arXiv subjects

E. Zeldov

Publications and source records attributed to E. Zeldov.

At least 37 records · Page 2Linked to original sources

arXiv2007

Interplay of Anisotropy and Disorder in the Doping-Dependent Melting and Glass Transitions of Vortices in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

We study the oxygen doping dependence of the equilibrium first-order melting and second-order glass transitions of vortices in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. Doping affects both anisotropy and disorder. Anisotropy scaling is shown to collapse the melting lines only where thermal fluctuations are dominant. Yet, in the region where disorder breaks that scaling, the glass lines are still collapsed. A quantitative fit to melting and replica symmetry breaking lines of a 2D Ginzburg-Landau model further reveals that disorder amplitude weakens with doping, but to a lesser degree than thermal fluctuations, enhancing the relative role of disorder.

cond-mat.supr-con↗

arXiv2007

Spatial Determination of Magnetic Avalanche Ignition Points

Using time-resolved measurements of local magnetization in the molecular magnet Mn12-ac, we report studies of the propagation of magnetic avalanches (fast magnetization reversals) that originate from points inside the crystals rather than at the edges. The curved nature of the fronts produced by avalanches is reflected in the time-of-arrival at micro-Hall sensors placed at the surface of the sample. Assuming that the avalanche interface is a spherical bubble that grows with a radius proportional to time, we are able to locate the approximate ignition point of each avalanche in a two-dimensional cross-section of the crystal. For the samples used in these studies, avalanches in a given crystal are found to originate in a small region with a radius of roughly 150 microns.

cond-mat.mtrl-sci↗

arXiv2006

Self field of ac current reveals voltage-current law in type-II superconductors

The distribution of ac magnetic fields in a thin superconducting strip is calculated when an ac current is applied to the sample. The edge barrier of the strip to flux penetration and exit is taken into account. The obtained formulas provide a basis to extract voltage-current characteristics in the bulk and at the edges of superconductors from the measured ac magnetic fields. Based on these results, we explain the spatial and temperature dependences of the profiles of the ac magnetic field measured in a Bi2Sr2CaCu2O8 strip [D.T. Fuchs et al., Nature 91, 373 (1998)].

cond-mat.supr-con↗

arXiv2006

Photon-Induced Magnetization Changes in Single-Molecule Magnets

Microwave radiation applied to single-molecule magnets can induce large magnetization changes when the radiation is resonant with transitions between spin levels. These changes are interpreted as due to resonant heating of the sample by the microwaves. Pulsed-radiation studies show that the magnetization continues to decrease after the radiation has been turned off with a rate that is consistent with the spin's characteristic relaxation rate. The measured rate increases with pulse duration and microwave power, indicating that greater absorbed radiation energy results in a higher sample temperature. We also performed numerical simulations that qualitatively reproduce many of the experimental results. Our results indicate that experiments aimed at measuring the magnetization dynamics between two levels resonant with the radiation must be done much faster than the >20-microsecond time scales probed in these experiments.

cond-mat.mes-hall↗

arXiv2005

Equilibrium First-Order Melting and Second-Order Glass Transitions of the Vortex Matter in Bi$_2$Sr$_2$CaCu$_2$O$_8$

The thermodynamic $H-T$ phase diagram of Bi$_2$Sr$_2$CaCu$_2$O$_8$ was mapped by measuring local \emph{equilibrium} magnetization $M(H,T)$ in presence of vortex `shaking'. Two equally sharp first-order magnetization steps are revealed in a single temperature sweep, manifesting a liquid-solid-liquid sequence. In addition, a second-order glass transition line is revealed by a sharp break in the equilibrium $M(T)$ slope. The first- and second-order lines intersect at intermediate temperatures, suggesting the existence of four phases: Bragg glass and vortex crystal at low fields, glass and liquid at higher fields.

cond-mat.supr-con↗

arXiv2005

Propagation of Avalanches in Mn$_{12}$-acetate: Magnetic Deflagration

Local time-resolved measurements of fast reversal of the magnetization of single crystals of Mn12-acetate indicate that the magnetization avalanche spreads as a narrow interface that propagates through the crystal at a constant velocity that is roughly two orders of magnitude smaller than the speed of sound. We argue that this phenomenon is closely analogous to the propagation of a flame front (deflagration) through a flammable chemical substance.

cond-mat.other↗

arXiv2005

Local Measurements of Magnetization in Mn12 Crystals

The spatial profile of the magnetization of Mn12 crystals in a swept magnetic field applied along the easy axis is determined from measurements of the local magnetic induction along the sample surface using an array of Hall sensors. We find that the magnetization is not uniform inside the sample, but rather shows some spatial oscillations which become more prominent around the resonance field values. Moreover, it appears that different regions of the sample are at resonance at different values of the applied field and that the sweep rate of the internal magnetic induction is spatially non-uniform. We present a model which describes the evolution of the non-uniformities as a function of the applied field. Finally we show that the degree of non-uniformity can be manipulated by sweeping the magnetic field back and forth through part of the resonance.

cond-mat.mes-hall↗

arXiv2005

Non-equilibrium Magnetization Dynamics in the Fe_8 Single-Molecule Magnet Induced by High-Intensity Microwave Radiation

Resonant microwave radiation applied to a single crystal of the molecular magnet Fe_8 induces dramatic changes in the sample's magnetization. Transitions between excited states are found even though at the nominal system temperature these levels have negligible population. We find evidence that the sample heats significantly when the resonance condition is met. In addition, heating is observed after a short pulse of intense radiation has been turned off, indicating that the spin system is out of equilibrium with the lattice.

cond-mat.mes-hall↗

arXiv2004

Experimental Upper Bound on Superradiance Emission from Mn12 Acetate

We used a Josephson junction as a radiation detector to look for evidence of the emission of electromagnetic radiation during magnetization avalanches in a crystal assembly of Mn_12-Acetate. The crystal assembly exhibits avalanches at several magnetic fields in the temperature range from 1.8 to 2.6 K with durations of the order of 1 ms. Although a recent study shows evidence of electromagnetic radiation bursts during these avalanches [J. Tejada, et al., Appl. Phys. Lett. {\bf 84}, 2373 (2004)], we were unable to detect any significant radiation at well-defined frequencies. A control experiment with external radiation pulses allows us to determine that the energy released as radiation during an avalanche is less than 1 part in 10^4 of the total energy released. In addition, our avalanche data indicates that the magnetization reversal process does not occur uniformly throughout the sample.

cond-mat.mes-hall↗

arXiv2004

Vortex nanoliquid in high-temperature superconductors

Using a differential magneto-optical technique to visualize flow of transport currents, we reveal a new delocalization line within the reversible vortex liquid region in the presence of a low density of columnar defects. This line separates a homogeneous vortex liquid, in which all the vortices are delocalized, from a heterogeneous "nanoliquid" phase, in which interconnected nanodroplets of vortex liquid are caged in the pores of a solid skeleton formed by vortices pinned on columnar defects. The nanoliquid phase displays high correlation along the columnar defects but no transverse critical current.

cond-mat.supr-con↗

arXiv2004

Photon-Induced Magnetization Reversal in Single-Molecule Magnets

We use millimeter wave radiation to manipulate the populations of the energy levels of a single crystal molecular magnet Fe8. When a continuous wave radiation is in resonance with the transitions from the ground state to the first excited state, the equilibrium magnetization exhibits a dip. The position of this dip varies linearly with the radiation frequency. Our results provide a lower bound of 0.17 ns for transverse relaxation time and suggest the possibility that single-molecule magnets might be utilized for quantum computation.

cond-mat.mes-hall↗

arXiv2003

Melting of 'porous' vortex matter

Bitter decoration and magneto-optical studies reveal that in heavy-ion irradiated superconductors, a 'porous' vortex matter is formed when vortices outnumber columnar defects (CDs). In this state ordered vortex crystallites are embedded in the 'pores' of a rigid matrix of vortices pinned on CDs. The crystallites melt through a first-order transition while the matrix remains solid. The melting temperature increases with density of CDs and eventually turns into a continuous transition. At high temperatures a sharp kink in the melting line is found, signaling an abrupt change from crystallite melting to melting of the rigid matrix.

cond-mat.supr-con↗

arXiv2002

First order phase transition from the vortex liquid to an amorphous solid

We present a systematic study of the topology of the vortex solid phase in superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ samples with low doses of columnar defects. A new state of vortex matter imposed by the presence of geometrical contours associated with the random distribution of columns is found. The results show that the first order liquid-solid transition in this vortex matter does not require a structural symmetry change.

cond-mat.supr-con↗

arXiv2002

More Evidence for a Distribution of Tunnel Splittings in Mn$_{12}$-acetate

In magnetic fields applied parallel to the anisotropy axis, the magnetization of Mn$_{12}$ has been measured in response to a field that is swept back and forth across the resonances corresponding to steps $N=4,5,...9$. The fraction of molecules remaining in the metastable well after each sweep through the resonance is inconsistent with expectations for an ensemble of identical molecules. The data are consistent instead with the presence of a broad distribution of tunnel splittings. A very good fit is obtained for a Gaussian distribution of the second-order anisotropy tunneling parameter $X_E=-\ln(\mid E\mid/2D)$. We show that dipolar shuffling is a negligible effect which cannot explain our data.

cond-mat.mes-hall↗

arXiv2002

Vortex avalanches and self organized criticality in superconducting niobium

In 1993 Tang proposed [1] that vortex avalanches should produce a self organized critical state in superconductors, but conclusive evidence for this has heretofore been lacking. In the present paper, we report extensive micro-Hall probe data from the vortex dynamics in superconducting niobium, where a broad distribution of avalanche sizes scaling as a power-law for more than two decades is found. The measurements are combined with magneto-optical imaging, and show that over a widely varying magnetic landscape the scaling behaviour does not change, hence establishing that the dynamics of superconducting vortices is a SOC phenomenon.

cond-mat.supr-con↗

arXiv2002

V-I characteristics in the vicinity of order-disorder transition in vortex matter

The shape of the V-I characteristics leading to a peak in the differential resistance r_d=dV/dI in the vicinity of the order-disorder transition in NbSe2 is investigated. r_d is large when measured by dc current. However, for a small Iac on a dc bias r_d decreases rapidly with frequency, even at a few Hz, and displays a large out-of-phase signal. In contrast, the ac response increases with frequency in the absence of dc bias. These surprisingly opposite phenomena and the peak in r_d are shown to result from a dynamic coexistence of two vortex matter phases rather than from the commonly assumed plastic depinning.

cond-mat.supr-con↗

arXiv2002

Dynamic Creation and Annihilation of Metastable Vortex Phase as a Source of Excess Noise

The large increase in voltage noise, commonly observed in the vicinity of the peak-effect in superconductors, is ascribed to a novel noise mechanism. A strongly pinned metastable disordered vortex phase, which is randomly generated at the edges and annealed into ordered phase in the bulk, causes large fluctuations in the integrated critical current of the sample. The excess noise due to this dynamic admixture of two distinct phases is found to display pronounced reentrant behavior. In the Corbino geometry the injection of the metastable phase is prevented and, accordingly, the excess noise disappears

cond-mat.supr-con↗

arXiv2002

Investigating The Vortex Melting Phenomenon In BSCCO Crystals Using Magneto-Optical Imaging Technique

Using a novel differential magneto-optical imaging technique we investigate the phenomenon of vortex lattice melting in crystals of Bi_2Sr_2CaCu_2O_8 (BSCCO). The images of melting reveal complex patterns in the formation and evolution of the vortex solid-liquid interface with varying field (H) or temperature (T). We believe that the complex melting patterns are due to a random distribution of material disorder or inhomogeneities across the sample, which create fluctuations in the local melting temperature or field value. To study the fluctuations in the local melting temperature / field, we have constructed maps of the melting landscape T_m(H,r), viz., the melting temperature (T_m) at a given location (r) in the sample at a given field (H). A study of these melting landscapes reveals an unexpected feature: the melting landscape is not fixed, but changes rather dramatically with varying field and temperature along the melting line. It is concluded that the changes in both the scale and shape of the landscape result from the competing contributions of different types of quenched disorder which have opposite effects on the local melting transition.

cond-mat.supr-con↗