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We study the local vortex-vortex interaction force fi of the structure frozen during a field-cooling process in an electron-irradiated Bi2Sr2CaCu2O8+δ sample. We compute this magnitude from snapshots of the vortex structure obtained via magnetic decoration experiments at various fields H in the same sample. Since the observed structures correspond to the equilibrium ones frozen at T∼Tirr(H)[1], at this temperature the local modulus of fi roughly equals the local pinning force at the decorated surface of the sample. We estimate the most probable local pinning force from the mode value of the fi(r)distribution, fpm. We found that fpm grows algebraically with H and in electron-irradiated samples is 50-20% smaller than for samples with columnar defects.

We study the entropy jump associated with the first-order vortex melting transition (FOT) in Bi2Sr2CaCu2O8+δ crystals by means of Hall probe magnetometry. The samples present a diluted distribution of columnar defects (CD) introduced by irradiation with Xe ions. The FOT is detected in ac transmittivity measurements as a paramagnetic peak, the height of which is proportional to the enthalpy difference entailed by the transition. By applying the Clausius-Clapeyron relation, we quantify the evolution of the entropy jump Δs as a function of the FOT temperature, TFOT, in both pristine crystals and crystals with CD. On increasing the density of CD, Δs decreases monotonically with respect to values found in pristine samples. The Δs versus TFOT dependence in the case of pristine samples follows reasonably well the theoretical prediction of dominant electromagnetic coupling for a model neglecting the effect of disorder. The data for samples with a diluted distribution of CD are not properly described by such a theoretical model.

We study the field for the penetration of a first vortex, HP, for vortex matter nucleated in micron-sized samples with edges aligned along the nodal and antinodal directions of the d-wave superconducting order parameter of Bi2Sr2CaCu2O8-δ. Here we present evidence that the HP for vortex matter nucleated in mesoscopic samples with edges parallel to the nodal direction is larger than for the antinodal case, ∼72% at low temperatures. This finding supports the theoretical proposal that surface Andreev bound states appearing in a sample with edges parallel to the nodal direction would produce an anomalous Meissner current that increases the Bean-Livingston barrier for vortex penetration. Fil: Dolz, Moira Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich". Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto de Física Aplicada "Dr. Jorge Andrés Zgrablich"; Argentina Fil: Cejas Bolecek, Néstor René. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Puig, Joaquin Roberto. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Pastoriza, Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Nieva, Gladys Leonor. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Guimpel, Julio Juan. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina Fil: Van Der Beek, Cornelis Jacominus. Ecole Polytechnique. Departement Physique. Laboratoire Des Solides Irradies; Francia. Centre National de la Recherche Scientifique; Francia Fil: Konczykowski, M.. Centre National de la Recherche Scientifique; Francia. Ecole Polytechnique. Departement Physique. Laboratoire Des Solides Irradies; Francia Fil: Fasano, Yanina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina. Universidad Nacional de Cuyo; Argentina

We report time- and angle-resolved spectroscopic measurements in optimally doped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$. The photoelectron intensity maps are monitored as a function of temperature, photoexcitation density, and delay time from the pump pulse. We evince that thermal fluctuations are effective only for temperatures near the critical value whereas photoinduced fluctuations scale linearly at low pumping fluence. The minimal energy to fully disrupt the superconducting gap slightly increases when moving off the nodal direction. No evidence of a pseudogap arising from other phenomena than pairing has been detected in the explored region of reciprocal space. On the other hand, a model accounting for the finite pair breaking explains the gap filling both in the near-nodal as well as in the off-nodal direction. Finally, we observed that nodal quasiparticles develop a faster dynamics when pumping the superconductor with fluence large enough to induce the total collapse of the gap.

We directly image individual vortex positions in nanocrystals in order to unveil the structural property that contributes to the depletion of the entropy jump entailed at the first-order transition. On reducing the nanocrystal size, the density of topological defects increases near the edges over a characteristic length. Within this "healing-length" distance from the sample edge, vortex rows tend to bend, while towards the center of the sample, the positional order of the vortex structure is what is expected for the Bragg-glass phase. This suggests that the healing length may be a key quantity to model confinement effects in the first-order transition of extremely layered vortex nanocrystals. Fil: Cejas Bolecek, Néstor René. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Dolz, Moira Ines. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto de Física Aplicada ; Argentina Fil: Pastoriza, Hernan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Konczykowski, M.. Centre National de la Recherche Scientifique; Francia Fil: Van Der Beek, Cornelis Jacominus. Centre National de la Recherche Scientifique; Francia Fil: Kolton, Alejandro Benedykt. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Fasano, Yanina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina

Small-angle neutron scattering on high quality single crystalline Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ reveals the transition from a low--field vortex solid phase with orientational order to a vortex polycrystal at high magnetic field. The vortex order-disorder transition is correlated with the second-peak feature in isothermal hysteresis loops, and is interpreted in terms of the generation of supplementary vortex solid dislocations. The sharp drop of the structure factor above the second peak field is explained by the dynamics of freezing of the vortex ensemble in the high field phase.

Despite extensive work on high-temperature superconductors, the critical behavior of an incipient condensate has so far been studied exclusively under equilibrium conditions. Here, we excite Bi2Sr2CaCu2O8+d with a femtosecond laser pulse and monitor the subsequent nonequilibrium dynamics of the mid-infrared conductivity. Our data allow us to discriminate temperature regimes where superconductivity is either coherent, fluctuating or vanishingly small. Above the transition temperature Tc, we make the striking observation that the relaxation to equilibrium exhibits power-law dynamics and scaling behavior, both for optimally and underdoped superconductors. Our findings can in part be modeled using time-dependent Ginzburg-Landau theory and provide strong indication of universality in systems far from equilibrium., Comment: 5 pages, 4 figures

We have measured the ultrasound velocity of quasi-two-dimensional superconductor CeCoIn 5 with extremely large Pauli paramagnetic susceptibility. The results indicate that the new high field superconducting phase, which is revealed by the recent heat capacity measurements, is characterized by the unusual softening of flux line lattice. The softening is most likely due to the collapse of the flux line lattice tilt modulus and transition to quasi-two-dimensional vortex state. These results provide a strong evidence that the high field phase is the Fulde–Ferrell–Larkin–Ovchinnikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicularly to the vortices.

Spontaneous ferromagnetic domains in lightly Ca-doped La1−xCaxMnO3 single crystals have been visualized and investigated by means of the magneto-optical technique. In marked difference to the magnetic contrast structures associated with magneto-crystalline anisotropy, which appear only in applied magnetic field, spontaneous ferromagnetic domains show up at low temperatures below the Curie temperature in zero applied field and are characterized by oppositely oriented spontaneous magnetization in adjacent domains. Ferromagnetic domains seen in zero field cooled samples take the form of almost periodic, corrugated stripe-like structures. Application of even a very weak magnetic field during cooling through the magnetic ordering transition changes the stripe domain structures into a bubble domain system.

It is widely accepted that the first-order vortex liquid-solid phase transition is associated with a crystalline solid phase and the second order transition with an amorphous one. The combination of a technique that determines the order of the transition with the visualization of the vortex structure has allowed the detection, for the first time, of a first-order liquid-solid transition without structural symmetry change. The results show that the quasi-long range order of the solid phase is not a necessary condition for the first-order phase transition to occur. This opens an important question on the microscopic origin of the liquid-solid phase transition in vortex matter.

The resistive and magnetic properties of La1-x Ca x MnO3 single crystals are investigated. In particular, properties of close-to-critically doped crystals with x = 0.22 are confronted with those of underdoped crystals with x = 0.18 and x = 0.2 and optimally doped crystals with x = 0.3. A systematic investigation of nonlinear transport shows that the critically doped crystal does not simply constitute an intermediate case between strongly nonlinear underdoped and linear, optimally doped one. The observed low-temperature resistivity increase can be interpreted in terms of orbital ordering or as a manifestation of an intrinsic tunnelling mechanism. The transport measurements have been completed by magneto-optic investigations which suggest that the low temperature resistivity of the underdoped x = 0.18 compound is dominated by tunnelling through intrinsic barriers associated with twin domains, while phase separation dominates in the critically doped x = 0.22 compound.

The dynamic behavior of the magnetization around the second peak was studied for strongly underdoped Bi2Sr2CaCu2O8+x single crystals by the local Hall probe technique. The strong material anisotropy results in a very low second peak field, as low as 65 G . The second peak in the magnetization curves is identified as the sudden change of the flux creep energy barrier versus screening current. This indicates the different electrodynamics of the low-field quasiordered versus the high-field disordered phase.

The Bose-glass transition in underdoped Bi 2 Sr 2 CaCu 2 O 8+ x single crystals heavy-ion irradiated at room temperature was investigated by using a local Hall probe magnetometer. It is shown that the introduction of columnar defects, not only significantly shifts the Bose-glass lines to much higher fields, but also results in a drastic increase of T c and decrease of the penetration depth λ due to the modification of the oxygen concentration during the irradiation.

Using two methods, the bolometric method and the cavity perturbation technique, we have observed the Josephson plasma resonance (JPR) in heavily underdoped Bi 2 Sr 2 CaCu 2 O 8+δ crystals (T c = 69.4 K) in the Bragg-glass phase in different magnetic fields. From these data, we obtain the penetration depth along the c-axis, λ c (0) = 229 ± 10 μm. The JPR results yielded an experimental estimate of the wandering length r w of vortex pancakes in the samples (r w 300-550 nm at 52 K depending on the magnetic field applied). We observe an increase of r w with increasing applied dc magnetic field applied.

A short overview of the present state of the magneto-optical imaging of magnetic flux structures in superconductors and magnetic materials is given. Modern applications of the technique use ferrimagnetic garnet films with in-plane anisotropy as magneto-optical indicators. Advantages and limitations of the technique are reviewed and its possible further development is discussed.

We study geometrical confinement effects in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8 +\delta}$ mesoscopic vortex-matter with edge-to-surface ratio of $7-12$%. Samples have in-plane square and circular edges, 30\,$\mu$m widths, and $\sim 2\,\mu$m thickness. Direct vortex imaging reveals the compact planes of the structure align with the sample edge by introducing topological defects. The defects density is larger for circular than for square edges. Molecular dynamics simulations suggest this density is not an out-of-equilibrium property but rather determined by the geometrical confinement., Comment: accepted in Journal of Low Temperature Physics

The low temperature heat capacity C p and microwave surface impedance Z s in the vortex state of YNi 2 B 2 C have been studied. In contrast to conventional s-wave superconductors, C p shows a nearly H 1/2 -dependence. This H 1/2 -dependence is little affected by the introduction of the columnar defects. On the other hand, flux flow resistivity obtained from Z s increases linearly with H . These results indicate that in the vortex state of YNi 2 B 2 C the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.

We measured the heat capacity ${C}_{p}$ and microwave surface impedance ${Z}_{s}$ in the vortex state of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$. In contrast to conventional $s$-wave superconductors, ${C}_{p}$ shows a $\sqrt{H}$ dependence. This $\sqrt{H}$ dependence persists even after the introduction of the columnar defects which change the electronic structure of the vortex core regime and destroy the regular vortex lattice. On the other hand, flux flow resistivity is nearly proportional to $H$. These results indicate that the vortex state of ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ is fundamentally different from the conventional $s$-wave counterparts, in that the delocalized quasiparticle states around the vortex core are important, similar to $d$-wave superconductors.

Dynamic and thermodynamic magnetization experiments on heavy-ion-irradiated single-crystalline ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ are correlated in order to clarify the nature of the mixed-state phase diagram. It is shown that whereas the entropy contribution to the free energy in the London regime plays a minor role in unirradiated crystals and irradiated crystals at fields close to or above the matching field ${B}_{\ensuremath{\varphi}},$ it becomes very important at low fields in irradiated crystals with high ${B}_{\ensuremath{\varphi}}.$ The direct determination of the entropy contribution to the free energy from the reversible magnetization allows one to determine not only the correct values of the pinning energy, but also to extract quite detailed information on pancake vortex alignment. The characteristic field ${H}_{\mathrm{int}}\ensuremath{\sim}\frac{1}{6}{B}_{\ensuremath{\varphi}}$ at which intervortex repulsion begins to determine the vortex arrangement and the reversible magnetization is shown to coincide with a sharp increase in the irreversibility field ${H}_{\mathrm{irr}}(T)$ and with the recoupling transition found in Josephson plasma resonance. Above ${H}_{\mathrm{int}},$ the repulsive interaction between vortices causes both the vortex mobility to decrease and pancake alignment to increase. At higher fields $\ensuremath{\gtrsim}\frac{1}{3}{B}_{\ensuremath{\varphi}}\ensuremath{\gg}{B}_{c1},$ free vortices outnumber those that are trapped on a columnar defect. This causes the decrease of the c-axis resistivity and a second crossover of the irreversibility field, to a regime where it is determined by plastic creep.

We have measured an angle-dependent contribution to the equilibrium magnetization of a YBa_2Cu_3O_{7-d} single crystal with columnar defects created by 5.8GeV Pb ions. This contribution manifests itself as a jump in the equilibrium torque signal, when the magnetic field crosses that of the defects. The magnitude of the signal, which is observed in a narrow temperature interval less than 2K and for fields up to about twice the irradiation field, is used to estimate the the energy gained by vortex pinning on the defects. The vanishing of the effective pinning energy at a temperature below Tc is attributed to its renormalization by thermal fluctuations.

Electron irradiation is investigated as a way to dope the topological insulator Bi${}_{2}$Te${}_{3}$. For this, $p$-type Bi${}_{2}$Te${}_{3}$ single crystals have been irradiated with 2.5 MeV electrons at room temperature and electrical measurements have been performed in situ as well as ex situ in magnetic fields up to 14 T. The defects created by irradiation act as electron donors, allowing the compensation of the initial hole-type conductivity of the material as well as the conversion of the conductivity from $p$ to $n$ type. The changes in carrier concentration are investigated using the Hall effect and Shubnikov--de Haas (SdH) oscillations, clearly observable in the $p$-type samples before irradiation, but also after the irradiation-induced conversion of the conductivity to $n$ type. The SdH patterns observed for the magnetic field along the trigonal axis can be entirely explained assuming the contributions of only one valence and one conduction band, respectively, and Zeeman splitting of the orbital levels.

We present a comprehensive overview of vortex pinning in single crystals of the isovalently substituted iron-based superconductor BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$, a material that qualifies as an archetypical clean superconductor, containing only sparse strong pointlike pins [in the sense of C. J. van der Beek et al., Phys. Rev. B 66, 024523 (2002)]. Widely varying critical current values for nominally similar compositions show that flux pinning is of extrinsic origin. Vortex configurations, imaged using the Bitter decoration method, show less density fluctuations than those previously observed in charge-doped Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$ single crystals. Analysis reveals that the pinning force and energy distributions depend on the P content $x$. However, they are always much narrower than in Ba(Fe${}_{1\ensuremath{-}x}$Co${}_{x}$)${}_{2}$As${}_{2}$, a result that is attributed to the weaker temperature dependence of the superfluid density on approaching ${T}_{c}$ in BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$. Critical current density measurements and pinning force distributions independently yield a mean distance between effective pinning centers $\overline{\mathcal{L}}\ensuremath{\sim}90$ nm, increasing with increasing P content $x$. This evolution can be understood as being the consequence of the P dependence of the London penetration depth. Further salient features are a wide vortex free ``Meissner belt'', observed at the edge of overdoped crystals, and characteristic chainlike vortex arrangements, observed at all levels of P substitution.

A good correlation of twins on the basal surface of flux-grown Bi2Sr2CaCu2Ox (BSCCO) single crystals with surface. growth steps is observed, the b-axis being perpendicular to the steps and, thus, parallel to the growth direction. It is found that mono-twin BSCCO single crystals produced by the travelling solvent floating zone method also grow preferentially along b, i.e. nearly perpendicularly to the boule axis, contrary to the common belief. This new understanding of the morphology of growth explains the nature of major defects in these crystals, which considerably change their measured superconducting properties, in a different way.

Frequency-dependent observations of magnetic flux structures are used to show that pinning plays a principal role in the whole mixed state in Bi2Sr2CaCu2O8 (BSCCO) single crystals. We speculate that the random pinning force on the moving vortices may dominate over thermal fluctuations and considerably modify the position of the vortex lattice phase transition.

We calculate the nonlinear AC response of a thin superconducting strip in perpendicular field, shielded by an edge current due to the geometrical barrier. A comparison with the results for infinite samples in parallel field, screened by a surface barrier, and with those for screening by a bulk current in the critical state, shows that the AC response due to a barrier has general features that are independent of geometry, and that are significantly different from those for screening by a bulk current in the critical state. By consequence, the nonlinear (global) AC susceptibility can be used to determine the origin of magnetic irreversibility. A comparison with experiments on a Bi 2 Sr 2 CaCu 2 O 8+δ crystal shows that in this material, the low-frequency AC screening at high temperature is mainly due to the screening by an edge current, and that this is the unique source of the nonlinear magnetic response at temperatures above 40 K.

SUMMARY Individual and average growth patterns of the facial dimensions Nasion-Gnathion and Sella-Gonion, as related to statural growth were studied. The sample consisted of 134 girls aged 7-14 years. Data were analysed using a multivariate extension of the multilevel model for longitudinal data. The results confirmed that the mean growth curves of Nasion-Gnathi on, Sella-Gonion and body height parallel each other to a large extent. At an individual level it appeared that the pubertal growth spurt of body height and the growth spurts of both facial dimensions are coincident. The major results of the present study pertain to the dynamic (age-dependent) relationship between statural growth and the growth of the face. It appears that there exists a stronger relationship between the growth velocities of standing height, Sella-Gonion and Nasion-Gnathion than between the actual lengths of the three variables themselves. The strongest relation was found between the growth velocity of body height and that of SellaGonion. These findings may be a contribution for diagnosis and treatment planning of individual cases.

We introduce a simple method of an MgB2 film preparation using sequential electron-beam evaporation of B-Mg two-layer (followed by in-situ annealing) on the NbN buffered Si(100) substrate. The Transmission Electron Microscopy analyses confirm a growth of homogeneous nanogranular MgB2 films without the presence of crystalline MgO. A sensitive measurement of temperature dependence of microwave losses shows a presence of intergranular weak links close the superconducting transition only. The MgB2 films obtained, about 200 nm thick, exhibit a maximum zero resistance critical temperature of 36 K and critical current density of 3x10^7 A/cm^2 at 13.2 K, Comment: 11 pages, 6 figures, submitted to Appl. Phys. Lett

We report on the temperature and field dependence of the ac susceptibility and magnetic hysteresis of a number of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} single crystals with very low twin boundary densities and with defects added in a controlled manner to the crystal surfaces. It is shown that surface damage is by far the most important source of flux pinning, and that substantial shifts in the irreversibility line can be controlled by the number and type of defects added to the surface.

The frequency-dependent shielding of an applied ac field is used to study the vortex dynamics in a Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystal irradiated with 5.8-GeV Pb ions. At fields below the matching field {ital B}{sub {Phi}}{approx}200 G, the sample exhibits strong pinning due to the irradiation-induced columnar defects. The dynamics in this field regime is shown to be in accurate agreement with the predictions of the Bose-glass theory of Nelson and Vinokur [Phys. Rev. B 48, 13 082 (1993)]. We propose to explain deviations from the Bose-glass predictions at fields greater than {ital B}{sub {Phi}}, and temperatures exceeding 85 K, by vortex decoupling and the appearance of a surface barrier, respectively. The crossover between creep of pancake vortices above {ital B}{sub {Phi}} and Bose-glass creep below {ital B}{sub {Phi}} is marked by a break in the irreversibility line. At all temperatures the irreversibility field is determined by the disappearance of the shielding current due to flux creep. No evidence for critical scaling of the resistivity was found.

International audience; Compensation of intrinsic charges is widely used to reduce the bulk conductivity of 3D topological insulators (TIs). Here we use low temperature electron irradiation-induced defects paired with in situ electrical transport measurements to fine-tune the degree of compensation in Bi2Te3. The coexistence of electrons and holes at the point of optimal compensation can only be explained by bulk carriers forming charge puddles. These need to be considered to understand the electric transport in compensated TI samples, irrespective of the method of compensation.

We report on the temperature and field dependencies of the resistivity of an untwinned $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ single crystal before and after 1 MeV electron irradiation in fields up to 8 T. We show, for the first time, direct suppression of the first order vortex liquid-to-solid phase transition due to the controlled introduction of point defects, and that transport in the vortex liquid is dominated by viscosity rather than by single vortex pinning. Furthermore, we demonstrate that the first order vortex transition can be recovered by reducing the density of point defects through subsequent annealing of the sample.

The angular dependence of the ac screening of Bi[sub 2]Sr[sub 2]CaCu[sub 2]O[sub 8+[delta]] single crystals irradiated with 5.8 GeV Pb ions is used to demonstrate the importance of interactions between vortex segments in different CuO[sub 2] layers. The field tilt angle below which pinning by the columnar defects is effective is very large: [Theta][sub [ital a]][approx]70[degree]. The magnitude and frequency dependence of the shielding current as well as the frequency dependence of the irreversibility line are self-consistently described by the Bose-glass theory. The irreversibility line is shown to be determined by the flux creep rate.

The surprisingly rapid relaxation of the sustainable current density in the critical state of single crystalline (Ba,K)Fe_2As_2 is investigated for magnetic fields oriented parallel to the c-axis and to the ab-plane respectively. Due to the inadequacy of standard analysis procedures developed for flux creep in the high temperature superconducting cuprates, we develop a simple, straightforward data treatment technique that reveals the creep mechanism and the creep exponent µ. At low magnetic fields, below the second magnetization peak, µ varies only slightly as function of temperature and magnetic flux density B. From the data, we determine the temperature- and field dependence of the effective activation barrier for creep. At low temperatures, the measured current density approaches the zero--temperature critical current density (in the absence of creep) to within a factor 2, thus lending credence to earlier conclusions drawn with respect to the pinning mechanism. The comparable values of the experimental screening current density and the zero-temperature critical current density reveals the limited usefulness of the widely used ''interpolation formula''.

We discuss the diffusion of magnetic flux in a field-cooled (“hard”) superconducting slab in a creep regime in which E ∝ | J | σ J . Bryksin and Dorogovtsev recently discussed flux diffusion in a pinningless (“soft”) superconductor in which E ∝ | B | J . This problem is closely related to the flux-creep one with σ =1, and provides additional insight into the possible types of behaviour. We list a series of possible long-term asymptotic solutions of a scaling form, which are either analytically exact or accurately calculated. We check numerically that the relevant long-term solution is approached after various initial conditions. Amongst other conclusions we find S =d(In| M |)/d(In t )→-1/ σ or -1/2 σ , after application and removal of an additional field, according to whether the disturbance has reached the sample centre or not. A relaxing sandpile model appears to have wide validity when creep is only a small correction to the critical state, as when σ→∞.

We have measured the low temperature heat capacity C p in the vortex state of YNi 2 B 2 C. In contrast to conventional s-wave superconductors, C p shows a nearly H -dependence. This H -dependence persists even after the introduction of the columnar defects which change the electronic structure of the vortex core regime dramatically and strongly disturb the regular vortex lattice. These results indicate that the vortex state of YNi 2 B 2 C is fundamentally different from the conventional s-wave counterparts, in that the delocalized quasiparticle states around the vortex core play a much more important role, similar to d-wave superconductors.

Miniature Hall-probe arrays were used to measure the critical current densities for the three main directions of vortex motion in the stoichiometric LiFeAs superconductor. These correspond to vortices oriented along the c-axis moving parallel to the ab-plane, and to vortices in the ab-plane moving perpendicular to, and within the plane, respectively. The measurements were carried out in the low-field regime of strong vortex pinning, in which the critical current anisotropy is solely determined by the coherence length anisotropy parameter, {\epsilon}_{\xi}. This allows extraction of {\epsilon}_{\xi} at magnetic fields far below the upper critical field B_c2. We find that increasing magnetic field decreases the anisotropy of the coherence length.

The interrelation between heterogeneity and flux pinning is studied in Ba(Fe1−xCox)2As2 single crystals with widely varying Co content x. Magnetic Bitter decoration of the superconducting vortex ensemble in crystals with x=0.075 and x=0.1 reveals highly disordered vortex structures. The width of the Meissner belt observed at the edges of the crystals, and above the surface steps formed by cleaving, as well as the width of the intervortex distance distribution, indicate that the observed vortex ensemble is established at a temperature just below the critical temperature Tc. The vortex interaction energy and pinning force distributions extracted from the images strongly suggest that the vortex lattice disorder is attributable to strong pinning due to spatial fluctuations of Tc and of the superfluid density. Correlating the results with the critical current density yields a typical length scale of the relevant disorder of 40-60 nm.

The pinning of quantized flux lines, or vortices, in the mixed state is used to quantify the effect of impurities in iron-based superconductors (IBS). Disorder at two length scales is relevant in these materials. Strong flux pinning resulting from nm-scale heterogeneity of the superconducting properties leads to the very disordered vortex ensembles observed in the IBS, and to the pronounced maximum in the critical current density jc at low magnetic fields. Disorder at the atomic scale, most likely induced by the dopant atoms, leads to "weak collective pinning" and a magnetic field-independent contribution jcoll. The latter allows one to estimate quasi-particle scattering rates., Comment: 4 pages; ECRYS 2011 - International School and Workshop on Electronic Crystals, Carg\`ese : France (2011)

We have studied the field, frequency, and amplitude dependence of the ac susceptibility on a single crystal of the electron-doped superconductor Pr[sub 1.85]Ce[sub 0.15]CuO[sub 4[minus][ital y]]. We show that a thermally activated flux motion over effective energy barriers [ital U]([ital T],[ital H])=[ital U][sub 0](1[minus][ital T]/[ital T][sub [ital c]])[ital H][sup [minus]0.6], with [ital U][sub 0][approx]40 meV, can account for the dc-field and frequency effects. The amplitude dependence of the ac response in this superconducting system is discussed and it is argued that it is a signature of the critical state.

International audience; The field-driven transition from an ordered Bragg glass to a disordered vortex phase in singlecrystalline MgB2 is tuned by an increasing density of point defects, introduced by electron irradiation. The discontinuity observed in magnetization attests to the first-order nature of the transition. The temperature and defect density dependences of the transition field point to vortex pinning mediated by fluctuations in the quasiparticle mean free path, and reveal the mechanism of the transition in the absence of complicating factors such as layeredness or thermal fluctuations.

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations of the dopant atom density on the scale of a few dozen to one hundred nm. These variations do not go beyond 5 % - we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-T fields is characterized by the presence of a peak effect, the location of which in the (B,T)--plane is consistent with an order-disorder transition of the vortex lattice.

International audience; We studied Aharonov-Bohm effect on thin graphite single crystals containing columnar defects (nanoholes) produced by irradiation with heavy ions. The thickness of irradiated single crystals has been varied from 50 nm down to less then 1 nm by thinning the thicker samples with beam-plasma etching. For irradiated samples we found the existence of oscillating, field-periodic contribution of magnetoresistance with a periodicity corresponding to the flux quantum hc/e per area of nanohole independently of sample thickness. The results point to the significant contribution of Dirac fermions to the oscillating part of magnetoresistance and also to the existence of the edge states located near the nanohole perimeter.

Flux motion in a ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ single crystal has been studied by using ac susceptibility experiments under a bias dc field for H parallel to the c axis. It is found that the irreversibility line is given by H(T)=H(0)(1-t${)}^{2}$. Analysis of the susceptibility curves within the framework of a thermally activated flux motion allows the extraction of the activation energies U(T,B). It turns out that U(T=0,B=1T)\ensuremath{\approxeq}40 meV, which is notably larger than the energies typically found in p-type high-temperature superconductors. Single-vortex and collective-pinning mechanisms by oxygen vacancies are investigated to account for the observed field dependence of the activation energies. The dependence of energy absorption on the ripple ac field is also investigated.

The magnetization curves of rapidly relaxing type-II superconductors are calculated by means of Monte Carlo simulations and numerical solutions of the partial differential equation for thermally activated flux-creep. Several models for the current dependence of the activation energy ( U ( j )= U c (1- j / j c ), U ( j )=( U c / μ )(( j c / j ) μ -1) and U ( j )= U c ( j c / j )) are considered. The calcu lated curves reproduce all the features exhibited by experimental magnetization curves, even when the critical current is assumed to be field-independent. This remarkable result shows explicitly that strong flux relaxation effects can lead to spurious field-dependent critical currents. The characteristic features of the magnetization curves are related to the relaxation behaviour of the corresponding flux density profiles. The dependence of hysteresis loops on the magnetic field sweep rate is investigated in detail and is shown to contain basically the same information as the time dependence of the magnetization during relaxation.

The temporal evolution of flux profiles in superconducting slabs and cylinders is investigated numerically for the situation where the flux-creep activation barrier U depends explicitly on current density j, e.g., U(j) ∞ [(jc/j)μ - 1] or U(j) ∞ln(jc/j). Both field-independent and field-dependent forms of the critical current density jc are considered. Exact numerical results for the time-dependent magnetization and relaxation time τ are compared to approximate analytical solutions. When the flux-creep activation barriers diverges as j→0, flux- and current-density profiles evolve towards an asymptotic behaviour that is independent of initial conditions. The time-dependent magnetization in the case of full flux penetration is then well described by the appropriate form for the magnetization of the sample in the critical state, but with a time-dependent surface current density replacing the critical current density. The current-dependent flux-creep activation barrier may be obtained from low-temperature experiments using the analysis method of Maley et al. [7], even when jc depends explicitly on B.

Measurements of the relaxation of the non-equilibrium magnetic moment of Bi2Sr2CaCu2O8+δ single crystals in fields between 0.1 and 12T are used to study the low-temperature vortex state in this extremely anisotropic material. From the data, we obtian (1) the critical current density at T=0, jc(0); (2) the flux-flow resistivity at j=jc(0)_; (3) the flux-creep activation barrier U as function of current density j, induction B and Temperature T. From collective pinning theory, it is found that (two-dimensional) point vortices in the CuO2-layers are prinned individually. The flux-creep activation barrier is well described by two-dimensional collective creep theory at sufficienty high current density. Although low-current U(j)-data are inconclusive as to the nature of the low-temperature vortex state, critical scaling shows that it is most likely a vortex glass, with vanishing linear restivity at zero current density. Above the second-order phase transition at TG, the same activation barrier is measured as in previously published AC-susceptibility experiments. Deviations from critical scaling at low magnetic fields find an explanation if the vortex-glass transitions is viewed as the phase-decoupling line suggested by Glazman and Koshelev [Phys. Rev. B 43 (1991) 2835], but in the presence of pinning.

Results on the driving-field amplitude dependence of the AC-susceptibility of a Bi2Sr2CaCu2O8+ delta -single crystal measured in an applied DC magnetic field are presented. The data obtained in DC fields higher than 2.5T strongly support 2D plastic creep as determining the diamagnetic transition. At lower fields the susceptibility behaviour is more complex, although at all fields flux motion seems to be diffusive.