arXiv subjects

A. M. Bratkovsky

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

At least 55 records · Page 3Linked to original sources

arXiv2004

High-frequency spin valve effect in ferromagnet-semiconductor-ferromagnet structure based on precession of injected spins

New mechanism of magnetoresistance, based on tunneling-emission of spin polarized electrons from ferromagnets (FM) into semiconductors (S) and precession of electron spin in the semiconductor layer under external magnetic field, is described. The FM-S-FM structure is considered, which includes very thin heavily doped (delta-doped) layers at FM-S interfaces. At certain parameters the structure is highly sensitive at room-temperature to variations of the field with frequencies up to 100 GHz. The current oscillates with the field, and its relative amplitude is determined only by the spin polarizations of FM-S junctions at relatively large bias voltage.

cond-mat.mtrl-sci↗

arXiv2003

Comment on `Origin of combination frequencies in quantum magnetic oscillations of two-dimensional multiband metals' by T. Champel [Phys. Rev. B 65, 153403 (2002); cond-mat/0110154]

We analyze the applicability of our analytical theory of combination harmonics in canonical low-dimensional multi-band Fermi liquids, which was recently criticized by Champel (Phys. Rev. B 65, 153403 (2002)). It is shown that his claim that our analytical theory does not apply at low temperatures and in clean samples, is incorrect. We demonstrate that the analytical theory of combination harmonics is in excellent agreement with the exact numerical results even at zero temperature and for clean systems, which are the most challenging for an analytical description.

cond-mat.mes-hall↗

arXiv2003

Spin extraction from a non-magnetic semiconductor

New efficient mechanism of obtaining spin polarization in_nonmagnetic_ semiconductors at arbitrary temperutures is described. The effect appears during tunneling of electrons from a nonmagnetic semiconductors (S) into ferromagnet (FM) through a Schottky barrier modified with very thin heavily doped interfacial layer. We show that electrons with a certain spin projection are extracted from S, while electrons with the opposite spins are accumulated in S. The spin density increases and spin penetration depth decreases with current.

cond-mat.mtrl-sci↗

arXiv2003

Current rectification by simple molecular quantum dots: an ab-initio study

We calculate a current rectification by molecules containing a conjugated molecular group sandwiched between two saturated (insulating) molecular groups of different length (molecular quantum dot) using an ab-initio non-equilibrium Green's function method. In particular, we study S-(CH2)m-C10H6-(CH2)n-S dithiol with Naphthalene as a conjugated central group. The rectification current ratio ~35 has been observed at m = 2 and n = 10, due to resonant tunneling through the molecular orbital (MO) closest to the electrode Fermi level (lowest unoccupied MO in the present case). The rectification is limited by interference of other conducting orbitals, but can be improved by e.g. adding an electron withdrawing group to the naphthalene.

cond-mat.mes-hall↗

arXiv2002

Bi-stable tunneling current through a molecular quantum dot

An exact solution is presented for tunneling through a negative-U d-fold degenerate molecular quantum dot weakly coupled to electrical leads. The tunnel current exhibits hysteresis if the level degeneracy of the negative-U dot is larger than two (d>2). Switching occurs in the voltage range V1 < V < V2 as a result of attractive electron correlations in the molecule, which open up a new conducting channel when the voltage is above the threshold bias voltage V2. Once this current has been established, the extra channel remains open as the voltage is reduced down to the lower threshold voltage V1. Possible realizations of the bi-stable molecular quantum dots are fullerenes, especially C60, and mixed-valence compounds.

cond-mat.mes-hall↗

arXiv2002

Current hysteresis and memory effect in a molecular quantum dot with strong electron-vibron interaction

Theory of current hysteresis for tunneling through a molecular quantum dot (MQD) with strong electron-vibron interactions and attractive electron-electron correlations is developed. The dot is modeled as a d-fold degenerate energy level weakly coupled to the leads. The effective attractive interaction between polarons in the dot results in a "switching" phenomenon in the current-voltage characteristics when d>2, in agreement with the results for the phenomenological negative-U model. The degenerate MQD with strong electron-vibron coupling has two stable current states in certain interval of the bias voltage below some critical temperature.

cond-mat.mes-hall↗

arXiv2002

Formation and rapid evolution of domain structure at phase transitions in slightly inhomogeneous ferroelectrics and ferroelastics

We present the analytical study of stability loss and evolution of domain structure in inhomogeneous ferroelectric and ferroelastic samples for exactly solvable models. The model assumes a short-circuited ferroelectric capacitor (free ferroelastic) with two regions with slightly different critical temperatures Tc1 Tc2, where Tc1-Tc2 << Tc1, Tc2. We show that even a tiny inhomogeneity like 10-5 K results in splitting the system into domains below the phase transition temperature. At T < Tc2 the domain width a is proportional to (Tc1-T)/(Tc1-Tc2) and quickly increases with lowering temperature. The minute inhomogeneities in Tc may result from structural (growth) inhomogeneities, which are always present in real samples, and a similar role can be played by inevitable temperature gradients.

cond-mat.stat-mech↗

arXiv2002

Current rectification by molecules with asymmetric tunneling barriers

A simple experimentally accessible realization of current rectification by molecules (molecular films) bridging metal electrodes is described. It is based on the spatial asymmetry of the molecule and requires only one resonant conducting molecular level (pi-orbital). The rectification, which is due to asymmetric coupling of the level to the electrodes by tunnel barriers, is largely independent of the work function difference between the two electrodes. Results of extensive numerical studies of the family of suggested molecular rectifiers HS-(CH2)_m-C6H4-(CH2)_n-SH are presented. The highest rectification ratio ~500 is achieved at m = 2 and n = 10.

cond-mat.soft↗

arXiv2002

Bistable molecular conductors with a field-switchable dipole group

A class of bistable "stator-rotor" molecules is proposed, where a stationary bridge (stator) connects the two electrodes and facilitates electron transport between them. The rotor part, which has a large dipole moment, is attached to an atom of the stator via a single sigma bond. Hydrogen bonds formed between the rotor and stator make the symmetric orientation of the dipole unstable. The rotor has two potential minima with equal energy for rotation about the sigma bond. The dipole orientation, which determines the conduction state of the molecule, can be switched by an external electric field that changes the relative energy of the two potential minima. Both orientation of the rotor correspond to asymmetric current-voltage characteristics that are the reverse of each other, so they are distinguishable electrically. Such bistable stator-rotor molecules could potentially be used as parts of molecular electronic devices.

cond-mat.soft↗

arXiv2002

Formation and rapid evolution of domain structure at phase transitions in slightly inhomogeneous ferroelectrics

We present the first analytical study of stability loss and evolution of domain structure in inhomogeneous ferroelectric samples for exactly solvable model. The model assumes a short-circuited capacitor with two regions with slightly different critical temperatures Tc1 > Tc2, where Tc1-Tc2 << Tc1, Tc2. Even a tiny inhomogeneity like 10-5 K may result in splitting the system into domains below the phase transition temperature. At T < Tc2 the domain width $a$ is proportional to (Tc1 - T)/(Tc1 - Tc2) and quickly increases with lowering temperature. The minute inhomogeneities in Tc may result from structural (growth) inhomogeneities which are always present in real samples and a similar role can be played by inevitable temperature gradients.

cond-mat.stat-mech↗

arXiv2002

Effects of gating and contact geometry on current through conjugated molecules covalently bonded to electrodes

We study the effects of gating and contact geometry on current through self-assembled monolayers of conjugated molecules strongly coupled to gold electrodes by sulfur ``anchor groups''. The current changes by more than an order of magnitude depending on the angle between the axis of the benzene-dithiolate molecules and the normal to the electrode on the less coordinated ``top site'' position. The effect of gating is also much stronger in this case compared to higher coordinated ``hollow site'' binding of the molecule on a Au(111) surface. The large hybridization of the molecular states with electrode states for the hollow site leads to practically ohmic current-voltage characteristics. Changes in molecule-electrode geometry accompanying the gating of the SAM may be the reason for strong changes of the conductance observed by Schoen et al. in the ``slot'' geometry.

cond-mat.soft↗

arXiv2001

Three-body scattering problem and two-electron tunneling in molecular wires

We solve the Lippmann-Schwinger equation describing elastic scattering of preformed pairs (e.g. bipolarons) off a short-range scattering center and find the two-particle transmission through a thin potential barrier. While the pair transmission is smaller than the single-electron transmission in the strong-coupling limit, it is remarkably larger in the weak coupling limit. We also calculate current-voltage characteristics of a molecule - barrier - molecule junction. They show unusual temperature and voltage behavior which are experimentally verifiable at low temperatures.

cond-mat.str-el↗

arXiv2001

Orientational dependence of current through molecular films

We study the current through molecular films as a function of orientation of the molecules in the film with respect to electrodes. It may change by more than an order of magnitude, depending on the angle between the axis of the molecules and the normal to the electrode. This is a consequence of a strong directional character of $p$-orbitals that determines the conductance through the molecules. We demonstrate this result on an exactly solvable model, and present the calculations for two different experimentally accessible molecular films sandwiched between gold electrodes.

cond-mat.mtrl-sci↗

arXiv2001

Angular dependence of novel magnetic quantum oscillations in a quasi-two-dimensional multiband Fermi liquid with impurities

The semiclassical Lifshitz-Kosevich-type description is given for the angular dependence of quantum oscillations with combination frequencies in a multiband quasi-two-dimensional Fermi liquid with a constant number of electrons. The analytical expressions are found for the Dingle, thermal, spin, and amplitude (Yamaji) reduction factors of the novel combination harmonics, where the latter two strongly oscillate with the direction of the field. At the "magic" angles those factors reduce to the purely two-dimensional expressions given earlier. The combination harmonics are suppressed in the presence of the non-quantized ("background") states, and they decay exponentially faster with temperature and/or disorder compared to the standard harmonics, providing an additional tool for electronic structure determination. The theory is applied to Sr$_2$RuO$_4$.

cond-mat.str-el↗

arXiv2001

Rigidity of the elastic domain structure near the boundary of its existence in thin epitaxial films

We consider an interesting and practically important case of elastic domain structure, which is the analogue of c/a domain pattern with 90$^\circ$ walls in perovskites, and is solvable analytically for arbitrary misfit strain. There is no critical thickness, below which the domain structure cannot exist, when the "extrinsic" misfit is zero and the domains are of equal width. At the boundary of polydomain-monodomain transition the period of the pattern diverges, as does the dynamic stiffness of the domain structure. It is unlikely, therefore, that one can achieve a softness of the dielectric response of the c/a elastic domains in ferroelectric-ferroelastic thin films.

cond-mat.stat-mech↗

arXiv2001

Charge dynamics and metal-insulator transition in Si$_{1-x}$Gd$_{x}$ and Si$_{1-x}$Y$_{x}$ alloys

Carrier dynamics in amorphous a-Si$_{1-x}$RE$_{x}$ (RE=Gd, Y) films has been studied in the doping regime close to the metal-insulator transition by means of infrared spectroscopy. Optical constants throughout the entire intra-gap region ($\hbar ω<$ 1 eV) have been found to be anomalously sensitive to changes of temperature and/or magnetic field. The observed behavior is consistent with the model of hopping transport where the interaction of carriers with both the lattice and large core spin of Gd ions is taken into account.

cond-mat.str-el↗

arXiv2001

Proper ferroelastic phase transitions in thin epitaxial films with symmetry-conserving and symmetry-breaking misfit strains

We study how the ferroelastic domain structure sets in in an epitaxial film of a material with second order proper ferroelastic transition. The domain structures considered are similar to either $a_{1}/a_{2}/a_{1}/a_{2}$ or $c/a/c/a$ structures in perovskite ferroelectrics. If the "extrinsic" misfit strain, not associated with the transition, does not break the symmetry of the high-temperature phase, the phase transition in the film occurs at somewhat lower temperature compared to the bulk. The loss of stability then occurs with respect to a sinusoidal strain wave, which evolves into the domain structure with practically the same geometry and approximately the same period. In the presence of the symmetry-breaking component of the misfit strain ("extrinsic" misfit) the character of the phase transition is qualitatively different. In this case it is a {\em topological} transition between single-domain and multi-domain states, which starts from a low density of the domain walls.

cond-mat.stat-mech↗

arXiv2001

Very large dielectric response of thin ferroelectric films with the dead layers

We study the dielectric response of ferroelectric (FE) thin films with "dead" dielectric layer at the interface with electrodes. The domain structure inevitably forms in the FE film in presence of the dead layer. As a result, the effective dielectric constant of the capacitor $ε_{eff}$ increases abruptly when the dead layer is thin and, consequently, the pattern of 180-degree domains becomes "soft". We compare the exact results for this problem with the description in terms of a popular "capacitor" model, which is shown to give qualitatively incorrect results. We relate the present results to fatigue observed in thin ferroelectric films.

cond-mat.stat-mech↗