arXiv subjects

S. Bandyopadhyay

Publications and source records attributed to S. Bandyopadhyay.

At least 37 records · Page 2Linked to original sources

arXiv2012

Sb concentration dependent structural and resistive properties of polycrystalline Bi-Sb alloys

Polycrystalline Bi-Sb alloys have been synthesized over a wide range of antimony concentration (8 at% to 20 at%) by solid state reaction method. In depth structural analysis using X-Ray diffraction (XRD) and temperature dependent resistivity measurement of synthesized samples have been performed. XRD data confirmed single phase nature of polycrystalline samples and revealed that complete solid solution is formed between bismuth and antimony. Rietveld refinement technique, utilizing MAUD software, has been used to perform detail structural analysis of the samples and lattice parameters of synthesized Bi-Sb alloys have been estimated. Lattice parameter and unit cell volume decreases monotonically with increasing antimony content. The variation of lattice parameters with antimony concentration depicts a distinct slope change beyond 12 at% Sb content sample. Band gap has been estimated from the thermal variation of resistivity data, with the 12% Sb content sample showing maximum value. It has been observed that, with increasing antimony concentration the transition from direct to indirect gap semiconductor is intimately related to the variation of the estimated lattice parameters. Band diagram for the polycrystalline Bi-Sb alloy system has also been proposed.

cond-mat.mtrl-sci↗

arXiv2012

Modification of structural and magnetic properties of Zn0.96 Mn0.04O samples by Li3+ ion irradiation

Zn0.96Mn0.04O samples were synthesized by solid state reaction technique to explore their magnetic behavior. Structural, morphological and magnetic properties of the samples have been found to be modified by 50 MeV Li+3 ion beam irradiation. The samples exhibit impurity phase and upon irradiation it disappears. Rietveld refinement analysis indicates that substitutional incorporation of Mn in the host lattice increases with irradiation. Grain size decreases with irradiation. Field dependent magnetization (M-H) measurement explicitly indicates ferromagnetic (FM) nature. It has been established from temperature dependent magnetization (M-T) measurement (500 Oe) and ac susceptibility (χ-T) measurement that ferromagnetism in the system seems to be mainly intrinsic; though superparamagnetic Mn nanoparticles also has a minor role. The analysis of M-T data at comparatively high field (5000-Oe) provides an estimation of antiferromagnetic (AFM) exchange, which acts as a reducing agent for observed magnetic moment. The value of saturation magnetization has been increased upon irradiation and is highly correlated with dissolution of impurity phase. Actually structural property has been modified with ion irradiation and this modification may cause some definite positive change in magnetic property.

cond-mat.mtrl-sci↗

arXiv2011

Effect of 50 MeV Li3+ irradiation on structural and electrical properties of Mn doped ZnO

The present work aims to study the effect of ion irradiation on structural and electrical properties and their correlation with the defects in Zn1-xMnxO type system. Zn1-xMnxO (x = 0.02, 0.04) samples have been synthesized by solid-state reaction method and have been irradiated with 50 MeV Li3+ ions. The concomitant changes have been probed by x-ray diffraction (XRD), temperature dependent electrical resistivity and positron annihilation lifetime (PAL) spectroscopy. XRD result shows single phase wurtzite structure for Zn0.98Mn0.02O, whereas for Zn0.96Mn0.04O sample an impurity phase has been found apart from the usual peaks of ZnO. Ion irradiation dissolves this impurity peak. Grain size of the samples found to be uniform. For Zn0.98Mn0.02O, the observed sharp decrease in room temperature resistivity (RhoRT) with irradiation is consistent with the lowering of FWHM of the XRD peaks. However for Zn0.96Mn0.04O, RhoRT decreases for initial fluence but increases for further increase of fluence. All the irradiated Zn0.98Mn0.02O samples show metal-semiconductor transition in temperature dependent resistivity measurement at low temperature. But all the irradiated Zn0.96Mn0.04O samples show semiconducting nature in the whole range of temperature. Results of room temperature resistivity, XRD and PAL measurements are consistent with each other.

cond-mat.mtrl-sci↗

arXiv2011

An ultrasensitive spintronic strain sensor

We propose a spintronic strain sensor capable of sensing strain with a sensitivity of 1E-13/sqrt{Hz} at room temperature with an active sensing area of 1 cmE2 and power dissipation of 1 watt. This device measures strain by monitoring the change in the spin-polarized current in a parallel array of free standing nanowire spin valves when the array is subjected to compressive or tensile stress along the wires' length. The change in the current is linearly proportional to the strain, which makes the sensor relatively distortion-free. Such a sensor can be fabricated using a variety of techniques involving nanolithography, self assembly and epitaxial growth.

cond-mat.mes-hall↗

arXiv2010

Defects induced ferromagnetism in Mn doped ZnO

Single phase Mn doped (2 at %) ZnO samples have been synthesized by solid-state reaction technique. Before the final sintering at 500 C, the mixed powders have been milled for different milling periods (6, 24, 48 and 96 hours). The grain sizes of the samples are very close to each other (~ 32 \pm 4 nm). However, the defective state of the samples is different from each other as manifested from the variation of magnetic properties and electrical resistivity with milling time. All the samples have been found to be ferromagnetic with clear hysteresis loops at room temperature. The maximum value for saturation magnetization (0.11 μ_B / Mn atom) was achieved for 96 hours milled sample. Electrical resistivity has been found to increase with increasing milling time. The most resistive sample bears the largest saturation magnetization. Variation of average positron lifetime with milling time bears a close similarity with that of the saturation magnetization. This indicates the key role played by open volume vacancy defects, presumably zinc vacancies near grain surfaces, in inducing ferromagnetic order in Mn doped ZnO. To attain optimum defect configuration favorable for ferromagnetism in this kind of samples proper choice of milling period and annealing conditions is required.

cond-mat.mtrl-sci↗

arXiv2010

Equivalence Checking in Embedded Systems Design Verification

In this report we focus on some aspects related to modeling and formal verification of embedded systems. Many models have been proposed to represent embedded systems. These models encompass a broad range of styles, characteristics, and application domains and include the extensions of finite state machines, data flow graphs, communication processes and Petri nets. In this report, we have used a PRES+ model (Petri net based Representation for Embedded Systems) as an extension of classical Petri net model that captures concurrency, timing behaviour of embedded systems; it allows systems to be representative in different levels of abstraction and improves expressiveness by allowing the token to carry information. Modeling using PRES+, as discussed above, may be convenient for specifying the input behaviour because it supports concurrency. However, there is no equivalence checking method reported in the literature for PRES+ models to the best of our knowledge. In contrast, equivalence checking of FSMD models exist. As a first step, therefore, we seek to devise an algorithm to translate PRES+ models to FSMD models.

cs.LO↗

arXiv2010

Bennett clocking of nanomagnetic logic using electrically induced rotation of magnetization in multiferroic single-domain nanomagnets

The authors show that it is possible to rotate the magnetization of a multiferroic (strain-coupled two-layer magnetostrictive-piezoelectric) nanomagnet by a large angle with a small electrostatic potential. This can implement Bennett clocking in nanomagnetic logic arrays resulting in unidirectional propagation of logic bits from one stage to another. This method of Bennett clocking is superior to using spin-transfer torque or local magnetic fields for magnetization rotation. For realistic parameters, it is shown that a potential of ~ 0.2 V applied to a multiferroic nanomagnet can rotate its magnetization by nearly 900 to implement Bennett clocking.

cond-mat.mes-hall↗

arXiv2010

Analysis of the two dimensional Datta-Das Spin Field Effect Transistor

An analytical expression is derived for the conductance modulation of a ballistic two dimensional Datta-Das Spin Field Effect Transistor (SPINFET) as a function of gate voltage. Using this expression, we show that the recently observed conductance modulation in a two-dimensional SPINFET structure does not match the theoretically expected result very well. This calls into question the claimed demonstration of the SPINFET and underscores the need for further careful investigation.

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

Synthesis and characterization of single phase Mn doped ZnO

Different samples of Zn1-xMnxO series have been prepared by conventional solid state sintering method. It has been identified, up to what extent of doping enable us to synthesize single-phase polycrystalline Mn doped ZnO samples which is one of the prerequisite for dilute magnetic semiconductor and we have analyzed its certain other physical aspects. In synthesizing the samples proportion of Mn varies from 1 at% to 5 at%. However the milling times have been varied (6, 12, 24, 48 & 96 hours) for only 2 at% Mn doped samples while for other samples (1, 3, 4 & 5 at% Mn doped) the milling time has been kept fixed at 96 hours. Room temperature X-Ray diffraction (XRD) data reveal that all of the prepared samples up to 3 at% of Mn doping exhibit wurtzite-type structure, no segregation of Mn and/or its oxides has been found. The 4 at% Mn doped samples show a weak peak of ZnMn2O4 apart from usual other peaks of ZnO and the intensity of this impurity peak has been further increased for 5 at% of Mn doping. So beyond 3 at% doping single-phase behavior is destroyed. Band gap for all the 2 at% Mn doped samples have been estimated as between 3.21 to 3.19 eV and reason for this low band gap values has been explained through the grain boundary trapping model. The room temperature resistivity measurement shows increase of resistivity up to 48 hours of milling and with further milling it saturates. The defect state of these samples has been investigated by using positron annihilation lifetime (PAL) spectroscopy technique. Here all the relevant lifetime parameters of positron i.e. free annihilation (tau 1), at defect site (tau 2) and average (tau av) increases with milling time.

cond-mat.mtrl-sci↗

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↗

arXiv2008

Single spin universal Boolean logic

Recent advances in manipulating single electron spins in quantum dots have brought us close to the realization of classical logic gates based on representing binary bits in spin polarizations of single electrons. Here, we show that a linear array of three quantum dots, each containing a single spin polarized electron, and with nearest neighbor exchange coupling, acts as the universal NAND gate. The energy dissipated during switching this gate is the Landauer-Shannon limit of kTln(1/p) [T = ambient temperature and p = intrinsic gate error probability]. With present day technology, p = 1E-9 is achievable above 1 K temperature. Even with this small intrinsic error probability, the energy dissipated during switching the NAND gate is only ~ 21 kT, while today's nanoscale transistors dissipate about 40,000 - 50,000 kT when they switch.

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↗

arXiv2006

Spin relaxation in a nanowire organic spin valve: Observation of extremely long spin relaxation times

We report spin valve behavior in an organic nanowire consisting of three layers - cobalt, Alq3 and nickel - all nominally 50 nm in diameter. Based on the data, we conclude that the dominant spin relaxation mechanism in Alq3 is the Elliott-Yafet mode. Despite the very short momentum relaxation time, the spin relaxation time is found to be very long - at least a few milliseconds - and relatively temperature independent up to 100 K. To our knowledge, this is the first demonstration of an organic nanoscale spin valve, as well as the first determination of the primary spin relaxation mechanism in organics. The unusually long spin relaxation time makes these materials ideal platforms for some areas of spintronics.

cond-mat.mes-hall↗

arXiv2006

Comment on "Performance of a spin based insulated gate field effect transistor" [cond-mat/0603260] [cond-mat/0603260]

In a recent e-print [cond-mat/0603260] Hall and Flatte claim that a particular spin based field effect transistor (SPINFET), which they have analyzed, will have a lower threshold voltage, lower switching energy and lower leakage current than a comparable metal oxide semiconductor field effect transistor (MOSFET). Here, we show that all three claims of HF are invalid.

cond-mat.mes-hall↗