arXiv subjects

David A. Ruiz-Tijerina

Publications and source records attributed to David A. Ruiz-Tijerina.

23 records · Page 2Linked to original sources

arXiv2017

Transport signatures of Kondo physics and quantum criticality in graphene with magnetic impurities

Localized magnetic moments have been predicted to develop in graphene samples with vacancies or adsorbates. The interplay between such magnetic impurities and graphene's Dirac quasiparticles leads to remarkable many-body phenomena, which have so far proved elusive to experimental efforts. In this article, we study the thermodynamic, spectral and transport signatures of quantum criticality and Kondo physics of a dilute ensemble of atomic impurities in graphene. We consider vacancies and adatoms that either break or preserve graphene's $C_{3v}$ and inversion symmetries. In a neutral graphene sample, all cases display symmetry-dependent quantum criticality, leading to enhanced impurity scattering for asymmetric impurities, in a manner analogous to bound-state formation by nonmagnetic resonant scatterers. Kondo correlations emerge only in the presence of a back gate, with estimated Kondo temperatures well within the experimentally accessible domain for all impurity types. For symmetry-breaking impurities at charge neutrality, quantum criticality is signaled by $T^{-2}$ resistivity scaling, leading to full insulating behavior at low temperatures, while low-temperature resistivity plateaus appear both in the non-critical and Kondo regimes. By contrast, the resitivity contribution from symmetric vacancies and hollow-site adsorbates vanishes at charge neutrality and for arbitrary back gate voltages, respectively. This implies that local probing methods are required for the detection of both Kondo and quantum critical signatures in these symmetry-preserving cases.

cond-mat.mes-hall↗

arXiv2016

Symmetry-protected coherent transport for diluted vacancies and adatoms in graphene

We study the effects of a low concentration of adatoms or single vacancies in the linear-response transport properties of otherwise clean graphene. These impurities were treated as localized orbitals, and for each type two cases with distinct coupling symmetries were studied. For adatoms, we considered top- and hollow-site adsorbates (TOP and HS). For vacancies, we studied impurity formation by soft bond reconstruction (REC), as well as the more symmetric case of charge accumulation in unreconstructed vacancies (VAC). Our results indicate that the transport is determined by usual impurity scattering when the graphene-impurity coupling does not possess $C_{3v}$ symmetry (TOP and REC). In contrast, VAC impurities decouple from the electronic states at the Dirac points, and yield no contribution to the resistivity for a sample in charge neutrality. Furthermore, the inversion-symmetry-conserving HS impurities also decouple from entire sets of momenta throughout the Brillouin zone, and do not contribute to the resistivity within a broad range of parameters. These behaviors are protected by $C_{3v}$ and inversion symmetry, respectively, and persist for more general impurity models.

cond-mat.mes-hall↗

arXiv2015

Interaction effects on a Majorana zero mode leaking into a quantum dot

We have recently shown [Phys. Rev. B {\bf 89}, 165314 (2014)] that a non--interacting quantum dot coupled to a one--dimensional topological superconductor and to normal leads can sustain a Majorana mode even when the dot is expected to be empty, \emph{i.e.}, when the dot energy level is far above the Fermi level of he leads. This is due to the Majorana bound state of the wire leaking into the quantum dot. Here we extend this previous work by investigating the low--temperature quantum transport through an {\it interacting} quantum dot connected to source and drain leads and side--coupled to a topological wire. We explore the signatures of a Majorana zero--mode leaking into the quantum dot for a wide range of dot parameters, using a recursive Green's function approach. We then study the Kondo regime using numerical renormalization group calculations. We observe the interplay between the Majorana mode and the Kondo effect for different dot-wire coupling strengths, gate voltages and Zeeman fields. Our results show that a "0.5" conductance signature appears in the dot despite the interplay between the leaked Majorana mode and the Kondo effect. This robust feature persists for a wide range of dot parameters, even when the Kondo correlations are suppressed by Zeeman fields and/or gate voltages. The Kondo effect, on the other hand, is suppressed by both Zeeman fields and gate voltages. We show that the zero--bias conductance as a function of the magnetic field follows a well--known universality curve. This can be measured experimentally, and we propose that the universal conductance drop followed by a persistent conductance of $0.5\,e^2/h$ is evidence of the presence of Majorana--Kondo physics. These results confirm that this "0.5" Majorana signature in the dot remains even in the presence of the Kondo effect.

cond-mat.mes-hall↗

arXiv2014

Capacitive interactions and Kondo effect tuning in double quantum impurity systems

We present a study of the correlated transport regimes of a double quantum impurity system with mutual capacitive interactions. Such system can be implemented by a double quantum dot arrangement or by a quantum dot and nearby quantum point contact, with independently connected sets of metallic terminals. Many--body spin correlations arising within each dot--lead subsystem give rise to the Kondo effect under appropriate conditions. The otherwise independent Kondo ground states may be modified by the capacitive coupling, decisively modifying the ground state of the double quantum impurity system. We analyze this coupled system through variational methods and the numerical renormalization group technique. Our results reveal a strong dependence of the coupled system ground state on the electron--hole asymmetries of the individual subsystems, as well as on their hybridization strengths to the respective reservoirs. The electrostatic repulsion produced by the capacitive coupling produces an effective shift of the individual energy levels toward higher energies, with a stronger effect on the `shallower' subsystem (that closer to resonance with the Fermi level), potentially pushing it out of the Kondo regime and dramatically changing the transport properties of the system. The effective remote gating that this entails is found to depend nonlinearly on the capacitive coupling strength, as well as on the independent subsystem levels. The analysis we present here of this mutual interaction should be important to fully characterize transport through such coupled systems.

cond-mat.str-el↗

arXiv2012

Dynamical magnetic anisotropy and quantum phase transitions in a vibrating spin-1 molecular junction

We study the electronic transport through a spin-1 molecule in which mechanical stretching produces a magnetic anisotropy. In this type of device, a vibron mode along the stretching axis will couple naturally to the molecular spin. We consider a single molecular vibrational mode and find that the electron-vibron interaction induces an effective correction to the magnetic anisotropy that shifts the ground state of the device toward a non-Fermi liquid phase. A transition into a Fermi liquid phase could then be achieved, by means of mechanical stretching, passing through an underscreened spin-1 Kondo regime. We present numerical renormalization group results for the differential conductance, the spectral density, and the magnetic susceptibility across the transition.

cond-mat.str-el↗