations without using high-frequency magnetic fields. On the other hand, fluctuating fields induce random geometric phases that lead to spin relaxation and dephasing, thus limiting the use of such spins as qubits. We estimate the decay rates due to piezoelectric phonons and conduction electrons in the circuit, both representing dominant electric noise sources with characteristically differing power spectra.

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arXiv2007

Spin dephasing due to a random Berry phase

We investigate relaxation and dephasing of an electron spin confined in a semiconductor quantum dot and subject to spin-orbit coupling. Even in vanishing magnetic field, B = 0, slow noise coupling to the electron's orbital degree of freedom leads to dephasing of the spin due to a random, in general non-Abelian Berry phase acquired by the spin. For illustration we first present a simple quasiclassical description, then consider a model with 2 orbital states only, and finally present a perturbative quantum treatment appropriate for an electron in a realistic (roughly parabolic, not too strongly confining) quantum dot. We further compare the effect of different sources of noise. While at large magnetic fields phonons dominate the relaxation processes, at low fields electron-hole excitations and possibly 1/f noise may dominate.

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arXiv2007

Pseudo-diffusive magnetotransport in graphene

Transport properties through wide and short ballistic graphene junctions are studied in the presence of arbitrary dopings and magnetic fields. No dependence on the magnetic field is observed at the Dirac point for any current cumulant, just as in a classical diffusive system, both in normal-graphene-normal and normal-graphene-superconductor junctions. This pseudo-diffusive regime is however extremely fragile respect to doping at finite fields. We identify the crossovers to a field-suppressed and a normal ballistic transport regime in the magnetic field - doping parameter space, and provide a physical interpretation of the phase diagram. Remarkably, pseudo-diffusive transport is recovered away from the Dirac point in resonance with Landau levels at high magnetic fields.

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arXiv2006

Geometrical spin dephasing in quantum dots

We study spin-orbit mediated relaxation and dephasing of electron spins in quantum dots. We show that higher order contributions provide a relaxation mechanism that dominates for low magnetic fields and is of geometrical origin. In the low-field limit relaxation is dominated by coupling to electron-hole excitations and possibly $1/f$ noise rather than phonons.

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arXiv2006

Effect of inelastic scattering on spin entanglement detection through current noise

We study the effect of inelastic scattering on the spin entanglement detection and discrimination scheme proposed by Egues, Burkard, and Loss [Phys. Rev. Lett. \textbf{89}, 176401 (2002)]. The finite-backscattering beam splitter geometry is supplemented by a phenomenological model for inelastic scattering, the charge-conserving voltage probe model, conveniently generalized to deal with entangled states. We find that the behavior of shot-noise measurements in one of the outgoing leads remains an efficient way to characterize the nature of the non-local spin correlations in the incoming currents for an inelastic scattering probability up to 50%. Higher order cumulants are analyzed, and are found to contain no additional useful information on the spin correlations. The technique we have developed is applicable to a wide range of systems with voltage probes and spin correlations.

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arXiv2004

Electron backscattering from dynamical impurities in a Luttinger liquid

Electron backscattering in a Luttinger liquid with an impurity is investigated in the presence of zero point motion of the phonon lattice. The impurity can mean either a mass defect, an elastic defect or a pinning defect. The phonon spectrum is then affected by the presence of the defect, which enters in the renormalization group equations for backscattering. The RG equation becomes dependent on the energy cutoff for finite Debye frequency $ω_D$ giving rise to finite energy effects. We compute the local density of states and show how the renormalization group flow is affected by a finite $ω_D$.

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arXiv2004

Interplay between exchange interactions and charging effects in metallic grains

We study the effect of the exchange interactions in small superconducting grains near the Coulomb blockade regime. We extend the standard description of the grain in terms of a single collective variable, the charge and its conjugated phase, to include the spin degree of freedom. The suppression of spin fluctuations enhances the tendency towards Coulomb blockade. The effective charging energy and conductance are calculated numerically in the regime of large grain-lead coupling.

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