Boldizsar Janko

Universal emission intermittency in quantum dots, nanorods and nanowires

Virtually all known fluorophores exhibit mysterious episodes of emission intermittency. A remarkable feature of the phenomenon is a power-law distribution of on- and off-times observed in colloidal semiconductor quantum dots, nanorods, nanowires and some organic dyes. For nanoparticles, the resulting power law extends over an extraordinarily wide dynamic range: nine orders of magnitude in probability density and five to six orders of magnitude in time. Exponents hover about the ubiquitous value of -3/2. Dark states routinely last for tens of seconds—practically forever on quantum mechanical timescales. Despite such infinite states of darkness, the dots miraculously recover and start emitting again. Although the underlying mechanism responsible for this phenomenon remains a mystery and many questions persist, we argue that substantial theoretical progress has been made.

Reducing vortex density in superconductors using the `ratchet effect.'

A serious obstacle impeding the application of low- and high-temperature superconductor devices is the presence of trapped magnetic flux,: flux lines or vortices can be induced by fields as small as the Earths magnetic field. Once present, vortices dissipate energy and generate internal noise, limiting the operation of numerous superconducting devices,. Methods used to overcome this difficulty include the pinning of vortices by the incorporation of impurities and defects, the construction of flux ‘dams’, slots and holes, and magnetic shields, which block the penetration of new flux lines in the bulk of the superconductor or reduce the magnetic field in the immediate vicinity of the superconducting device. The most desirable method would be to remove the vortices from the bulk of the superconductor, but there was hitherto no known phenomenon that could form the basis for such a process. Here we show that the application of analternating current to a superconductor patterned with an asymmetric pinning potential can induce vortex motion whose direction is determined only by the asymmetry of the pattern. The mechanism responsible for this phenomenon is the so-called ‘ratchet effect’, and its working principle applies to both low- and high-temperature superconductors. We demonstrate theoretically that, with an appropriate choice of pinning potential, the ratchet effect can be used to remove vortices from low-temperature superconductors in the parameter range required for various applications.

Pairing Fluctuation Theory of Superconducting Properties in Underdoped to Overdoped Cuprates

Pseudogap phenomena in the cuprates are of interest not only because of the associated unusual normal-state properties, but more importantly because of the constraints which these phenomena impose on the nature of the superconductivity and its associated high Tc. Moreover, this superconducting state presents an interesting challenge to theory: while the normal state is highly unconventional, the superconducting phase exhibits some features of traditional BCS superconductivity along with others which are strikingly different. Thus far, there is no consensus on a theory of cuprate superconductivity. Scenarios which address the pseudogap state below Tc can be distinguished by the character of the excitations responsible for destroying superconductivity. In the theory of Lee and Wen [1], the destruction of the superconducting phase is associated with the excitation of the low-lying quasiparticles near the d-wave gap nodes. By contrast, Emery and Kivelson [2] argue that the destruction of the superconductivity is associated with low frequency, long wavelength phase fluctuations within a microscopically inhomogeneous model, based on one dimensional “stripes.” In the present paper, we present an alternative scenario in which, along with the quasiparticles of the usual BCS theory, there are additionally incoherent (but not preformed) pair excitations of finite momentum q, which assist in the destruction of superconductivity. This approach is based on a self-consistent treatment of the coupling of single particle and pair states. It represents a natural extension of BCS theory to the short coherence length (j) regime and provides a quantitative framework for addressing cuprate superconductivity. Here, we find a pronounced departure from BCS behavior in the underdoped limit which is continuously reduced with increasing hole concentration x. We derive a phase diagram for Tc and the zero temperature gap, Ds0d, as a function of x, which is in semiquantitative agreement with (the anomalous) behavior observed in cuprate experiments, and we compute properties of the associated superconducting state such as the superfluid density rs and Josephson critical current Ic. When these are plotted as rssT dyrss0d and IcsT dyIcs0d ,a s a function of T yTc, we deduce a quite remarkable, nearly

Pseudogap effects induced by resonant pair scattering

We demonstrate how resonant pair scattering of correlated electrons above T_c can give rise to pseudogap behavior. This resonance in the scattering T-matrix appears for superconducting interactions of intermediate strength, within the framework of a simple fermionic model. It is associated with a splitting of the single peak in the spectral function into a pair of peaks separated by an energy gap. Our physical picture is contrasted with that derived from other T-matrix schemes, with superconducting fluctuation effects, and with preformed pair (boson-fermion) models. Implications for photoemission and tunneling experiments in the cuprates are discussed.

Collective Interaction-Driven Ratchet for Transporting Flux Quanta

We propose and study a novel way to produce a dc transport of vortices when applying an ac electrical current to a sample. Specifically, we study superconductors with a graduated random pinning density, which transports interacting vortices as a ratchet system. We show that a ratchet effect appears as a consequence of the long range interactions between the vortices. The pinned vortices create an asymmetric periodic flux density profile, which results in an asymmetric effective potential for the unpinned interstitial vortices. The latter exhibit a net longitudinal rectification under an applied transverse ac electric current.

Strongly Enhanced Pinning of Magnetic Vortices in Type-II Superconductors by Conformal Crystal Arrays

Conformal crystals are non-uniform structures created by a conformal transformation of regular two-dimensional lattices. We show that gradient-driven vortices interacting with a conformal pinning array exhibit substantially stronger pinning effects over a much larger range of field than found for random or periodic pinning arrangements. The pinning enhancement is partially due to matching of the critical flux gradient with the pinning gradient, but the preservation of the sixfold ordering in the conformally transformed hexagonal lattice plays a crucial role. Our results can be generalized to a wide class of gradient-driven interacting particle systems such as colloids on optical trap arrays.

Magnetic scattering of spin polarized carriers in (In, Mn)Sb dilute magnetic semiconductor.

Magnetoresistance measurements on the magnetic semiconductor (In, Mn)Sb suggest that magnetic scattering in this material is dominated by isolated Mn2+ ions located outside the ferromagnetically ordered regions when the system is below T(c). A model is proposed, based on the p-d exchange between spin-polarized charge carriers and localized Mn2+ ions, which accounts for the observed behavior both below and above the ferromagnetic phase transition. The suggested picture is further verified by high-pressure experiments, in which the degree of magnetic interaction can be varied in a controlled way.

Manipulating spin and charge in magnetic semiconductors using superconducting vortices

The continuous need for miniaturization and increase in device speed drives the electronics industry to explore new avenues of information processing. One possibility is to use electron spin to store, manipulate and carry information. All such ‘spintronics’ applications are faced with formidable challenges in finding fast and efficient ways to create, transport, detect, control and manipulate spin textures and currents. Here we show how most of these operations can be performed in a relatively simple manner in a hybrid system consisting of a superconducting film and a paramagnetic diluted magnetic semiconductor (DMS) quantum well. Our proposal is based on the observation that the inhomogeneous magnetic fields of the superconducting film create local spin and charge textures in the DMS quantum well, leading to a variety of effects such as Bloch oscillations and an unusual quantum Hall effect. We exploit recent progress in manipulating magnetic flux bundles (vortices) in superconductors and show how these can create, manipulate and control the spin textures in DMSs.

Superconducting transitions from the pseudogap state: d -wave symmetry, lattice, and low-dimensional effects

We investigate the behavior of the superconducting transition temperature within a previously developed BCS-Bose Einstein crossover picture. This picture, based on a decoupling scheme of Kadanoff and Martin, further extended by Patton, can be used to derive a simple form for the superconducting transition temperature in the presence of a pseudogap. We extend previous work which addressed the case of s-wave pairing in jellium, to explore the solutions for T_c as a function of variable coupling in more physically relevant situations. We thereby ascertain the effects of reduced dimensionality, periodic lattices and a d-wave pairing interaction. Implications for the cuprate superconductors are discussed.

BCS SUPERCONDUCTIVITY WITH FIXED NUMBER PARITY

We investigate superconductivity in a grand canonical ensemble with [ital fixed] [ital number] [ital parity] (even or odd). In the low-temperature limit we find small corrections to the BCS gap equation and dispersion [ital E]([ital k]). The even-odd free-energy difference in the same limit decreases linearly with temperature, in accordance with the behavior observed experimentally and previously arrived at from a quasiparticle model. The theory yields deviations from the BCS predictions for the specific heat, ultrasound attenuation, nucleon spin-relaxation rate, and electromagnetic absorption.