Bratkovsky and Levanyuk Reply:The main new result of our paper [1] is that the ferroelectric (FE) with the dead layer of thickness d is always split into domains, no matter how thin the layer is. We have found that the width of the domains a depends exponentially on d 22 when the dead layer is thin. We have also evaluated the response of the structure to external field (Fig. 2 in [1], and Ref. [2]). In the Comment [3] Tagantsev has tried to interpret our approximate Eq. (14), which he misrepresented as the main result of the Letter, within the "capacitors in series" model, by assuming that the dielectric constant of the FE is infinite, e f `.However, Tagantsev has failed to notice that e f , as found in the "capacitor" model, is not infinite, but finite and actually negative. Indeed, a simple calculation in the capacitor model gives [2]

A novel, general Green's function technique for elastic spin-dependent transport calculations is presented, which (i) scales linearly with system size and (ii) allows straightforward application to general tight-binding Hamiltonians (spd in the present work). The method is applied to studies of conductance and giant magnetoresistance (GMR) of magnetic multilayers in CPP (current perpendicular to planes) geometry in the limit of large coherence length. The magnetic materials considered are Co and Ni, with various non-magnetic materials from the 3d, 4d, and 5d transition metal series. Realistic tight-binding models for them have been constructed with the use of density functional calculations. We have identified three qualitatively different cases which depend on whether or not the bands (densities of states) of a non-magnetic metal (i) form an almost perfect match with one of spin sub-bands of the magnetic metal (as in Cu/Co spin valves); (ii) have almost pure sp character at the * Fermi level (e.g. Ag); (iii) have almost pure d character at the Fermi energy (e.g. Pd, Pt). The key parameters which give rise to a large GMR ratio turn out to be (i) a strong spin polarization of the magnetic metal, (ii) a large energy offset between the conduction band of the non-magnetic metal and one of spin sub-bands of the magnetic metal, and (iii) strong interband scattering in one of spin sub-bands of a magnetic metal. The present results show that GMR oscillates with variation of the thickness of either non-magnetic or magnetic layers, as observed experimentally.

No abstract

A novel ferromagnetic transition, accompanied by carrier density collapse, is found in doped chargetransfer insulators with strong electron-phonon coupling. The transition is driven by an exchange interaction of polaronic carriers with localized spins; the strength of the interaction determines whether the transition is first or second order. A giant drop in the number of current carriers during the transition, which is a consequence of bound pairs formation in the paramagnetic phase close to the transition, is extremely sensitive to an external magnetic field. This carrier density collapse describes the resistivity peak and the colossal magnetoresistance of doped manganites. 71.30.+h, 71.38.+i, 72.20.Jv, 75.50.Pp, 75.70.Pa The interplay of the electron-phonon and exchange interactions [1-3] is thought to be responsible for many exotic properties of oxides ranging from high-T c superconductivity in cuprates [4] to colossal magnetoresistance (CMR) and ferromagnetism in doped manganites [5][6][7][8][9]. A huge negative magnetoresistance was observed in doped perovskite manganites La 1−x D x MnO 3 (D=Ca, Sr, Ba) close to the ferromagnetic transition in a certain range of doping x ≈ 0.15 − 0.4 [6][7][8][9], and this raised a question of possible applications.The metal-insulator transition in lanthanum manganites has long been thought to be the consequence of a double exchange (DEX), which results in a varying band width of holes doped into the Mn 3+ d-shell [10], as function of the doping concentration and temperature. Recently it has been realized [11], however, that the effective carrier-spin interaction in DEX model is too weak to lead to a significant reduction of the electron band width and, therefore, cannot account for the observed scattering rate [12] (see also Ref. [13]) or for localization induced by slowly fluctuating spin configurations [14]. In view of this problem, it has been suggested [11] that the essential physics of perovskite manganites lies in the strong coupling of carriers to Jahn-Teller lattice distortions. The argument [11] was that in the high-temperature state the electron-phonon coupling constant λ is large (so that the carriers are polarons) while the growing ferromagnetic order increases the bandwidth and thus decreases λ sufficiently for metallic behavior to set in below the Curie temperature T c . A giant isotope effect [15], the sign anomaly of the Hall effect, and the Arrhenius behavior of the drift and Hall mobilities [16] over a temperature range from 2T c to 4T c unambiguously confirmed the polaronic nature of carriers in manganites.However, an early established unusual relation between magnetization and transport below T c have led to a conclusion that the polaronic hopping is the prevalent conduction mechanism also below T c [17]. Lowtemperature optical [18][19][20], electron-energy-loss (EELS) [21] and photoemission spectroscopies [22] showed that the idea [11,14] of a 'metalization' of manganites below T c is not tenable. A broad incoherent spectral feature [18][19][20]22]...

Here we demonstrate a molecular trap structure that can be formed to capture analyte molecules in solution for detection and identification. The structure is based on gold-coated nanoscale polymer fingers made by nanoimprinting technique. The nanofingers are flexible and their tips can be brought together to trap molecules, while at the same time the gold-coated fingertips form a reliable Raman hot spot for molecule detection and identification based on surface enhanced Raman spectroscopy (SERS). The molecule self-limiting gap size control between fingertips ensures ultimate SERS enhancement for sensitive molecule detection. Furthermore, these type of structures, resulting from top-down meeting self-assembly, can be generalized for other applications, such as plasmonics, meta-materials, and other nanophotonic systems.

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 (π 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.

No abstract

We study the domain structure in ferroelectric thin films with a 'passive' layer (material with damaged ferroelectric properties) at the interface between the film and electrodes within a continuous medium approximation. An abrupt transition from a monodomain to a polydomain state has been found with the increase of the 'passive' layer thickness d. The domain width changes very quickly at the transition (exponentially with d −2 ). We have estimated the dielectric response dP/dE (the slope of the hysteresis loop) in the 'fatigued' multidomain state and found that it is in agreement with experiment, assuming realistic parameters of the layer. We derive a simple universal relation for the dielectric response, which scales as 1/d, involving only the properties of the passive layer. This relation qualitatively reproduces the evolution of the hysteresis loop in fatigued samples and it could be tested with controlled experiments. It is expected that the coercive field should increase with decreasing lateral size of the film. We believe that specific properties of the domain structure under bias voltage in ferroelectrics with a passive layer can resolve the long-standing 'paradox of the coercive field 77.80.Dj, 84.32.Tt, 85.50.+k Recent studies of thin ferroelectric films have revived interest in properties of the ferroelectric domain structures. It became clear that some basic aspects of these properties remain unexplored, whereas they are of key importance for applications. For instance, the progressive loss of switchable polarization after repeated switching cycles, i.e. polarization fatigue, is a serious problem in device applications of ferroelectrics. In spite of extensive studies, the physics of the fatigue remains poorly understood. Various mechanisms were considered over the years [1][2][3][4][5][6][7][8]. In many cases the deterioration of the switching behavior, like the loss of the coercive force and of the squareness of hysteresis loop, were attributed to the growth of a 'passive layer' at the ferroelectric-electrode interface [1,3,6], or to the pinning of domain walls by defects [2,4]. In this paper, we study the effects of a passive layer (material with damaged ferroelectric properties). We assume an ideal infinite ferroelectric film and treat it within the continuous medium approximation. The model exhibits very interesting properties relevant for real systems.The passive layer leads to the appearance of a depolarizing field in the ferroelectric, so that the system would tend to transform to a polydomain state in order to reduce the field. There is, therefore, a transition between the monodomain state, when the thickness d of the passive layer is zero, to a polydomain state otherwise. What is the nature of this transition? This is the main theoretical question we address in this paper.For a short-circuited infinite ferroelectric plate, as we shall show, the domain structure exists for any value of the passive layer thickness, in disagreement with some earlier results [9]. A surprising feature of the ...