arXiv subjects
Publications and source records attributed to Mukunda P. Das.
We give a short overview of the role of microscopic conservation in charge transport at small scales, and at driving fields beyond the linear-response limit. As a practical example we recall the measurement and theory of interband coupling effects in a quantum point contact driven far from equilibrium.
Practical mesoscopic devices based on quantum point contacts (QPCs) must function at operating point involving large internal driving fields. Experimental evidence has accumulated to display anomalous nonlinear features of QPC response beyond the capacities of accepted tunnelling-based models of nonlinear quantum transport. Here we recall the physical setting of three anomalous QPC experiments and review how, for two of them, a microscopically based nonequilibrium quantum kinetic description (the correct physical boundary conditions being crucial) has already overcome the predictive limitations of standard nonequilibrium mesoscopic models. The third experiment remains a significant challenge to all theorists.
Over the last decade, interest in one-dimensional charge transport has progressed from the seminal discovery of Landauer quantization of conductance, as a function of carrier density, to finer-scale phenomena at the onset of quantization. This has come to be called the "0.7 anomaly", rather connoting a theoretical mystery of some profundity and universality, which remains open to date. Its somewhat imaginative appellation may tend to mislead, since the anomaly manifests itself over a range of conductance values: anywhere between 0.25 to 0.95 Landauer quanta. In this paper we offer a critique of the 0.7 anomaly and discuss the extent to which it represents a deep question of physics.
We report on a striking departure from the canonical step sequence of quantized conductance in a ballistic, quasi-one-dimensional metallic channel. Ideally, in such a structure, each sub-band population contributes its Landauer conductance quantum independently of the rest. In a picture based exclusively on coherent single-carrier transmission, unitary back-scattering can lower a conductance step below ideal, but it is absolutely impossible for it to enhance the ideal Landauer conductance of a sub-band. Precisely such an anomalous and robust nonlinear enhancement has already been observed over the whole density range between sub-band thresholds (de Picciotto R et al., Phys. Rev. Lett. 92, 036805 (2004) and J. Phys. Condens. Matter 20, 164204 (2008)). We show theoretically that the anomalous enhancement of ideal Landauer conductance is the hallmark of carrier transitions coupling the discrete sub-bands.
An earthquake-proof seismic negative belt of an artificial seismic shadow zone is introduced. The belt is composed of acoustic materials which has one of the constituent parameter between density and modulus is negative effectively. It converts the velocity of the seismic wave imaginary, and then creates a stop-band for the seismic frequency range. The belt is an attenuator of a seismic wave that reduces the amplitude of the wave exponentially. Passing the belt underground, the seismic energy turns into sound and heat in air and the magnitude of the seismic wave is weakened to be defended by conventional method. Models of the negative density and negative modulus for engineering are suggested.
A principle of an acoustic Eaton Lens array and its application as a removable tsunami wall is proposed theoretically. The lenses are made of expandable rubber pillars or balloons and create a stop-band by the rotating the incoming tsunami wave and reduce the pressure by canceling each other. The diameter of each lens is larger than the wavelength of the tsunami near the coast, that is, order of a kilometer. The impedance matching on the border of the lenses results in little reflection. Before a tsunami, the balloons are buried underground in shallow water near the coast in folded or rounded form. Upon sounding of the tsunami alarm, water and air are pumped into the pillars, which expand and erect the wall above the sea level within a few hours. After the tsunami, the water and air are released from the pillars, which are then buried underground for reuse. Electricity is used to power the entire process. A numerical simulation with a linear tsunami model was carried out.
The Fermi surface is an abstract object in the reciprocal space of a crystal lattice, enclosing the set of all those electronic band states that are filled according to the Pauli principle. Its topology is dictated by the underlying lattice structure and its volume is the carrier density in the material. The Fermi surface is central to predictions of thermal, electrical, magnetic, optical and superconducting properties in metallic systems. Density functional theory is a first-principles method used to estimate the occupied-band energies and, in particular, the iso-energetic Fermi surface. In this review we survey several key facts about Fermi surfaces in complex systems, where a proper theoretical understanding is still lacking. We address some critical difficulties.
The recently discovered multiband superconductors have created a new class of novel superconductors. In these materials multiple superconducting gaps arise due to the formation of Cooper pairs on different sheets of the Fermi surfaces. An important feature of these superconductors is the interband couplings, which not only change the individual gap properties, but also create new collective modes. Here we investigate the effect of the interband couplings in the Ginzburg-Landau theory. We produce a general $τ^{(2n+1)/2}$ expansion ($τ= 1-T/T_c$) and show that this expansion has unexpected behaviour for $n\geq 2$. This point emphasises the weaker validity of the GL theory for lower temperatures and gives credence to the existence of hidden criticality near the critical temperature of the uncoupled subdominant band.
In three-band BCS superconductors with repulsive interband interactions, frustration between the bands can lead to an inherently complex gap function, arising out of a phase difference between the bands in the range 0 and π. Since the complex conjugate of this state is also a solution, the ground state is degenerate, and there appears a time-reversal-symmetry-broken state. In this paper we investigate the existence of this state as a function of interband coupling strength and show how a new phase transition appears between the TRSB and conventional BCS states.
We developed a new method of earthquakeproof engineering to create an artificial seismic shadow zone using acoustic metamaterials. By designing huge empty boxes with a few side-holes corresponding to the resonance frequencies of seismic waves and burying them around the buildings that we want to protect, the velocity of the seismic wave becomes imaginary. The meta-barrier composed of many meta-boxes attenuates the seismic waves, which reduces the amplitude of the wave exponentially by dissipating the seismic energy. This is a mechanical method of converting the seismic energy into sound and heat. We estimated the sound level generated from a seismic wave. This method of area protection differs from the point protection of conventional seismic design, including the traditional cloaking method. The meta-barrier creates a seismic shadow zone, protecting all the buildings within the zone. The seismic shadow zone is tested by computer simulation and compared with a normal barrier.
The recent article "Ohm's Law Survives to the Atomic Scale" by Weber et al. [Science 335, 64 (1021)] reveals ohmic transport in quantized P-in-Si wires. We argue that their results have two main deficiencies: (a) the interpretation of conductance data is inadequate for serious systematics; (b) metallic-like structures hold few implications for quantum computing.
Models of nonequilibrium quantum transport underpin all modern electronic devices, from the largest scales to the smallest. Past simplifications such as coarse graining and bulk self-averaging served well to understand electronic materials. Such particular notions become inapplicable at mesoscopic dimensions, edging towards the truly quantum regime. Nevertheless a unifying thread continues to run through transport physics, animating the design of small-scale electronic technology: microscopic conservation and nonequilibrium dissipation. These fundamentals are inherent in quantum transport and gain even greater and more explicit experimental meaning in the passage to atomic-sized devices. We review their genesis, their theoretical context, and their governing role in the electronic response of meso- and nanoscopic systems.
We have developed a new method of an earthquake-resistant design to support conventional aseismic designs using acoustic metamaterials. We suggest a simple and practical method to reduce the amplitude of a seismic wave exponentially. Our device is an attenuator of a seismic wave. Constructing a cylindrical shell-type waveguide that creates a stop-band for the seismic wave, we convert the wave into an evanescent wave for some frequency range without touching the building we want to protect.
The magnetic vortices in superconductors usually repel each other. Several cases are discussed when the vortex interaction has an attractive tail and thus a minimum, leading to vortex clusters and chains. Decoration pictures then typically look like in the intermediate state of type-I superconductors, showing lamellae or islands of Meissner state or surrounded by Meissner state, but with the normal regions filled with Abrikosov vortices that are typical for type-II superconductors in the mixed state. Such intermediate-mixed state was observed and investigated in detail in pure Nb, TaN and other materials 40 years ago; last year it was possibly also observed in MgB$_2$, where it was called "a totally new state" and ascribed to the existence of two superconducting electron bands, one of type-I and one of type-II. The complicated electronic structure of MgB$_2$ and its consequences for superconductivity and vortices are discussed. It is shown that for the real superconductor MgB$_2$ which possesses a single transition temperature, the assumption of two independent order parameters with separate penetration depths and separate coherence lengths is unphysical.
Having driven a large part of the decade's progress in physics, nanoelectronics is now passing from today's realm of the extraordinary to tomorrow's commonplace. This carries the problem of turning proofs of concept into practical artefacts. Better and more sharply focussed predictive modelling will be the ultimate guide to optimising mesoscopic technology as it matures. Securing this level of understanding needs a reassessment of the assumptions at the base of the present state of the field. We offer a brief overview of the underlying assumptions of mesoscopic transport.
Where, and how, does energy dissipation of electrical energy take place in a ballistic wire? Fully two decades after the advent of the transmissive phenomenology of electrical conductance, this deceptively simple query remains unanswered. We revisit the quantum kinetic basis of dissipation and show its power to give a definitive answer to our query. Dissipation leaves a clear, quantitative trace in the non-equilibrium current noise of a quantum point contact; this signature has already been observed in the laboratory. We then highlight the current state of accepted understandings in the light of well-known yet seemingly contradictory measurements. The physics of mesoscopic transport rests not in coherent carrier transmission through a perfect and dissipationless metallic channel, but explicitly in their dissipative inelastic scattering at the wire's interfaces and adjacent macroscopic leads.
Divergence exponents of the first-order quantum correction of a two-dimensional hard-sphere Bose atoms are obtained by an effective field theory method. The first-order correction to the ground-state energy density with respect to the zeroth-order is given by $\cale_1/\cale_0 \sim |D-2|^{-\al}|\lnγ|^{-\al'}$, where $D$ is the spatial dimension, and $γ$ is the gas parameter ($γ=n a^D$). As $D \to 2$, $\al =\al'=1$. We show that the first-order quantum correction of the energy density is not perturbative in low dimensions of $D < 2.2$ regardless of any gas parameter which is much less that 1.
A ground state energy density of an interacting dilute Bose gas system is studied in the canonical transformation scheme. It is shown that the transformation scheme enables us to calculate a higher order correction of order $n a^3$ in the particle depletion and ground state energy density of a dilute Bose gas system, which corresponds to the density fluctuation contribution from the excited states. The coefficient of $n a^3$ term is shown to be $2(π- 8/3)$ for the particle depletion, and $16(π- 8/3)$ for the ground state energy density.