arXiv subjects

Talieh S. Ghiasi

Publications and source records attributed to Talieh S. Ghiasi.

16 recordsLinked to original sources

arXiv2025

Tunable Kondo effect in a bilayer graphene quantum channel

The interaction between itinerant electrons and localized spins is key to a wide range of electronic phenomena. Of particular interest is the regime where the interacting electrons exhibit both spin and valley degeneracy, resulting in SU(4) Kondo physics. However, this regime is challenging to realize in typical mesoscopic systems because it requires a strong interaction between electrons, resulting in a Kondo temperature ($T_\mathrm{K}$) significantly larger than the spin and valley splittings. Here, we present conductance measurements of a quantum point contact (QPC) in bilayer graphene (BLG). Beyond the expected quantized conductance plateaus, which reflect spin and valley degeneracy, we observe an additional subband, known as `0.7 anomaly' exhibiting signatures of Kondo physics and a $T_\mathrm{K}$ ranging from approximately 0.5 up to 2.4 K at zero magnetic field, corresponding to Kondo energies between 40 and 200 $μ$eV. Given that the spin-orbit splitting in BLG is between 40 and 80 $μ$eV, we argue that these results are consistent with a transition between four-fold degenerate SU(4) and two-fold degenerate spin-valley locked SU(2) Kondo effects. Furthermore, we break the valley degeneracy of the lowest subband by an out-of-plane magnetic field and show that Kondo signatures remain present, indicating a transition from SU(4) to a valley-polarized SU(2) Kondo effect, and showing the versatility of BLG QPCs for exploring many-body effects.

cond-mat.mes-hall↗

arXiv2025

Interplay of energy and charge transfer in WSe2/CrSBr heterostructures

Van der Waals heterostructures (vdWHs) composed of transition-metal dichalcogenides (TMDs) and layered magnetic semiconductors offer great opportunities to manipulate exciton and valley properties of TMDs. Here, we present magneto-photoluminescence (PL) studies in a WSe2 monolayer (ML) on a CrSBr crystal, an anisotropic layered antiferromagnetic semiconductor. Our results reveal unique behavior of each of the ML-WSe2 PL peaks under magnetic field that is distinct from the pristine case. An intriguing feature is the clear enhancement of the PL intensity that we observe each time the external magnetic field tunes the energy of an exciton in CrSBr into resonance with one of the optical states of WSe2. This result suggests a magnetic field-controlled resonant energy transfer (RET) beyond other effects reported in similar structures. Our work provides deep insight on the importance of different mechanisms into magnetic vdWHs and underscores its great potential for light harvesting and emission enhancement of two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2024

Quantum Spin Hall Effect in Magnetic Graphene

A promising approach to attain long-distance coherent spin propagation is accessing topological spin-polarized edge states in graphene. Achieving this without external magnetic fields necessitates engineering graphene band structure, obtainable through proximity effects in van der Waals heterostructures. In particular, proximity-induced staggered potentials and spin-orbit coupling are expected to form a topological bulk gap in graphene with gapless helical edge states that are robust against disorder. In this work, we detect the spin-polarized helical edge transport in graphene at zero external magnetic field, allowed by the proximity of an interlayer antiferromagnet, CrPS$_4$. We show the coexistence of the quantum spin Hall (QSH) states and magnetism in graphene, where the induced spin-orbit and exchange couplings also give rise to a large anomalous Hall (AH) effect. The detection of the QSH states at zero external magnetic field, together with the AH signal that persists up to room temperature, opens the route for practical applications of magnetic graphene in quantum spintronic circuitries.

cond-mat.mes-hall↗

arXiv2023

A ballistic electron source with magnetically-controlled valley polarization in bilayer graphene

The achievement of valley-polarized electron currents is a cornerstone for the realization of valleytronic devices. Here, we report on ballistic coherent transport experiments where two opposite quantum point contacts (QPCs) are defined by electrostatic gating in a bilayer graphene (BLG) channel. By steering the ballistic currents with an out-of-plane magnetic field we observe two current jets, a consequence of valley-dependent trigonal warping. Tuning the BLG carrier density and number of QPC modes (m) with a gate voltage we find that the two jets are present for m=1 and up to m=6, indicating the robustness of the effect. Semiclassical simulations which account for size quantization and trigonal warping of the Fermi surface quantitatively reproduce our data without fitting parameters, confirming the origin of the signals. In addition, our model shows that the ballistic currents collected for non-zero magnetic fields are valley-polarized independently of m, but their polarization depends on the magnetic field sign, envisioning such devices as ballistic current sources with tuneable valley-polarization.

cond-mat.mes-hall↗

arXiv2023

Nitrogen-vacancy magnetometry of CrSBr by diamond membrane transfer

Magnetic imaging using nitrogen-vacancy (NV) spins in diamonds is a powerful technique for acquiring quantitative information about sub-micron scale magnetic order. A major challenge for its application in the research on two-dimensional (2D) magnets is the positioning of the NV centers at a well-defined, nanoscale distance to the target material required for detecting the small magnetic fields generated by magnetic monolayers. Here, we develop a diamond 'dry-transfer' technique, akin to the state-of-the-art 2D-materials assembly methods, and use it to place a diamond micro-membrane in direct contact with the 2D interlayer antiferromagnet CrSBr. We harness the resulting NV-sample proximity to spatially resolve the magnetic stray fields generated by the CrSBr, present only where the CrSBr thickness changes by an odd number of layers. From the magnetic stray field of a single uncompensated ferromagnetic layer in the CrSBr, we extract a monolayer magnetization of $M_\mathrm{CSB}$ = 0.46(2) T, without the need for exfoliation of monolayer crystals or applying large external magnetic fields. The ability to deterministically place NV-ensemble sensors into contact with target materials and detect ferromagnetic monolayer magnetizations paves the way for quantitative analysis of a wide range of 2D magnets assembled on arbitrary target substrates.

cond-mat.mes-hall↗

arXiv2023

MoRe Electrodes with 10-nm Nanogaps for Electrical Contact to Atomically Precise Graphene Nanoribbons

Atomically precise graphene nanoribbons (GNRs) are predicted to exhibit exceptional edge-related properties, such as localized edge states, spin polarization, and half-metallicity. However, the absence of low-resistance nano-scale electrical contacts to the GNRs hinders harnessing their properties in field-effect transistors. In this paper, we make electrical contact with 9-atom-wide armchair GNRs using superconducting alloy MoRe as well as Pd (as a reference), which are two of the metals providing low-resistance contacts to carbon nanotubes. We take a step towards contacting a single GNR by fabrication of electrodes with a needle-like geometry, with about 20 nm tip diameter and 10 nm separation. To preserve the nano-scale geometry of the contacts, we develop a PMMA-assisted technique to transfer the GNRs onto the pre-patterned electrodes. Our device characterizations as a function of bias-voltage and temperature, show a thermally-activated gate-tunable conductance in the GNR-MoRe-based transistors.

cond-mat.mes-hall↗

arXiv2023

Specular electron focusing between gate-defined quantum point contacts in bilayer graphene

We report on multiterminal measurements in a ballistic bilayer graphene (BLG) channel where multiple spin and valley-degenerate quantum point contacts (QPCs) are defined by electrostatic gating. By patterning QPCs of different shapes and along different crystallographic directions, we study the effect of size quantization and trigonal warping on the transverse electron focusing (TEF) spectra. Our TEF spectra show eight clear peaks with comparable amplitude and weak signatures of quantum interference at the lowest temperature, indicating that reflections at the gate-defined edges are specular and transport is phase coherent. The temperature dependence of the scattering rate indicates that electron-electron interactions play a dominant role in the charge relaxation process for electron doping and temperatures below 100 K. The achievement of specular reflection, which is expected to preserve the pseudospin information of the electron jets, is promising for the realization of ballistic interconnects for new valleytronic devices.

cond-mat.mes-hall↗

arXiv2022

Towards fully two-dimensional spintronic devices

Within the field of spintronics major efforts are directed towards developing applications for spin-based transport devices made fully out of two-dimensional (2D) materials. In this work we present an experimental realization of a spin-valve device where the generation of the spin signal is exclusively attributed to the spin-dependent conductivity of the magnetic graphene resulting from the proximity of an interlayer antiferromagnet, CrSBr. We clearly demonstrate that the usage of the conventional 3D magnetic contacts, that are commonly air-sensitive and incompatible with practical technologies, can be fully avoided when graphene/CrSBr heterostructures are employed. Moreover, apart from providing exceptionally long spin relaxation length, the usage of graphene for both generation and transport of the spin allows to automatically avoid the conductivity mismatch between the source and the channel circuits that has to be considered when using conventional low-resistive contacts. Our results address a necessary step in the engineering of spintronic circuitry out of layered materials and precede further developments in the area of complex spin-logic devices. Moreover, we introduce a fabrication procedure where we designed and implemented a recipe for the preparation of electrodes via a damage-free technique that offers an immediate advantage in the fields of air-sensitive and delicate organic materials.

cond-mat.mes-hall↗

arXiv2020

The role of device asymmetries and Schottky barriers on the helicity-dependent photoresponse of 2D phototransistors

Circular photocurrents (CPC), namely circular photogalvanic (CPGE) and photon drag effects, have recently been reported both in monolayer and multilayer transition metal dichalcogenide (TMD) phototransistors. However, the underlying physics for the emergence of these effects are not yet fully understood. In particular, the emergence of CPGE is not compatible with the D3h crystal symmetry of two-dimensional TMDs, and should only be possible if the symmetry of the electronic states is reduced by influences such as an external electric field or mechanical strain. Schottky contacts, nearly ubiquitous in TMD-based transistors, can provide the high electric fields causing a symmetry breaking in the devices. Here, we investigate the effect of these Schottky contacts on the CPC by characterizing the helicity-dependent photoresponse of monolayer MoSe2 devices both with direct metal-MoSe2 Schottky contacts and with h-BN tunnel barriers at the contacts. We find that, when Schottky barriers are present in the device, additional contributions to CPC become allowed, resulting in emergence of CPC for illumination at normal incidence.

cond-mat.mtrl-sci↗

arXiv2020

Electrical and Thermal Generation of Spin Currents by Magnetic Graphene

The demand for compact, high-speed and energy-saving circuitry urges higher efficiency of spintronic devices that can offer a viable alternative for the current electronics. The route towards this goal suggests implementing two-dimensional (2D) materials that provide large spin polarization of charge current together with the long-distance transfer of the spin information. Here, for the first time, we experimentally demonstrate a large spin polarization of the graphene conductivity ($\approx 14\%$) arising from a strong induced exchange interaction in proximity to a 2D layered antiferromagnetic. The strong coupling of charge and spin currents in graphene with high efficiency of spin current generation, comparable to that of metallic ferromagnets, together with the observation of spin-dependent Seebeck and anomalous Hall effects, all consistently confirm the magnetic nature of graphene. The high sensitivity of spin transport in graphene to the magnetization of the outermost layer of the adjacent interlayer antiferromagnet, also provides a tool to read out a single magnetic sub-lattice. The first time observations of the electrical and thermal generation of spin currents by magnetic graphene suggest it as the ultimate building block for ultra-thin magnetic memory and sensory devices, combining gate tunable spin-dependent conductivity, long-distance spin transport and spin-orbit coupling all in a single 2D material.

cond-mat.mes-hall↗

arXiv2019

Charge-to-Spin Conversion by the Rashba-Edelstein Effect in 2D van der Waals Heterostructures up to Room Temperature

The proximity of a transition metal dichalcogenide (TMD) to graphene imprints a rich spin texture in graphene and complements its high quality charge/spin transport by inducing spin-orbit coupling (SOC). Rashba and valley-Zeeman SOCs are the origin of charge-to-spin conversion mechanisms such as Rashba-Edelstein effect (REE) and spin Hall effect (SHE). In this work, we experimentally demonstrate for the first time charge-to-spin conversion due to the REE in a monolayer WS2-graphene van der Waals heterostructure. We measure the current-induced spin polarization up to room temperature and control it by a gate electric field. Our observation of the REE and inverse of the effect (IREE) is accompanied by the SHE which we discriminate by symmetry-resolved spin precession under oblique magnetic fields. These measurements also allow for quantification of the efficiencies of charge-to-spin conversion by each of the two effects. These findings are a clear indication of induced Rashba and valley-Zeeman SOC in graphene that lead to generation of spin accumulation and spin current without using ferromagnetic electrodes. These realizations have considerable significance for spintronic applications, providing accessible routes towards all-electrical spin generation and manipulation in two-dimensional materials.

cond-mat.mes-hall↗

arXiv2019

Semiconductor channel mediated photodoping in h-BN encapsulated monolayer MoSe2 phototransistors

In optically excited two-dimensional phototransistors, charge transport is often affected by photodoping effects. Recently, it was shown that such effects are especially strong and persistent for graphene/h-BN heterostructures, and that they can be used to controllably tune the charge neutrality point of graphene. In this work we investigate how this technique can be extended to h BN encapsulated monolayer MoSe_2 phototransistors at room temperature. By exposing the sample to 785 nm laser excitation we can controllably increase the charge carrier density of the MoSe_2 channel by Δn {\approx} 4.45 {\times} 10^{12} cm^{-2}, equivalent to applying a back gate voltage of 60 V. We also evaluate the efficiency of photodoping at different illumination wavelengths, finding that it is strongly correlated with the light absorption by the MoSe_2 layer, and maximizes for excitation on-resonance with the A exciton absorption. This indicates that the photodoping process involves optical absorption by the MoSe_2 channel, in contrast with the mechanism earlier described for graphene/h-BN heterostroctures.

physics.app-ph↗

arXiv2018

Bilayer h-BN Barriers for Tunneling Contacts in Fully-Encapsulated Monolayer $\mathbf{MoSe_2}$ Field-Effect Transistors

The performance of electronic and spintronic devices based on two-dimensional semiconductors (2D SC) is largely dependent on the quality and resistance of the metal/SC electrical contacts, as well as preservation of the intrinsic properties of the SC channel. Direct Metal/SC interaction results in highly resistive contacts due to formation of large Schottky barriers and considerably affects the properties of the 2D SC. In this work, we address these two important issues in monolayer $\mathrm{MoSe_2}$ Field-Effect transistors (FETs). We encapsulate the $\mathrm{MoSe_2}$ channel with hexagonal Boron Nitride (h-BN), using bilayer h-BN at the metal/SC interface. The bilayer h-BN eliminates the metal/$\mathrm{MoSe_2}$ chemical interactions, preserves the electrical properties of $\mathrm{MoSe_2}$ and reduces the contact resistances by prevention of Fermi-level pinning. We investigate electrical transport in the monolayer $\mathrm{MoSe_2}$ FETs that yields close to intrinsic electron mobilities ($\approx 26\ \mathrm{cm^2 V^{-1} s^{-1}}$) even at room temperature. Moreover, we experimentally study the charge transport through Metal/h-BN/$\mathrm{MoSe_2}$ tunnel contacts and we explicitly show that the dielectric bilayer of h-BN provides highly efficient gating (tuning the Fermi energy) of the $\mathrm{MoSe_2}$ channel at the contact regions even with small biases. Also we provide a theoretical model that allows to understand and reproduce the experimental $I-V$ characteristics of the contacts. These observations give an insight into the electrical behavior of the metal/h-BN/2D SC heterostructure and introduce bilayer h-BN as a suitable choice for high quality tunneling contacts that allows for low energy charge and spin transport.

cond-mat.mes-hall↗

arXiv2018

Symmetry regimes for circular photocurrents in monolayer MoSe2

In monolayer transition metal dichalcogenides helicity-dependent charge and spin photocurrents can emerge, even without applying any electrical bias, due to circular photogalvanic and photon drag effects. Exploiting such circular photocurrents (CPC) in devices, however, requires better understanding of their behavior and physical origin. Here, we present symmetry, spectral, and electrical characteristics of CPC from excitonic interband transitions in a MoSe2 monolayer. The dependence on bias and gate voltages reveals two different CPC contributions, dominant at different voltages and with different dependence on illumination wavelength and incidence angles. We theoretically analyze symmetry requirements for effects that can yield CPC and compare these with the observed angular dependence and symmetries that occur for our device geometry. This reveals that the observed CPC effects require a reduced device symmetry, and that effects due to Berry curvature of the electronic states do not give a significant contribution.

cond-mat.mtrl-sci↗

arXiv2017

Observation of bright and dark exciton transitions in monolayer MoSe2 by photocurrent spectroscopy

We investigate the excitonic transitions in single- and few-layer MoSe2 phototransistors by photocurrent spectroscopy. The measured spectral profiles show a well-defined peak at the optically active (bright) A0 exciton resonance. More interestingly, when a gate voltage is applied to the MoSe2 to bring its Fermi level near the bottom of the conduction band, another prominent peak emerges at an energy 30 meV above the A0 exciton. We attribute this second peak to a gate-induced activation of the spin-forbidden dark exciton transition, AD0. Additionally, we evaluate the thickness-dependent optical bandgap of the fabricated MoSe2 crystals by characterizing their absorption edge.

cond-mat.mtrl-sci↗

arXiv2017

Large Proximity-Induced Spin Lifetime Anisotropy in Transition Metal Dichalcogenide/Graphene Heterostructures

Van-der-Waals heterostructures have become a paradigm for designing new materials and devices, in which specific functionalities can be tailored by combining the properties of the individual 2D layers. A single layer of transition metal dichalcogenide (TMD) is an excellent complement to graphene (Gr), since the high quality of charge and spin transport in Gr is enriched with the large spin-orbit coupling of the TMD via proximity effect. The controllable spin-valley coupling makes these heterostructures particularly attractive for spintronic and opto-valleytronic applications. In this work, we study spin precession in a monolayer MoSe2/Gr heterostructure and observe an unconventional, dramatic modulation of the spin signal, showing one order of magnitude longer lifetime of out-of-plane spins (40 ps) compared with that of in-plane spins (3.5 ps). This demonstration of a large spin lifetime anisotropy in TMD/Gr heterostructures, is a direct evidence of induced spin-valley coupling in Gr and provides an accessible route for manipulation of spin dynamics in Gr, interfaced with TMDs.

cond-mat.mes-hall↗