arXiv subjects

Supriyo Datta

Publications and source records attributed to Supriyo Datta.

At least 73 records · Page 4Linked to original sources

arXiv2008

Spin Transfer Torque as a Non-Conservative Pseudo-Field

In this paper we show that the spin transfer torque can be described by a pseudo magnetic field, proportional to the magnetic moment of the itinerant electrons that enters the Landau-Lifshitz-Gilbert equation in the same way as other external or internal magnetic fields. However, unlike an ordinary magnetic field, which is always conservative in nature, the spin torque induced pseudo field may have both conservative and non-conservative components. We further show that the magnetic moment of itinerant electrons develops an out-of-plane component only at non-equilibrium and this component is responsible for the Slonczewski type switching that acts against the damping and is always non-conservative. On the other hand, the in-plane components of the pseudo field exist both at equilibrium and out-of-equilibrium, and are responsible for the field like term. For tunnel based devices, this term results in lower switching current for anti-parallel (AP) to parallel (P) switching compared to P to AP, even when the torque magnitudes are completely symmetric with voltage.

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arXiv2008

Influence of Dimensionality on Thermoelectric Device Performance

The role of dimensionality on the electronic performance of thermoelectric devices is clarified using the Landauer formalism, which shows that the thermoelectric coefficients are related to the transmission, T(E), and how the conducing channels, M(E), are distributed in energy. The Landauer formalism applies from the ballistic to diffusive limits and provides a clear way to compare performance in different dimensions. It also provides a physical interpretation of the "transport distribution," a quantity that arises in the Boltzmann transport equation approach. Quantitative comparison of thermoelectric coefficients in one, two, and three dimension shows that the channels may be utilized more effectively in lower-dimensions. To realize the advantage of lower dimensionality, however, the packing density must be very high, so the thicknesses of the quantum wells or wires must be small. The potential benefits of engineering M(E) into a delta-function are also investigated. When compared to a bulk semiconductor, we find the potential for ~50 % improvement in performance. The shape of M(E) improves as dimensionality decreases, but lower dimensionality itself does not guarantee better performance because it is controlled by both the shape and the magnitude of M(E). The benefits of engineering the shape of M(E) appear to be modest, but approaches to increase the magnitude of M(E) could pay large dividends.

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arXiv2008

Nanoelectronic Devices: A Unified View

Nanoscale electronic devices are of great interest for all kinds of applications like switching, energy conversion and sensing. The objective of this chapter, however, is not to discuss specific devices or applications. Rather it is to convey the conceptual framework that has emerged over the last twenty years, which is important not only because of the practical insights it provides into the design of nanoscale devices, but also because of the conceptual insights it affords regarding the meaning of resistance and the essence of all non-equilibrium phenomena in general. We present a unified description applicable to a wide variety of devices from molecular conductors to carbon nanotubes to silicon transistors covering different transport regimes from the ballistic to the diffusive limit, based on what we call the NEGF-Landauer approach.

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arXiv2008

Nanodevices and Maxwell's Demon

In the last twenty years there has been significant progress in our understanding of quantum transport far from equilibrium and a conceptual framework has emerged through a combination of the Landauer approach with the non-equilibrium Green function (NEGF) method, which is now being widely used in the analysis and design of nanoscale devices. It provides a unified description for all kinds of devices from molecular conductors to carbon nanotubes to silicon transistors covering different transport regimes from the ballistic to the diffusive limit. In this talk I use a simple version of this model to analyze a specially designed device that could be called an electronic Maxwell's demon, one that lets electrons go preferentially in one direction over another. My objective is to illustrate the fundamental role of contacts and demons in transport and energy conversion. The discussion is kept at an academic level steering clear of real world details, but the illustrative devices we use are very much within the capabilities of present-day technology. For example, recent experiments on thermoelectric effects in molecular conductors agree well with the predictions from our model. The Maxwell's demon device itself is very similar to the pentalayer spin-torque device which has been studied by a number of groups though we are not aware of any discussion of the possibility of using the device as a nanoscale heat engine or as a refrigerator as proposed here. However, my objective is not to evaluate possible practical applications. Rather it is to introduce a simple transparent model showing how out-of-equibrium demons suitably incorporated into nanodevices can achieve energy conversion.

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arXiv2008

Spin Dephasing and Hot Spins

At this summer school, I have tried to describe a general approach to quantum transport problems based on the Non-equilibrium Green Function (NEGF) method . I will not repeat this treatment here. Instead I will use this article to draw attention to a core issue in quantum transport and how it is modeled within the NEGF method. Specifically, I will discuss what constitutes a dephasing process and how the standard descriptions may need to be extended to include what I will call hot spin effects. These involve a number of subtle issues that tend to get hidden in a purely formal description. So I have chosen instead to illustrate the concepts using a concrete device, which is specially engineered to enhance its sensitivity to spin dephasing and hot spin effects.

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arXiv2008

Atomistic Non-equilibrium Green s Function Simulations of Graphene Nano-Ribbons in the Quantum Hall Regime

The quantum Hall effect in Graphene nano-ribbons (GNR) is investigated with the non-equilibrium Green s function (NEGF) based quantum transport model in the ballistic regime. The nearest neighbor tight-binding model based on pz orbital constructs the device Hamiltonian. GNRs of different edge geometries (Zigzag and Armchair) are considered. The magnetic field is included in both the channels and contact through Peierls substitution. Efficient algorithms for calculating the surface Green function are used to save computation time while simulating realistically large dimensions comparable to those used in experiments. Hall resistance calculations exactly reproduce the quantum Hall plateaus observed in the experiments. Use of large dimensions in the simulation is crucial in order to capture the quantum Hall effect in magnetic fields within experimentally relevant 10-20T.

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arXiv2007

Non-equilibrium Green s function based model for dephasing in quantum transport

The objective of this paper is to describe a simple phenomenological approach for including incoherent dephasing processes in quantum transport models. The presented illustrative numerical results show this model provides the flexibility of adjusting the degree of phase and momentum relaxation independently that is not currently available in mesoscopic physics and in device simulations while retaining the simplicity of other phenomenological models.

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arXiv2007

Effect of contact induced states on minimum conductivity in graphene

The objective of this paper is to point out that contact induced states can help explain the structure dependence of the minimum conductivity observed experimentally even if the samples were purely ballistic. Contact induced states are similar to the well-known metal induced gap states (MIGS) in metal-semiconductor Schottky junctions, which typically penetrate only a few atomic lengths into the semiconductor, while the depth of penetration decreases with increasing band gap. However, in graphene we find that these states penetrate a much longer distance of the order of the width of the contacts. As a result, ballistic graphene samples with a length less than their width can exhibit a resistance proportional to length that is not Ohmic in origin, but arises from a reduced role of contact-induced states. While actual samples are probably not ballistic and involve scattering processes, our results show that these contact induced effects need to be taken into account in interpreting experiments and minimum conductivity depends strongly on the structure and configuration (two- vs. four-terminal) used.

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arXiv2007

Use of negative capacitance to provide a sub-threshold slope lower than 60 mV/decade

It is well known that conventional Field Effect Transistors (FET's) require a change in the channel potential of at least 60 mV at 300K to effect a change in the current by a factor of ten, and this minimum subthreshold slope S puts a lower limit of fundamental nature on the operating voltage and hence the power dissipation in standard FET based switches. Here we show that by replacing the standard insulator with a ferroelectric insulator of the right thickness it should be possible to implement a step-up voltage transformer that will amplify the gate voltage thus leading to values of S lower than 60 mV/decade and enabling low voltage/low power operation. The voltage transformer action can be understood intuitively as the result of an effective negative capacitance provided by the ferroelectric capacitor which arises from an internal positive feedback that in principle could be obtained from other microscopic mechanisms as well. Unlike other proposals to reduce S this involves no change in the basic physics of the FET and thus does not affect its current drive or impose other restrictions.

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arXiv2007

A Generic Model for Current Collapse in Spin Blockaded Transport

A decrease in current with increasing voltage, often referred to as negative differential resistance (NDR), has been observed in many electronic devices and can usually be understood within a one-electron picture. However, NDR has recently been reported in nanoscale devices with large single-electron charging energies which require a many-electron picture in Fock space. This paper presents a generic model in this transport regime leading to a simple criterion for the conditions required to observe NDR and shows that this model describes the recent observation of multiple NDR's in Spin Blockaded transport through weakly coupled-double quantum dots quite well. This model shows clearly how a delicate interplay of orbital energy offset, delocalization and Coulomb interaction lead to the observed NDR under the right conditions, and also aids in obtaining a good match with experimentally observed features. We believe the basic model could be useful in understanding other experiments in this transport regime as well.

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arXiv2007

Spin-Hall Effect: From the Ballistic to the Diffusive Regime

The spin-Hall effect has recently attracted a lot of attention and a central question is whether the effect is due to the intrinsic spin-orbit interaction or due to spin-asymmetric scattering by extrinsic impurities. The objective of this paper is to shed light on this question using a new approach based on the non-equilibrium Green function (NEGF) formalism, which allows us to go continuously from the ballistic to the diffusive limit and we present approximate analytical expressions that describe our results fairly well. Our model suggests that a spin accumulation proportional to the current should be observed even in clean ballistic sample. We further show good quantitative agreement with recent experimental observations in GaAs suggesting that these can be understood in terms of an intrinsic effect driven by the Rashba interaction, although experiments on ZnSe likely have a different origin.

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arXiv2007

Quantum Transport with Spin Dephasing: A Nonequilibrium Green's Function Approach

A quantum transport model incorporating spin scattering processes is presented using the non-equilibrium Green's function (NEGF) formalism within the self-consistent Born approximation. This model offers a unified approach by capturing the spin-flip scattering and the quantum effects simultaneously. A numerical implementation of the model is illustrated for magnetic tunnel junction devices with embedded magnetic impurity layers. The results are compared with experimental data, revealing the underlying physics of the coherent and incoherent transport regimes. It is shown that small variations in magnetic impurity spin-states/concentrations could cause large deviations in junction magnetoresistances.

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arXiv2006

Interacting Systems for Self-Correcting Low Power Switching

In this paper we first show that dynamic switching schemes can be used to reduce energy dissipation below the thermodynamic minimum of NkTlnr (N= number of state variables, 1/r=error probability), but only at the expense of the error immunity inherent in thermodynamic processes for which the final state is insensitive to the switching dynamics. It is further shown that, for a system which has internal feedback, e.g. nanomagnets, such that all N spins act in concert, it should be possible to switch with an energy dissipation of the order of kTlnr (considerably less than the thermodynamic limit of NkTlnr), while retaining an error immunity comparable to thermodynamic switching.

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arXiv2006

Theory of high bias Coulomb Blockade in ultrashort molecules

We point out that single electron charging effects such as Coulomb Blockade (CB) and high-bias staircases play a crucial role in transport through single ultrashort molecules. A treatment of Coulomb Blockade through a prototypical molecule, benzene, is developed using a master-equation in its complete many-electron Fock space, evaluated through exact diagonalization or full Configuration Interaction (CI). This approach can explain a whole class of non-trivial experimental features including vanishing zero bias conductances, sharp current onsets followed by ohmic current rises, and gateable current levels and conductance structures, most of which cannot be captured even qualitatively within the traditional Self Consistent Field (SCF) approach coupled with perturbative transport theories. By comparing the two approaches, namely SCF and CB, in the limit of weak coupling to the electrode, we establish that the inclusion of strong-correlations within the molecule becomes critical in addressing the above experiments. Our approach includes on-bridge-correlations fully, and is therefore well-suited for describing transport through short molecules in the limit of weak coupling to electrodes.

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arXiv2006

Self Consistent NEGF-LLG Model for Spin-Torque Based Devices

We present here a self consistent solution of quantum transport, using the Non Equilibrium Green's Function (NEGF) method, and magnetization dynamics, using the Landau-Lifshitz-Gilbert (LLG) formulation. We have applied this model to study current induced magnetic switching due to `spin torque' in a device where the electronic transport is ballistic and the free magnetic layer is sandwiched between two anti-parallel ferromagnetic contacts. The device shows clear hysteretic current-voltage characteristics, at room temperature, with a sharp transition between the bistable states and hence can be used as a non-volatile memory. We show that the proposed design may allow reducing the switching current by an order of magnitude.

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arXiv2006

Fock Space formulation for Nanoscale transport

In this paper we present a general formulation for electronic transport that combines strong correlation effects with broadening and quantum coherence, and illustrate it with a simple example ("spin blockade") that clearly demonstrates all three effects. The standard master equation method for Coulomb blockade captures the first effect, while a number of different approaches such as the non-equilibrium Green function (NEGF) method are available for handling the last two. But there is a need for a method that captures all three and in this paper we propose an approach that appears to fulfill this need. The equations look similar to the NEGF formalism but one basic distinction is that all quantities (like the Hamiltonian) appearing in our formalism are defined in Fock space and as such are matrices of dimension (2^N by 2^N), N being the number of basis functions describing the one-electron Hilbert space. Similar quantities appearing in the NEGF formalism are of dimension (N by N). Other important differences arise from the fact that in Fock space there is no need to include the exclusion principle explicitly and that elastic processes in one-electron space look like inelastic processes in Fock space. A simple numerical example is presented to show that our approach includes strong electron-electron interaction, broadening due to coupling to contacts and quantum coherence effects due to spin polarization in directions other than the z-direction. The basic equations for this "Fock space Green's function" (FSGF) are quite general and should be applicable to more interesting and exotic transport problems involving the entanglement of many-electron states. How effective it will be remains to be assessed for different problems, but our preliminary results for the Kondo peak look encouraging.

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arXiv2005

Electrical Detection of Spin Excitations

The objective of this paper is to draw attention to a possible new approach for measuring the spin excitation spectrum of a spin array by placing it in intimate contact with the channel of a spin-valve device in an anti-parallel (AP) configuration. We show, using realistic device parameters, that the second derivative of the current $\partial^2I/\partial V^2$ should exhibit peaks whose location corresponds to the energy of excitation and whose height is its strength. The effect is maximum for ideal half metallic contacts in AP configuration but we show that even with contacts that are less than ideal, the spin signals can be extracted using a de-embedding scheme. Compared to existing techniques our proposal has three attractive features: (a) high sensitivity down to single spins, (b) high spatial resolution limited only by the wavefunctions of the individual spins and (c) a relatively common device structure to which a lot of current research is devoted

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arXiv2005

A Theoretical Investigation of Surface Roughness Scattering in Silicon Nanowire Transistors

In this letter, we report a three-dimensional (3D) quantum mechanical simulation to investigate the effects of surface roughness scattering (SRS) on the device characteristics of Si nanowire transistors (SNWTs). We treat the microscopic structure of the Si/SiO2 interface roughness directly by using a 3D finite element technique. The results show that 1) SRS reduces the electron density of states in the channel, which increases the SNWT threshold voltage, and 2) the SRS in SNWTs becomes more effective when more propagating modes are occupied, which implies that SRS is more important in planar metal-oxide-semiconductor field-effect-transistors with many transverse modes occupied than in small-diameter SNWTs with few modes conducting.

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