Bhaskaran Muralidharan

Probing electronic excitations in molecular conduction

We identify experimental signatures in the current-voltage (I-V) characteristics of weakly contacted molecules directly arising from excitations in their many electron spectrum. The current is calculated using a multielectron master equation in the Fock space of an exact diagonalized model many-body Hamiltonian for a prototypical molecule. Using this approach, we explain several nontrivial features in frequently observed I-Vs in terms of a rich spectrum of excitations that may be hard to describe adequately with standard one-electron self-consistent field theories.

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 NDRs 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.

Performance analysis of an interacting quantum dot thermoelectric setup

In the absence of phonon contribution, a weakly coupled single orbital noninteracting quantum dot thermoelectric setup is known to operate reversibly as a Carnot engine. This reversible operation, however, occurs only in the ideal case of vanishing coupling to the contacts, wherein the transmission function is delta shaped, and under open-circuit conditions, where no electrical power is extracted. In this paper, we delve into the thermoelectric performance of quantum dot systems by analyzing the power output and efficiency directly evaluated from the nonequilibrium electric and energy currents across them. In the case of interacting quantum dots, the onequilibrium currents in the limit of weak coupling to the contacts are evaluated using the Pauli master equation approach. The following fundamental aspects of the thermoelectric operation of a quantum dot setup are discussed in detail: (a) With a finite coupling to the contacts, a thermoelectric setup always operates irreversibly under open-circuit conditions, with a zero efficiency. (b) Operation at a peak efficiency close to the Carnot value is possible under a finite power operation. In the noninteracting single orbital case, the peak efficiency approaches the Carnot value as the coupling to the contacts becomes smaller. In the interacting case, this trend depends nontrivially on the interaction parameter U. (c) The evaluated trends of the maximum efficiency derived from the nonequilibrium currents deviate considerably from the conventional figure of merit zT -based results. Finally, we also analyze the interacting quantum dot setup for thermoelectric operation at maximum power output.

Enhancement of Spin-transfer torque switching via resonant tunneling

We propose the use of resonant tunneling as a route to enhance the spin-transfer torque switching characteristics of magnetic tunnel junctions. The proposed device structure is a resonant tunneling magnetic tunnel junction based on a MgO-semiconductor heterostructure sandwiched between a fixed magnet and a free magnet. Using the non-equilibrium Greens function formalism coupled self consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation, we demonstrate enhanced tunnel magneto-resistance characteristics as well as lower switching voltages in comparison with traditional trilayer devices. Two device designs based on MgO based heterostructures are presented, where the physics of resonant tunneling leads to an enhanced spin transfer torque thereby reducing the critical switching voltage by up to 44%. It is envisioned that the proof-of-concept presented here may lead to practical device designs via rigorous materials and interface studies.

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 CB 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 nontrivial 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.

Power and efficiency analysis of a realistic resonant tunneling diode thermoelectric

Low-dimensional systems with sharp features in the density of states have been proposed as a means for improving the efficiency of thermoelectric devices. Quantum dot systems, which offer the sharpest density of states achievable, however, suffer from low power outputs while bulk (3-D) thermoelectrics, while displaying high power outputs, offer very low efficiencies. Here, we analyze the use of a resonant tunneling diode structure that combines the best of both aspects, that is, density of states distortion with a finite bandwidth due to confinement that aids the efficiency and a large number of current carrying transverse modes that enhances the total power output. We show that this device can achieve a high power output (∼0.3 MW∕m2) at efficiencies of ∼40% of the Carnot efficiency due to the contribution from these transverse momentum states at a finite bandwidth of kT∕2. We then provide a detailed analysis of the physics of charge and heat transport with insights on parasitic currents that reduce the e...

Proposal for a Domain Wall Nano-Oscillator driven by Non-uniform Spin Currents

We propose a new mechanism and a related device concept for a robust, magnetic field tunable radio-frequency (rf) oscillator using the self oscillation of a magnetic domain wall subject to a uniform static magnetic field and a spatially non-uniform vertical dc spin current. The self oscillation of the domain wall is created as it translates periodically between two unstable positions, one being in the region where both the dc spin current and the magnetic field are present, and the other, being where only the magnetic field is present. The vertical dc spin current pushes it away from one unstable position while the magnetic field pushes it away from the other. We show that such oscillations are stable under noise and can exhibit a quality factor of over 1000. A domain wall under dynamic translation, not only being a source for rich physics, is also a promising candidate for advancements in nanoelectronics with the actively researched racetrack memory architecture, digital and analog switching paradigms as candidate examples. Devising a stable rf oscillator using a domain wall is hence another step towards the realization of an all domain wall logic scheme.

Rectification by charging: Contact-induced current asymmetry in molecular conductors

We outline the qualitatively different physics behind charging-induced current asymmetries in molecular conductors operating in the weakly interacting self-consistent field (SCF) and the strongly interacting Coulomb blockade (CB) regimes. The SCF conductance asymmetry originates in the asymmetric shifts of the closed-shell molecular conducting levels, driven by unequal mean-field potentials for positive and negative biases. A very different current asymmetry arises for CB due to the unequal number of open-shell excitation channels at opposite bias voltages. The CB regime, dominated by single charge effects, typically requires a computationally demanding many-electron or Fock-space description to do justice to its complex excitation spectrum. However, our analysis of molecular CB measurements reveals that many novel signatures can be explained using a simpler orthodox model that involves an incoherent sum of Fock-space excitations and hence treats the molecule as a metallic dot. This also reduces the complexity of the Fock-space description by including charge configurations alone, somewhat underscoring the richness of its electronic structure while retaining the essential single charge nature of the transport process. The inclusion of electronic structure with well-resolved Fock space excitations is, however, crucial in some notable examples.

Thermoelectric study of dissipative quantum-dot heat engines

This paper examines the thermoelectric response of a dissipative quantum-dot heat engine based on the Anderson-Holstein model in two relevant operating limits, (i) when the dot phonon modes are out of equilibrium, and (ii) when the dot phonon modes are strongly coupled to a heat bath. In the first case, a detailed analysis of the physics related to the interplay between the quantum-dot level quantization, the on-site Coulomb interaction, and the electron-phonon coupling on the thermoelectric performance reveals that an

Ultrasensitive Nanoscale Magnetic-Field Sensors Based on Resonant Spin Filtering

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