arXiv subjects

N. Sandler

Publications and source records attributed to N. Sandler.

15 recordsLinked to original sources

arXiv2024

Valley-dependent transport through graphene quantum dots due to proximity-induced, staggered spin-orbit couplings

We study a system composed of graphene decorated with an array of islands with C_3v symmetry that induce quantum dot (IQD) regions via proximity effects and give rise to several spin-orbit couplings (SOCs). We evaluate transport properties for an array of IQDs and analyze the conditions for realizing isolated valley conductances and valley-state localization. The resulting transmission shows a square-type behavior with wide gaps that can be tuned by adjusting the strength of the staggered intrinsic SOCs. Realistic proximity effects are characterized by weak SOC strengths, and the analysis of our results in this regime shows that the Rashba coupling is the important interaction controlling valley properties. As a consequence, a top gate voltage can be used to tune the valley polarization and switch the valley scattering for positive or negative incident energies. A proper choice of SOC strengths leads to higher localization of valley states around the linear array of IQDs. These systems can be implemented in heterostructures composed of graphene and semiconducting transition-metal dichalcogenides (TMDs) such as MoSe2, WSe2, MoS2, or WS2. In these setups, the magnitudes of induced SOCs depend on the twist angle, and due to broken valley degeneracy, valley-polarized currents at the edges can be generated in a controllable manner as well as localized valley states. Our findings suggest an alternative approach for producing valley-polarized currents and propose a corresponding mechanism for valley-dependent electron optics and optoelectronic devices.

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arXiv2019

Valley polarization braiding in strained graphene

Previous works on deformed graphene predict the existence of valley-polarized states, however, optimal conditions for their detection remain challenging. We show that in the quantum Hall regime, edge-like states in strained regions can be isolated in energy within Landau gaps. We identify precise conditions for new conducting edges-like states to be valley polarized, with the flexibility of positioning them at chosen locations in the system. A map of local density of states as a function of energy and position reveals a unique braid pattern that serves as a fingerprint to identify valley polarization.

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arXiv2018

Quantum Wires and Waveguides Formed in Graphene by Strain

Confinement of electrons in graphene to make devices has proven to be a challenging task. Electrostatic methods fail because of Klein tunneling, while etching into nanoribbons requires extreme control of edge terminations, and bottom-up approaches are limited in size to a few nanometers. Fortunately, its mechanical flexibility raises the possibility of using strain to alter graphene's properties and create novel straintronic devices. Here, we report transport studies of nanowires created by linearly-shaped strained regions resulting from individual folds formed by layer transfer onto hexagonal boron nitride. Conductance measurements across the folds reveal Coulomb blockade signatures, indicating confined charges within these structures, which act as quantum dots. Along folds, we observe sharp features in traverse resistivity measurements, attributed to an amplification of the dot conductance modulations by a resistance bridge incorporating the device. Our data indicates ballistic transport up to ~1 um along the folds. Calculations using the Dirac model including strain are consistent with measured bound state energies and predict the existence of valley-polarized currents. Our results show that graphene folds can act as straintronic quantum wires.

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arXiv2016

Strained fold assisted transport in graphene systems

Deformations in graphene systems are central elements in the novel field of {\it straintronics}. Various strain geometries have been proposed to produce specific properties but their experimental realization has been limited. Because strained folds can be engineered on graphene samples on appropriate substrates, we study their effects on graphene transport properties. We show the existence of an enhanced local density of states (LDOS) along the direction of the strained fold that originates from localization of higher energy states, and provides extra conductance channels at lower energies. In addition to exhibit sublattice symmetry breaking, these states are valley polarized, with quasi-ballistic properties in smooth disorder potentials. We confirmed that these results persist in the presence of strong edge disorder, making these geometries viable electronic waveguides. These findings could be tested in properly engineered experimental settings.

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arXiv2015

Fano resonances in hexagonal zigzag graphene rings under external magnetic flux

We study transport properties of hexagonal zigzag graphene quantum rings connected to semi-infinite nanoribbons. Open two-fold symmetric structures support localized states that can be traced back to those existing in the isolated six-fold symmetric rings. Using a tight-binding Hamiltonian within the Green's function formalism, we show that an external magnetic field promotes these localized states to Fano resonances with robust signatures in transport. Local density of states and current distributions of the resonant states are calculated as a function of the magnetic flux intensity. For structures on corrugated substrates we analyze the effect of strain by including an out-of-plane centro-symmetric deformation in the model. We show that small strains shift the resonance positions without further modifications, while high strains introduce new ones.

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arXiv2014

RKKY interaction and intervalley processes in p-doped transition metal dichalcogenides

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in p-doped transition metal dichalcogenides such as MoS$_2$ and WS$_2$. We consider magnetic impurities hybridized to the Mo d-orbitals characteristic of the valence bands. Using the Matsubara Green's function formalism, we obtain the two-impurity interaction vs their separation and chemical potential of the system, accounting for the important angular dependence which reflects the underlying triangular lattice symmetry. The inclusion of the valence band valley at the $Γ$ point results in a strong enhancement of the interaction. Electron scattering processes transferring momentum between valleys at different symmetry points give rise to complex spatial oscillation patterns. Variable doping would allow the exploration of rather interesting behavior in the interaction of magnetic impurities on the surfaces of these materials, including the control of the interaction symmetry, which can be directly probed in STM experiments.

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arXiv2014

Gaussian deformations in graphene ribbons: flowers and confinement

The coupling of geometrical and electronic properties is a promising venue to engineer conduction properties in graphene. Confinement added to strain allows for interplay of different transport mechanisms with potential device applications. To investigate strain signatures on transport in confined geometries, we focus on graphene nanoribbons (GNR) with circularly symmetric deformations. In particular, we study GNR with an inhomogeneous, out of plane Gaussian deformation, connected to reservoirs. We observe an enhancement of the density of states in the deformed region, accompanied with a decrease in the conductance, signaling the presence of confined states. The local density of states exhibits a six-fold symmetric structure with an oscillating sub-lattice occupation asymmetry, that persist for a wide range of energy and model parameters.

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arXiv2013

Currents and pseudomagnetic fields in strained graphene rings

We study the effects of strain on the electronic properties and persistent current characteristics of a graphene ring using the Dirac representation. For a slightly deformed graphene ring flake, one obtains sizable pseudomagnetic (gauge) fields that may effectively reduce or enhance locally the applied magnetic flux through the ring. Flux-induced persistent currents in a flat ring have full rotational symmetry throughout the structure; in contrast, we show that currents in the presence of a circularly symmetric deformation are strongly inhomogeneous, due to the underlying symmetries of graphene. This result illustrates the inherent competition between the `real' magnetic field and the `pseudo' field arising from strains, and suggest an alternative way to probe the strength and symmetries of pseudomagnetic fields on graphene systems.

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arXiv2009

Kondo screening suppression by spin-orbit interaction in quantum dots

We study the transport properties of a quantum dot embedded in an Aharonov-Bohm ring in the presence of spin-orbit interactions. Using a numerical renormalization group analysis of the system in the Kondo regime, we find that the competition of Aharonov-Bohm and spin-orbit dynamical phases induces a strong suppression of the Kondo state singlet, somewhat akin to an effective intrinsic magnetic field in the system. This effective field breaks the spin degeneracy of the localized state and produces a finite magnetic moment in the dot. By introducing an {\em in-plane} Zeeman field we show that the Kondo resonance can be fully restored, reestablishing the spin singlet and a desired spin filtering behavior in the Kondo regime, which may result in full spin polarization of the current through the ring.

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arXiv2009

Kondo regime in triangular arrangements of quantum dots: Molecular orbitals, interference and contact effects

Transport properties of an interacting triple quantum dot system coupled to three leads in a triangular geometry has been studied in the Kondo regime. Applying mean-field finite-U slave boson and embedded cluster approximations to the calculation of transport properties unveils a set of rich features associated to the high symmetry of this system. Results using both calculation techniques yield excellent overall agreement and provide additional insights into the physical behavior of this interesting geometry. In the case when just two current leads are connected to the three-dot system, interference effects between degenerate molecular orbitals are found to strongly affect the overall conductance. An S=1 Kondo effect is also shown to appear for the perfect equilateral triangle symmetry. The introduction of a third current lead results in an `amplitude leakage' phenomenon, akin to that appearing in beam splitters, which alters the interference effects and the overall conductance through the system.

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arXiv2007

Phonon-assisted tunneling regimes in diatomic molecules

Electronic transport in diatomic molecules (two-level systems) connected to metallic contacts is analyzed in the presence of competing electron-electron and electron-phonon interactions. We show that phonon emission and absorption processes are strongly modified when a Coulomb energy $U$ is included, as the phonons open channels that can result in destructive or constructive interference effects. Resonance conditions for these processes produce dramatic effects both in the density of states at the molecular sites, as well as in the conductance through the system. We find in particular an enhanced {\it Rabi-assisted tunneling} due to phonons, as the resonance conditions are met, which is made more evident for increasing temperatures. These effects are controllable by voltage gating of the molecular sites, and should be accessible in current experiments.

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arXiv2005

Kondo Regime of a Quantum Dot Molecule: A Finite-U Slave-Boson Approach

We study the electronic transport in a double quantum dot structure connected to leads in the Kondo regime for both series and parallel arrangements. By applying a finite-U slave boson technique in the mean field approximation we explore the effect of level degeneracy in the conductance through the system. Our results show that for the series connection, as the energy difference of the localized dot levels increases, the tunneling via the Kondo state is destroyed. For the parallel configuration, we find an interesting interplay of state symmetry and conductance. Our results are in good agrement with those obtained with other methods, and provide additional insights into the physics of the Kondo state in the double dot system.

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arXiv2005

Phonon Rabi-assisted tunneling in diatomic molecules

We study electronic transport in diatomic molecules connected to metallic contacts in the regime where both electron-electron and electron-phonon interactions are important. We find that the competition between these interactions results in unique resonant conditions for interlevel transitions and polaron formation: the Coulomb repulsion requires additional energy when electrons attempt phonon-assisted interlevel jumps between fully or partially occupied levels. We apply the equations of motion approach to calculate the electronic Green's functions. The density of states and conductance through the system are shown to exhibit interesting Rabi-like splitting of Coulomb blockade peaks and strong temperature dependence under the it interacting resonant conditions.

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arXiv2003

Quantum and classical localization in the lowest Landau level

Spatial correlations of occupation probabilities, if their decay is not too fast, can change the critical exponents for classical percolation. From numerical studies of electron dynamics in the lowest Landau level (LLL) we demonstrate the quantum analog of this effect. Similar to classical percolation, we find that the extended Harris criterion applies to localization in the LLL. These results suggest experiments that might probe new quantum critical points in the integer quantum Hall setting.

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arXiv2000

Fractional quantum Hall junctions and two-channel Kondo models

A mapping between fractional quantum Hall (FQH) junctions and the two-channel Kondo model is presented. We discuss in detail this relation for the particular case of a junction of a FQH state at $ν= 1/3$ and a normal metal. We show that in the strong coupling regime this junction has a non-Fermi liquid fixed point. At this fixed point the electron Green's function has a branch cut, and the entropy has a non-zero value equal to $S={1/2} \ln{2}$. We construct the space of perturbations at the strong coupling fixed point and find that the dimensions of the tunneling operator is 1/2. These behaviors are strongly reminiscent of the non-Fermi liquid fixed point of a number of quantum impurity models, particularly the two-channel Kondo model. However we have found that, in spite of these similarities, the Hilbert spaces of these two systems are quite different. In particular, although in a special limit the Hamiltonians of both models are the same, their Hilbert spaces are not since they are determined by physically distinct boundary conditions. As a consequence the spectrum of operators in both models is different.

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