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M. Cahay

Publications and source records attributed to M. Cahay.

At least 19 recordsLinked to original sources
arXiv2017

Dependence of the 0.5(2e2/h) conductance plateau on the aspect ratio of InAs quantum point contacts with in-plane side gates

The observation of a 0.5 conductance plateau in asymmetrically biased quantum point contacts with in-plane side gates has been attributed to the onset of spin-polarized current through these structures. For InAs quantum point contacts with the same width but longer channel length, there is roughly a fourfold increase in the range of common sweep voltage applied to the side gates over which the 0.5 conductance plateau is observed when the QPC aspect ratio (ratio of length over width of the narrow portion of the structure) is increased by a factor 3. Non-equilibrium Green s function simulations indicate that the increase in the size of the 0.5 conductance plateau is due to an increased importance, over a larger range of common sweep voltage, of the effects of electron-electron interactions in QPC devices with larger aspect ratio. The use of asymmetrically biased QPCs with in-plane side gates and large aspect ratio could therefore pave the way to build robust spin injectors and detectors for the successful implementation of spin field effect transistors

cond-mat.mes-hall↗
arXiv2014

Hysteresis in the Conductance of Asymmetrically Biased GaAs Quantum Point Contacts with in-plane Side Gates

We have observed hysteresis between the forward and reverse sweeps of a common mode bias applied to the two in-plane side gates of an asymmetrically biased GaAs quantum point contact. The size of the hysteresis loop increases with the amount of bias asymmetry between the two side gates and depends on the polarity of the bias asymmetry. It is argued that hysteresis may constitute another indirect proof of spontaneous spin polarization in the narrow portion of the quantum point contact.

cond-mat.mes-hall↗
arXiv2013

Intrinsic Bistability In Quantum Point Contacts with in-plane Side Gates

We study the onset of intrinsic bistability and accompanying hysteresis in a single quantum point contact (QPC) with in-plane side gates in the presence of lateral spin-orbit coupling. The hysteresis in the conductance versus common gate voltage applied to the two side gates exists only if the narrow portion of the QPC is long enough. The hysteresis is absent if the effects of electron-electron interaction are neglected but increases with the strength of the electron-electron interaction. The hysteresis appears in the region of conductance anomalies, i.e., less than 2e2/h, and is due to multistable spin textures in these regions.

cond-mat.mes-hall↗
arXiv2013

Tunable All Electric Spin Polarizer

We propose a new device to create a tunable all-electric spin polarizer: a quantum point contact (QPC) with four gates -- two in-plane side gates in series. The first pair of gates, near the source, is asymmetrically biased to create spin polarization in the QPC channel. The second set of gates, near the drain, is symmetrically biased and that bias is varied to maximize the amount of spin polarization in the channel. The range of common mode bias on the first set of gates over which maximum spin polarization can be achieved is much broader for the four gate structure compared with the case of a single pair of in-plane side gates.

cond-mat.mes-hall↗
arXiv2012

Spin Polarization in a AlGaAs/GaAs Quantum Point Contact with in-plane side gates

We report the observation of an anomalous conductance plateau near G = 0.5 G0 (G0 = 2e2/h) in asymmetrically biased AlGaAs/GaAs quantum point contacts (QPCs), with in-plane side gates in the presence of lateral spin-orbit coupling. This is a signature of spin polarization in the narrow portion of the QPC. The appearance and evolution of the conductance anomaly has been studied at T=4.2K as a function of the potential asymmetry between the side gates. The observation of spontaneous spin polarization in a side-gated GaAs QPC could eventually lead to the realization of an all-electric spin-valve at tens of degrees Kelvin.

cond-mat.mes-hall↗
arXiv2011

Anamolous conductance plateau in an asymmetrically biased InAs/InAlAs quantum point contact

The appearance and evolution of an anomalous conductance plateau at 0.4 (in units of 2e2/h) in an In0.52Al0.48As/InAs quantum point contact (QPC), in the presence of lateral spin-orbit coupling, has been studied at T=4.2K as a function of the potential asymmetry between the in-plane gates of the QPC. The anomalous plateau, a signature of spin polarization in the channel, appears only over an intermediate range (around 3 V) of bias asymmetry. It is quite robust, being observed over a maximum range of nearly 1V of the sweep voltage common to the two in-plane gates. Our conductance measurements show evidence of surface roughness scattering from the side walls of the QPC. We show that a strong perpendicular magnetic field leads to magnetic confinement in the channel which reduces the importance of scattering from the side walls and favors the onset of near ballistic transport through the QPC.

cond-mat.mes-hall↗
arXiv2011

Improving the efficiency of organic light emitting diodes by use of a diluted light-emitting layer

The use of a thin mixed layer consisting of an inert diluent material and a light emitting material between the hole-transport layer and electron-transport layer of organic light-emitting diodes leads to an increase in the external quantum efficiency. The efficiency improvement is highly dependent on the thickness of the diluted light-emitting layer and driving current. Significant improvement seen at low current densities is explained in terms of effective hole confinement by the mixed layer while a modest decreases in efficiency at higher current densities may be attributed to luminescence quenching at the hole-transport layer/inert diluents material interface. The phenomena are demonstrated with three different inert diluents materials. A maximum external quantum efficiency improvement of about 40% is found for a diluted light-emitting layer thickness between 40 Å and 60 Å.

cond-mat.soft↗
arXiv2011

Rare-earth monosulfides as durable and efficient cold cathodes

In their rocksalt structure, rare-earth monosulfides offer a more stable alternative to alkali metals to attain low or negative electron affinity when deposited on various III-V and II-VI semiconductor surfaces. In this article, we first describe the successful deposition of Lanthanum Monosulfide via pulsed laser deposition on Si and MgO substrates and alumina templates. These thin films have been characterized by X-ray diffraction, atomic force microscopy, high resolution transmission electron microscopy, ellipsometry, Raman spectroscopy, ultraviolet photoelectron spectroscopy and Kelvin probe measurements. For both LaS/Si and LaS/MgO thin films, the effective work function of the submicron thick thin films was determined to be about 1 eV from field emission measurements using the Scanning Anode Field Emission Microscopy technique. The physical reasons for these highly desirable low work function properties were explained using a patchwork field emission model of the emitting surface. In this model, nanocrystals of low work function materials having a <100> orientation perpendicular to the surface and outcropping it are surrounded by a matrix of amorphous materials with higher work function. To date, LaS thin films have been used successfully as cold cathode emitters with measured emitted current densities as high as 50 A/cm2. Finally, we describe the successful growth of LaS thin films on InP substrates and, more recently, the production of LaS nanoballs and nanoclusters using Pulsed Laser Ablation.

cond-mat.mtrl-sci↗
arXiv2011

Influence of Impurity Scattering on the Conductance Anomalies of Quantum Point Contacts with Lateral Spin-Orbit Coupling

We have recently shown that asymmetric lateral spin orbit coupling (LSOC) resulting from the lateral in-plane electric field of the confining potential of a side-gated quantum point contact (QPC) can be used to create a strongly spin- polarized current by purely electrical means1 in the absence of applied magnetic field. Using the non-equilibrium Green function formalism (NEGF) analysis of a small model QPC2, three ingredients were found to be essential to generate the strong spin polarization: an asymmetric lateral confinement, a LSOC induced by the lateral confining potential of the QPC, and a strong electron-electron (e-e) interaction. In this paper, NEGF is used to study how the spin polarization is affected by the presence of impurities in the central portion of the QPC. It is found that the number, location, and shape of the conductance anomalies, occurring below the first quantized conductance plateau (G0=2e2/h), are strongly dependent on the nature (attractive or repulsive) and the locations of the impurities. We show that the maximum of the conductance spin polarization is affected by the presence of impurities. For QPCs with impurities off-center, a conductance anomaly appears below the first integer step even for the case of symmetric bias on the two side gates. These results are of practical importance if QPCs in series are to be used to fabricate all-electrical spin valves with large ON/OFF conductance ratio.

cond-mat.mes-hall↗
arXiv2010

Spin Texture in Quantum Point Contacts in the Presence of Lateral Spin Orbit Coupling

A non-equilibrium Green's function formalism is used to study in detail the ballistic conductance of asymmetrically biased side-gated quantum point contacts (QPCs) in the presence of lateral spin-orbit coupling and electron-electron interaction for a wide range of QPC dimensions and gate bias voltage. Various conductance anomalies are predicted below the first quantized conductance plateau (G0=2e2/h) which occur due to spontaneous spin polarization in the narrowest portion of the QPC. The number of observed conductance anomalies increases with increasing aspect ratio (length/width) of the QPC constriction. These anomalies are fingerprints of spin textures in the narrow portion of the QPC.

cond-mat.mes-hall↗
arXiv2009

The effective spin concept to analyze coherent charge transport in mesoscopic systems

An effective spin concept is introduced to examine the mathematical and physical analogy between phase coherent charge transport in mesoscopic systems and quantum operations on spin based qubits. When coupled with the Bloch sphere concept, this isomorphism allows formulation of transport problems in a language more familiar to researchers in the field of spintronics and quantum computing. We exemplify the synergy between charge tunneling and spin qubit unitary operations by recasting well-known problems of tunneling through a delta scatterer, a resonant tunneling structure, a superlattice structure, and arrays of elastic scatterers, in terms of specific unitary operations (rotations) of a spinor on the Bloch sphere.

cond-mat.mes-hall↗
arXiv2009

Possible origin of the 0.5 plateau in the ballistic conductance of quantum point contacts

A non-equilibrium Green function formalism (NEGF) is used to study the conductance of a side-gated quantum point contact (QPC) in the presence of lateral spin-orbit coupling (LSOC). A small difference of bias voltage between the two side gates (SGs) leads to an inversion asymmetry in the LSOC between the opposite edges of the channel. In single electron modeling of transport, this triggers a spontaneous but insignificant spin polarization in the QPC. However, the spin polarization of the QPC is enhanced substantially when the effect of electron-electron interaction is included. The spin polarization is strong enough to result in the occurrence of a conductance plateau at 0.5G0 (G0 = 2e2/h) in the absence of any external magnetic field. In our simulations of a model QPC device, the 0.5 plateau is found to be quite robust and survives up to a temperature of 40K. The spontaneous spin polarization and the resulting magnetization of the QPC can be reversed by flipping the polarity of the source to drain bias or the potential difference between the two SGs. These numerical simulations are in good agreement with recent experimental results for side-gated QPCs made from the low band gap semiconductor InAs.

cond-mat.mes-hall↗
arXiv2008

Transverse spin relaxation time in organic molecules: A possible platform for fault tolerant room temperature quantum computing

We report measurement of the ensemble averaged transverse spin relaxation time (T2*) in bulk and few molecules of the organic semiconductor tris(8-hydroxyquinolinolato aluminum) or Alq3. This system exhibits two characteristic T2* times, the longer of which is temperature-independent and the shorter is temperature-dependent, indicating that the latter is most likely limited by spin-phonon interaction. Based on the measured data, we infer that the single particle T2 time is long enough to meet Knill's criterion for fault tolerant quantum computing, even at room temperature. Alq3 is also an optically active organic and we propose a simple optical scheme for spin qubit read out. Moreover, we found that the temperature-dependent T2* time is considerably shorter in bulk Alq3 powder than in few molecules confined in 1-2 nm sized cavities, which is suggestive of a new type of ``phonon bottleneck effect''. This is very intriguing for organic molecules where carriers are always localized over individual molecules but the phonons are delocalized.

cond-mat.mes-hall↗
arXiv2008

The inequality of charge and spin diffusion coefficients

Since spin and charge are both carried by electrons (or holes) in a solid, it is natural to assume that charge and spin diffusion coefficients will be the same. Drift-diffusion models of spin transport typically assume so. Here, we show analytically that the two diffusion coefficients can be vastly different in quantum wires. Although we do not consider quantum wells or bulk systems, it is likely that the two coefficients will be different in those systems as well. Thus, it is important to distinguish between them in transport models, particularly those applied to quantum wire based devices

cond-mat.mes-hall↗
arXiv2007

A Dual Gate Spin Field Effect Transistor With Very Low Switching Voltage and Large ON-to-OFF Conductance Ratio

We propose and analyze a novel dual-gate Spin Field Effect Transistor (SpinFET) with half-metallic ferromagnetic source and drain contacts. The transistor has two gate pads that can be biased independently. It can be switched ON or OFF with a few mV change in the differential bias between the two pads, resulting in extremely low dynamic power dissipation during switching. The ratio of ON to OFF conductance remains fairly large (~ 60) up to a temperature of 10 K. This device also has excellent inverter characteristics, making it attractive for applications in low power and high density Boolean logic circuits.

cond-mat.mes-hall↗
arXiv2007

A Digital Switch and Femto-Tesla Magnetic Field Sensor Based on Fano Resonance in a Spin Field Effect Transistor

We show that a Spin Field Effect Transistor, realized with a semiconductor quantum wire channel sandwiched between half-metallic ferromagnetic contacts, can have Fano resonances in the transmission spectrum. These resonances appear because the ferromagnets are half-metallic, so that the Fermi level can be placed above the majority but below the minority spin band. In that case, the majority spins will be propagating, but the minority spins will be evanescent. At low temperatures, the Fano resonances can be exploited to implement a digital binary switch that can be turned on or off with a very small gate voltage swing of few tens of microvolts, leading to extremely small dynamic power dissipation during switching. An array of 500,000 x 500,000 such transistors can detect ultrasmall changes in a magnetic field with a sensitivity of 1 femto-Tesla/sqrt{Hz}, if each transistor is biased near a Fano resonance.

cond-mat.mes-hall↗
arXiv2006

Response to "Response to the Comment on 'Performance of a Spin Based Insulated Gate Field Effect Transistor' "

We show that the arguments in the posting cond-mat/0607432 by Flatte and Hall are flawed and untenable. Their spin based transistor cannot work as claimed because of fundamental scientific barriers, which cannot be overcome now, or ever. Their device is not likely to work as a transistor at room temperature, let alone outperform the traditional MOSFET, as claimed.

cond-mat.mes-hall↗