arXiv subjects

A. M. Bratkovsky

Publications and source records attributed to A. M. Bratkovsky.

At least 37 records · Page 2Linked to original sources

arXiv2007

Comment on ``Elastic Stabilization of a Single-Domain Ferroelectric State in Nanoscale Capacitors and Tunnel Junctions" [N.A. Pertsev and H. Kohlstedt, Phys. Rev. Lett. 98, 257603 (2007).]

In a recent Letter [N.A. Pertsev and H. Kohlstedt, Phys. Rev. Lett. 98, 257603 (2007)] the authors claim that "even nanoscale capacitors and tunnel junctions may have out of plane polarization sufficient for memory applications." Here we show in an elementary way that this conclusion is not substantiated by their calculations and that they should have come to the opposite conclusion within their approximations.

cond-mat.mtrl-sci↗

arXiv2007

Ensembles of plasmonic nanospheres at optical frequencies and a problem of negative index behavior

Arrays of metallic nanoparticles support individual and collective plasmonic excitations that contribute to unusual phenomena like surface enhanced Raman scattering, anomalous transparency, negative index, and subwavelength resolution in various metamaterials. We have examined the electromagnetic response of dual Kron's lattice and films containing up to three monolayers of metallic nanospheres. It appears that open cubic Kron's lattice exhibits `soft' electromagnetic response but no negative index behavior. The close-packed arrays behave similarly: there are plasmon resonances and very high transmission at certain wavelengths that are much larger than the separation between the particles, and a `soft' magnetic response, with small but positive effective index of refraction. It would be interesting to check those predictions experimentally.

cond-mat.mes-hall↗

arXiv2006

Current rectification, switching, polarons, and defects in molecular electronic devices

Devices for nano- and molecular size electronics are currently a focus of research aimed at an efficient current rectification and switching. A few generic molecular scale devices are reviewed here on the basis of first-principles and model approaches. Current rectification by (ballistic) molecular quantum dots can produce the rectification ratio ~100. Current switching due to conformational changes in the molecules is slow, on the order of a few kHz. Fast switching (~1THz) may be achieved, at least in principle, in a degenerate molecular quantum dot with strong coupling of electrons with vibrational excitations. We show that the mean-field approach fails to properly describe intrinsic molecular switching and present an exact solution to the problem. Defects in molecular films result in spurious peaks in conductance, apparent negative differential resistance, and may also lead to unusual temperature and bias dependence of current. The observed switching in many cases is_extrinsic_, caused by changes in molecule-electrode geometry, molecule reconfiguration, metallic filament formation through, and/or changing amount of disorder in a molecular film. We give experimental examples of telegraph "switching" and "hot spot" formation in the molecular films.

cond-mat.soft↗

arXiv2006

Optical Metamaterials at Near and Mid IR Range Fabricated by Nanoimprint Lithography

Two types of optical metamaterials operating at near-IR and mid-IR frequencies, respectively, have been designed, fabricated by nanoimprint lithography (NIL), and characterized by laser spectroscopic ellipsometry. The structure for the near-IR range was a metal/dielectric/metal stack "fishnet" structure that demonstrated negative permittivity and permeability in the same frequency region and hence exhibited a negative refractive index at a wavelength near 1.7 um. In the mid-IR range, the metamaterial was an ordered array of four-fold symmetric L-shaped resonators (LSRs) that showed both a dipole plasmon resonance resulting in negative permittivity and a magnetic resonance with negative permeability near wavelengths of 3.7 um and 5.25 um, respectively. The optical properties of both metamaterials are in agreement with theoretical predictions. This work demonstrates the feasibility of designing various optical negative-index metamaterials and fabricating them using the nanoimprint lithography as a low-cost, high-throughput fabrication approach.

cond-mat.mtrl-sci↗

arXiv2006

Fast polaron switching in degenerate molecular quantum dots

Devices for nano- and molecular size electronics are currently a focus of research aimed at an efficient current rectification and switching. Current switching due to conformational changes in the molecules is slow, on the order of a few kHz. Fast switching on the order of 1 THz may be achieved, at least in principle, in a degenerate molecular quantum dot with strong coupling of electrons with vibrational excitations. We show that the mean-field approach fails to properly describe intrinsic molecular switching and present an exact solution to the problem.

cond-mat.mes-hall↗

arXiv2006

Depolarizing field and "real" hysteresis loops in nanometer-scale ferroelectric films

We give detailed analysis of the effect of depolarizing field in nanometer-size ferroelectric capacitors studied by Kim et al. [Phys. Rev. Lett. 95, 237602 (2005)]. We calculate a critical thickness of the homogeneous state and its stability with respect to domain formation for strained thin films of BaTiO3 on SrRuO3/SrTiO3 substrate within the Landau theory. While the former (2.5nm) is the same as given by ab-initio calculations, the actual critical thickness is set by the domains at 1.6nm. There is a large Merz's activation field for polarization relaxation. Remarkably, the results show a_negative_ slope of the "actual" hysteresis loops, a hallmark of the domain structures in ideal thin films with imperfect screening.

cond-mat.mtrl-sci↗

arXiv2006

Polydomain ferroelectricity in very thin films [D.J. Kim et al., Phys.Rev.Lett. 95, 237602 (2005)]

T.W. Noh and his group (Kim et al.: PRL, 2005) have recently published a seminal experimental study of very thin ferroelectric (FE) BaTiO3 capacitors. They found an evidence that all their samples, including the thinnest one (d=5 nm), are multi-domain (MD). Under a high enough external field, the MD state goes over into polarization saturated state, perhaps a single domain (SD) one, which relaxes back due to domain wall motion if the external field E_0 is reduced below a certain value E_0=E_{0r}. Kim et al. have approximately identified this field as a depolarizing field due to incomplete screening by electrodes and claimed that it coincides with the one estimated from electrostatics. We found that such an interpretation does not apply and there is actually a very instructive disagreement between the theory and experiment. The data indicates (i) a presence of substantial Merz's activation field, (ii) first direct observation of negative susceptibility of multidomain films that we predicted some while ago.

cond-mat.mtrl-sci↗

arXiv2006

Phase transitions and ferroelectricity in very thin films: single- versus multidomain state

We discuss ferroelectric phase transitions into single- and multidomain states in very thin films using continuous theory. It is shown that in nearly cubic ferroelectrics the domain state may survive down to atomic film thicknesses, unlike the single domain state, which is almost always unstable or metastable. This conclusion is valid almost irrespective of the nature of electrodes (metallic or semiconducting) and whether or not the screening carriers may be present in the ferroelectric itself.

cond-mat.mtrl-sci↗

arXiv2006

Comment on 'Hysteresis, Switching, and Negative Differential Resistance in Molecular Junctions: a Polaron Model', by M. Galperin, M.A. Ratner, and A. Nitzan, Nano Lett. 5, 125 (2005)

It is shown that the ``hysteresis'' in a polaron model of electron transport through the molecule found by M.Galperin et al. [Nano Lett. 5, 125 (2005)] is an artefact of their ``mean-field'' approximation. The reason is trivial: after illegitimate replacement $\hat{n}^{2}=\hat{n}n_{0},$ where \hat{n} is the electron number operator, n_{0} the average molecular level occupation, Galperin et al. obtained non-physical dependence of a renormalized molecular energy level on the non-integer mean occupation number n_{0} (i.e. the electron self-interaction) and the resulting non-linearity of current. The exact theory of correlated polaronic transport through molecular quantum dots (MQDs) that we proposed earlier [Phys. Rev. B67, 235312 (2003)] proved that there is no hysteresis or switching in current-voltage characteristics of non-degenerate, d=1, or double degenerate, d=2, molecular bridges, contrary to the mean-field result. Switching could only appear in multiply degenerate MQDs with d>2 due to electron correlations. Most of the molecular quantum dots are in the regime of weak coupling to the electrodes addressed in our formalism.

cond-mat.mtrl-sci↗

arXiv2006

Phase coexistence and resistivity near the ferromagnetic transition of manganites

Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase [the so-called current-carrier density collapse (CCDC)] has accounted for the first-order ferromagnetic phase transition, colossal magnetoresistance (CMR), isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of CCDC. The present quantitative description of resistivity is obtained without any fitting parameters by using the experimental resistivities far away from the transition and the experimental magnetization, and essentially model independent.

cond-mat.str-el↗

arXiv2005

Spin accumulation in degenerate semiconductors near modified Schottky contact with ferromagnets

We study spin transport in forward and reverse biased junctions between a ferromagnetic metal and a degenerate semiconductor with a delta-doped layer near the interface at relatively low temperatures. We show that spin polarization of electrons in the semiconductor, Pn, near the interface increases both with the forward and reverse current and reaches saturation at certain relatively large current while the spin injection coefficient, Gamma, increases with reverse current and decreases with the forward current. We analyze the condition for efficient spin polarization of electrons in degenerate semiconductor near interface with ferromagnet. We compare the accumulation of spin polarized electrons in degenerate semiconductors at low temperatures with that in nondegenerate semiconductors at relatively high, room temperatures.

cond-mat.mtrl-sci↗

arXiv2005

Effect of impurities on transport through organic molecular films from first principles

We calculate electron transport through molecular monolayers of saturated alkanes with point defects from first principles. Single defects (incorporated Au ions, kinks, dangling bonds) produce deep localized levels in the gap between occupied and unoccupied molecular levels. Single defects produce steps on the I-V curve, whereas pairs of (unlike and like) defects give negative differential resistance peaks. The results are discussed in relation to the observed unusual transport behavior of organic monolayers and compared with transport through conjugated polythiophenes.

cond-mat.soft↗

arXiv2005

Strong effect of surfaces on resolution limit of negative-index "superlens"

We show that subwavelength imaging by negative index materials (NIM), related to their "soft" electromagnetic response, is very (and non trivially) sensitive to the surface properties. A minute deviation of dielectric permittivity epsilon or magnetic permeability mu from the ideal values epsilon = mu = -1 in thin surface layer(s) results in drastic reduction of the resolution limit of a NIM slab. There may be a gap in the polariton spectrum and this would allow establishment of a stationary regime even without losses.

cond-mat.mtrl-sci↗

arXiv2004

Smearing of phase transition due to surface effect in ferroelectric nanostructures

The boundary conditions, customarily used in the Landau-type approach to ferroelectric thin films and nanostructures, have to be modified to take into account that a surface of a ferroelectric (FE) is a defect of a ``field'' type. The surface (interface) field is coupled to a normal component of polarization and, as a result, the second order phase transitions are generally suppressed and anomalies in response are washed out, as observed experimentally.

cond-mat.mtrl-sci↗

arXiv2004

Efficient nonlinear room-temperature spin injection from ferromagnets into semiconductors through a modified Schottky barrier

We suggest a consistent microscopic theory of spin injection from a ferromagnet (FM) into a semiconductor (S). It describes tunneling and emission of electrons through modified FM-S Schottky barrier with an ultrathin heavily doped interfacial S layer . We calculate nonlinear spin-selective properties of such a reverse-biased FM-S junction, its nonlinear I-V characteristic, current saturation, and spin accumulation in S. We show that the spin polarization of current, spin density, and penetration length increase with the total current until saturation. We find conditions for most efficient spin injection, which are opposite to the results of previous works, since the present theory suggests using a lightly doped resistive semiconductor. It is shown that the maximal spin polarizations of current and electrons (spin accumulation) can approach 100% at room temperatures and low current density in a nondegenerate high-resistance semiconductor.

cond-mat.mtrl-sci↗

arXiv2004

Highly polarized injection luminescence in forward-biased ferromagnetic-semiconductor junctions at low spin polarization of current

We consider electron tunneling from a nonmagnetic $n$-type semiconductor ($n$-S) into a ferromagnet (FM) through a very thin forward-biased Schottky barrier resulting in efficient extraction of electron spin from a thin $n$-S layer near FM-S interface at low spin polarization of the current. We show that this effect can be used for an efficient polarization radiation source in a heterostructure where the accumulated spin polarized electrons are injected from $n$-S and recombine with holes in a quantum well. The radiation polarization depends on a bias voltage applied to the FM-S junction.

cond-mat.mtrl-sci↗

arXiv2004

A class of spin injection-precession ultrafast nanodevices

Spin valve ultrafast spin injection devices are described: an amplifier, a frequency multiplier, and a square-law detector. Their operation is based on injection of spin polarized electrons from one ferromagnet to another through a semiconductor layer and spin precession of the electrons in the semiconductor layer in a magnetic field induced by a (base) current in an adjacent nanowire. The base current can control the emitter current between the magnetic layers with frequencies up to several 100 GHz.

cond-mat.mtrl-sci↗

arXiv2004

Smearing of phase transition due to a surface effect or a bulk inhomogeneity in ferroelectric nanostructures

The boundary conditions, customarily used in the Landau-type approach to ferroelectric thin films and nanostructures, have to be modified to take into account that a surface of a ferroelectric (FE) is a defect of the ``field'' type. The surface (interface) field is coupled to a normal component of polarization and, as a result, the second order phase transitions are generally suppressed and anomalies in response are washed out. In FE films with a compositional (grading) or some other type of inhomogeneity, the transition into a monodomain state is suppressed, but a transition with formation of a domain structure may occur.

cond-mat.mtrl-sci↗