Boldizsár Jankó - Academia.edu (original) (raw)

Papers by Boldizsár Jankó

Bulletin of the American Physical Society, Mar 17, 2010

ABSTRACT Clear correlations have been found recently between the on- and off-events in the intens... more ABSTRACT Clear correlations have been found recently between the on- and off-events in the intensity trajectories of single colloidal quantum dots. These so-called memory effects have been found by the application of the commonly used threshold analysis. We propose, for the first time, a theoretical explanation for these correlations by using the theoretical framework of multiple recombination centers for fluorescence intermittency (blinking). The correlations are found to be threshold dependent and the Pearson correlation coefficient is shown to be both positive (on-on and off-off) or negative (on-off). We demonstrate that the model of multiple recombination centers can easily reproduce both the correct trends and signs of the correlations. The long correlations, as opposed to other models of blinking, are intrinsic to our model.

Bulletin of the American Physical Society, Mar 18, 2010

present a new physical model resolving a long-standing mystery of the power-law distributions of ... more present a new physical model resolving a long-standing mystery of the power-law distributions of the blinking times in single colloidal quantum dot fluorescence. The model considers the non-radiative relaxation of the exciton through multiple recombination centers. Each center is allowed to switch between two quasi-stationary states. We point out that the conventional threshold analysis method used to extract the exponents of the distributions for the on-times and off-times has a serious flaw: The qualitative properties of the distributions strongly depend on the threshold value chosen for separating the on and off states. Our new model explains naturally this threshold dependence, as well as other key experimental features of the single quantum dot fluorescence trajectories, such as the power-law power spectrum (1/f noise).

Nano Letters, Jan 24, 2013

A variety of optically active nanoscale objects show extremely long correlations in the fluctuati... more A variety of optically active nanoscale objects show extremely long correlations in the fluctuations of fluorescence intensity (blinking). Here we performed a systematic study to quantitatively estimate the power spectral density (PSD) of the fluorescence trajectories of colloidal and self-assembled quantum dots (QDs), nanorods (NRs), nanowires (NWs), and organic molecules. We report for the first time a statistically correct method of PSD estimation suitable for these systems. Our method includes a detailed analysis of the confidence intervals. The striking similarity in the spectra of these nanoscale systems, including even a "nonblinking" quantum dot investigated by Wang and collaborators (Nature 2009, 459, 685−689), is powerful evidence for the existence of a universal physical mechanism underlying the blinking phenomenon in all of these fluorophores (Frantsuzov et al. Nat. Phys. 2008, 4, 519−522). In this paper we show that the features of this universal mechanism can be captured phenomenologically by the multiple recombination center model (MRC) we suggested recently for explaining single colloidal QD intermittency. Within the framework of the MRCs we qualitatively explain all of the important features of fluorescence intensity fluctuations for a broad spectrum of nanoscale emitters.

Nano Letters, Jun 19, 2015

ACS Nano, Sep 21, 2012

Cross-sectional SEM images Figure S1. Cross-sectional SEM images illustrating obtained PMMA layer... more Cross-sectional SEM images Figure S1. Cross-sectional SEM images illustrating obtained PMMA layer thicknesses. Observed values range from 4.5-5.5 µm. Microscopy movies Along with this document, we provide two movies taken at 20 frames per second to illustrate the observed I em enhancement/modulation effect. The underlying V a waveform is a f=1 Hz triangle wave with 200 V pp. 1f emission modulation is observed for both single nanowires and with multiple nanowires within the same field of view. The first movie (MOV_wire 62_regular_bias.avi) illustrates the magnitude of the modulation in a single wire. The second movie (MOV_wire27_regular bias.avi) shows synchronous modulation of two wires in the field of view.

Nano Letters, Jul 13, 2010

Journal of Physics and Chemistry of Solids, Dec 1, 2002

In the last few years evidence has been accumulating that there are a multiplicity of energy scal... more In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature Tc is different from the energy gap onset temperature T *. In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter ∆sc is to be distinguished from the excitation gap ∆; in this way, pseudogap effects persist below Tc. It has been argued by many in the community that the presence of these distinct energy scales demonstrates that the pseudogap is unrelated to superconductivity. In this paper we show that this inference is incorrect. We demonstrate that the difference between the order parameter and excitation gap and the contrasting dependences of T * and Tc on hole concentration x and magnetic field H follow from a natural generalization of BCS theory. This simple generalized form is based on a BCS-like ground state, but with self consistently determined chemical potential in the presence of arbitrary attractive coupling g. We have applied this mean field theory with some success to tunneling, transport, thermodynamics and magnetic field effects. We contrast the present approach with the phase fluctuation scenario and discuss key features which might distinguish our precursor superconductivity picture from that involving a competing order parameter. cond-mat/0107275

APS March Meeting Abstracts, Mar 1, 2002

Physical Review Letters, Aug 7, 1995

We nd that if two superconducting islands of di erent number parity are linked by a tunnel juncti... more We nd that if two superconducting islands of di erent number parity are linked by a tunnel junction the unpaired electron in the odd island has a tendency to tunnel into the even island. This process leads to uctuations in time of the number parity of each island, giving rise to a random telegraph noise spectrum with a characteristic frequency that has an unusual temperature dependence. This new phenomenon should be observable in a Cooper-pair pump and similar single-electron tunneling devices.

We present a new physical model resolving a long-standing mystery of the power-law distributions ... more We present a new physical model resolving a long-standing mystery of the power-law distributions of the blinking times in single colloidal quantum dot fluorescence. The model considers the non-radiative relaxation of the exciton through multiple recombination centers. Each center is allowed to switch between two quasi-stationary states. We point out that the conventional threshold analysis method used to extract the

Journal of Applied Physics, Feb 1, 2010

We investigate the effect of single and multiple impurities on the Zeeman-localized, spin polariz... more We investigate the effect of single and multiple impurities on the Zeeman-localized, spin polarized bound states in dilute magnetic semiconductor hybrid system. Such bound states appear whenever a dilute magnetic semiconductor showing giant Zeeman effect is exposed to an external magnetic field showing nanoscale inhomogeneity. We consider the specific example of a superconductor-dilute magnetic semiconductor hybrid, calculate the energy spectrum and the wave functions of the bound states in the presence of a single impurity, and monitor the evolution of the bound state as a function of the impurity strength and impurity location with respect to the center of the Zeeman trapping potential. Our results have important experimental implications as they predict robust spin textures even for than than ideal samples. We find that for all realistic impurity strengths the Zeeman bound state survives the presence of the impurity. We also investigate the effect of a large number of impurities and perform ensemble averages with respect to the impurity locations. We find that the spin polarized Zeeman bound states are very robust, and they remain bound to the external field inhomogeneity throughout the experimentally relevant region of impurity concentration and scattering strength.

Physical Review B, Sep 14, 2006

We explore the possibility of using the inhomogeneous magnetic field carried by an Abrikosov vort... more We explore the possibility of using the inhomogeneous magnetic field carried by an Abrikosov vortex in a type-II superconductor to localize spin-polarized textures in a nearby magnetic semiconductor quantum well. We show how Zeeman-induced localization induced by a single vortex is indeed possible, and use these results to investigate the effect of a periodic vortex array on the transport properties of the magnetic semiconductor. In particular, we find an unconventional Integer Quantum Hall regime, and predict directly testable experimental consequences due to the presence of the periodic spin polarized structure induced by the superconducting vortex lattice in the magnetic semiconductor.

Superconductor Science and Technology, Jan 19, 2011

ABSTRACT Using a combination of the phenomenological Ginzburg–Landau theory and micromagnetic sim... more ABSTRACT Using a combination of the phenomenological Ginzburg–Landau theory and micromagnetic simulations, we study properties of a superconducting film with an array of soft magnetic dots on top. An external in-plane magnetic field gradually drives the magnets from an out-of-plane or magnetic vortex state to an in-plane single-domain state, which changes spatially the distribution of the superconducting condensate. If induced by the magnets, the vortex–antivortex molecules exhibit rich transitions as a function of the applied in-plane field. At the same time, we show how the magnetic dots act as very effective dynamic pinning centers for vortices in an applied perpendicular magnetic field.

investigate the electronic structure of a superconductor in proximity of nanoscale ferromagnets (... more investigate the electronic structure of a superconductor in proximity of nanoscale ferromagnets (SC/FM hybrids) and find that a wide variety of superconducting order parameter landscape can be achieved by varying the parameters and the magnetic state of the nanomagnets. In particular, we propose an energy-angular momentum dispersion ε(l) with the energy minimum at l ̸ = 0 in systems with superconductivity suppressed in a ring. This resembles to Landau's energy-momentum dispersion ε(k) for the roton in superfluid, a dispersion that also exhibits a local minimum at k ̸ = 0. We show how such structures will emerge in specific examples SC/FM hybrids and investigate these systems via Ginzburg-Landau and Bogoliubov-de Gennes calculations. Our results show that the low-lying bound states correspond to a non-zero angular momentum. In the presence of injected current, our system becomes analogous to an inverted pendulum, which could be driven towards equilibrium with an AC magnetic field.

Nucleation and Atmospheric Aerosols, 2005

We investigated possibility of using local magnetic field originating from ferromagnetic island d... more We investigated possibility of using local magnetic field originating from ferromagnetic island deposited on the top of semiconductor quantum well to produce zero-and one-dimensional traps for quasi-particles with spin. In particular we considered two shapes of experimentally made magnets-cylindrical and rectangular. In the case of ferromagnetic micro-disk the trap can localize spin in three dimensions, contrary to the rectangular micro-magnet which creates a trap that allows free propagation in one direction. We present in detail prediction for absorption spectrum around the main absorption edge in both type of micro-magnets.

Nature, May 1, 2005

The continuous need for miniaturization and increase in device speed 1 exerts pressure on the ele... more The continuous need for miniaturization and increase in device speed 1 exerts pressure on the electronics industry to explore new avenues of information processing. One possibility is to use the spin to store, manipulate and carry information. Indeed, spintronics may hold the promise of providing such a new paradigm 2. However, all spintronics applications are faced with formidable challenges in attempting to find 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 (SC) film and a paramagnetic diluted magnetic semiconductor (DMS) quantum well (QW). Our proposal is based on the observation that the inhomogeneous magnetic fields of the SC create local spin and charge textures in the DMS, leading to effects such as Bloch oscillations, an unusual Quantum Hall Effect, etc. We exploit the recent progress in manipulating magnetic flux bundles (vortices) in superconductors 3,4 and show how these can create, manipulate and control the spin textures in DMS.

Physical Review Letters, Jun 19, 2003

We investigate the possibility of charge carrier localization in magnetic semiconductors due to t... more We investigate the possibility of charge carrier localization in magnetic semiconductors due to the presence of a highly inhomogeneous external magnetic field. As an example, we study in detail the properties of a magnetic semiconductor-permalloy disk hybrid system. We find that the giant Zeeman response of the magnetic semiconductor in conjunction with the highly non-uniform magnetic field created by the vortex state of a permalloy disk can lead to Zeeman localized states at the interface of the two materials. These trapped states are chiral, with chirality controlled by the orientation of the core magnetization of the permalloy disk. We calculate the energy spectrum and the eigenstates of these Zeeman localized states, and discuss their experimental signatures in spectroscopic probes.

Scientific Reports, Apr 1, 2014

Despite its apparent simplicity, the idealized model of a particle constrained to move on a circl... more Despite its apparent simplicity, the idealized model of a particle constrained to move on a circle has intriguing dynamic properties and immediate experimental relevance. While a rotor is rather easy to set up classically, the quantum regime is harder to realize and investigate. Here we demonstrate that the quantum dynamics of quasiparticles in certain classes of nanostructured superconductors can be mapped onto a quantum rotor. Furthermore, we provide a straightforward experimental procedure to convert this nanoscale superconducting rotor into a regular or inverted quantum pendulum with tunable gravitational field, inertia, and drive. We detail how these novel states can be detected via scanning tunneling spectroscopy. The proposed experiments will provide insights into quantum dynamics and quantum chaos.

arXiv (Cornell University), May 6, 2008

We study the effect of magnetic Mn ions on the two-band superconductor MgB 2 , and compute both t... more We study the effect of magnetic Mn ions on the two-band superconductor MgB 2 , and compute both the total and spin resolved scanning tunneling spectrum in the vicinity of the magnetic impurity. We show that when the internal structure of the Mn ion's d-shell is taken into account, multiple Shiba states appear in the spectrum. The presence of these multiplets could alter significantly the overall interpretation of local tunneling spectra for a wide range of superconducting hosts and magnetic impurities.

arXiv (Cornell University), Jun 30, 2011

The ground state with vorticity larger than one in mesoscopic superconductors in applied magnetic... more The ground state with vorticity larger than one in mesoscopic superconductors in applied magnetic field may manifest as a 'giant'-vortex, where all vortices coalesce into a single singularity of the order parameter. Such a multi-quanta vortex may split into individual vortices (and vice versa) as a function of e.g. applied current, magnetic field or temperature. Here we show that such transitions can be identified by heat-capacity measurements, as the formation or splitting of a giant-vortex results in a clear jump in measured heat capacity vs. external drive. We attribute this phenomenon to an abrupt change in the density of states of the quasiparticle excitations in the vortex core(s), and further link it to a sharp change of the magnetic susceptibility at the transition-proving that formation of a giant-vortex can also be detected by conventional magnetometry.