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Solid 4He may acquire superfluid characteristics due to the frustration of the solid phase at grain boundaries. Here, we show that an analogous effect occurs in systems with competition among charge-density-waves (CDWs) and superconductivity in the presence of disorder, as cuprate or dichalcogenide superconductors. The CDWs breaks apart in domains with topologically protected filamentary superconductivity (FSC) at the interfaces. Transport experiments carried out in underdoped cuprates with the magnetic field acting as a control parameter are shown to be in excellent agreement with the theoretical expectation. At high temperature and low fields we find a transition from CDWs to fluctuating superconductivity, weakly affected by disorder, while at high field and low temperature the protected filamentary superconducting phase appears in close analogy with "glassy" supersolid phenomena in 4He.

Magnetic properties of single crystals of Mn$_{1-x}$Fe$_x$Si solid solutions with $x < 0.2$ are investigated by pulsed field technique in magnetic fields up to 50 T. It is shown that magnetization of Mn$_{1-x}$Fe$_x$Si in the paramagnetic phase follows power law $M(B) \sim B^\alpha$ with the exponents $\alpha \sim 0.33-0.5$, which starts above characteristic fields $B_c \sim 1.5-7$ T depending on the sample composition and lasts up to highest used magnetic field. Analysis of magnetization data including SQUID measurements in magnetic fields below 5 T suggests that this anomalous behavior may be likely attributed to the formation of a field-induced Griffiths phase in the presence of spin-polaron effects., Comment: to be published in JETP Letters

We report measurements on magnetization reversal in the Fe$_8$ molecular magnet using fast pulsed magnetic fields of 1.5 kT/s and in the temperature range of 0.6-4.1 K. We observe and analyze the temperature dependence of the reversal process, which involves in some cases several resonances. Our experiments allow observation of resonant quantum tunneling of magnetization up to a temperature of $\sim$ 4 K. We also observe shifts of the resonance fields in temperature that suggest the emergence of a thermal instability---a combination of spin reversal and self-heating that may result in a magnetic deflagration process. The results are mainly understood in the framework of thermally-activated quantum tunneling transitions in combination with emergence of a thermal instability., Comment: 6 pages 5 figs

Driving a two-dimensional superconductor normal by applying a high magnetic field may lead to Cooper pair localization. In this case, there should be a quantum critical point associated with specific scaling laws. Such a transition has been evidenced in a number of low critical temperature superconducting thin films and has been suggested to occur also in high temperature cuprate superconductors. Here we show experimental evidence for two distinct quantum critical regimes when applying perpendicular magnetic fields to underdoped La2-xSrxCuO4 thin films. At intermediate values of the magnetic field (18T-20T), a "ghost" QCP is observed, for which the values of the related critical exponents point towards a fermionic -as opposed to bosonic- scenario. At higher (about 37 T) magnetic field, another QCP is observed, which suggests the existence of either a 2D/3D or a clean/dirty temperature crossover., Comment: arXiv admin note: substantial text overlap with arXiv:1204.3493

The dynamics of magnetic hysteresis, including the training effect and the field sweep rate dependence of the exchange bias, is experimentally investigated in exchange-coupled potassium split graphene nanoribbons (GNRs). We find that, at low field sweep rate, the pronounced absolute training effect is present over a large number of cycles. This is reflected in a gradual decrease of the exchange bias with the sequential field cycling. However, at high field sweep rate above 0.5 T/min, the training effect is not prominent. With the increase in field sweep rate, the average value of exchange bias field grows and is found to follow power law behavior. The response of the exchange bias field to the field sweep rate variation is linked to the difference in the time it takes to perform a hysteresis loop measurement compared with the relaxation time of the anti-ferromagnetically aligned spins. The present results may broaden our current understanding of magnetism of GNRs and would be helpful in establishing the GNRs based spintronic devices., Comment: Accepted Applied Physics Letters (In press)

Systematic measurements pertinent to the magnetocaloric effect and nature of magnetic transition around the transition temperature are performed in the 10 nm Pr0.5Ca0.5MnO3 nanoparticles (PCMO10) . Maxwell relation is employed to estimate the change in magnetic entropy. At Curie temperature TC, 83.5 K, the change in magnetic entropy discloses a typical variation with a value 0.57 J/kg K, and is found to be magnetic field dependent. From the area under the curve Delta S vs T, the refrigeration capacity is calculated at TC, 83.5 K and it is found to be 7.01 J/kg. Arrott plots infer that due to the competition between the ferromagnetic and anti ferromagnetic interactions, the magnetic phase transition in PCMO10 is broadly spread over both in temperature as well as in magnetic field coordinates. Upon tuning the particle size, size distribution, morphology, and relative fraction of magnetic phases, it may be possible to enhance the magnetocalorific effect further in PCMO10., Comment: Accepted (Journal of Applied Physics) (In press)

We report on isothermal pulsed (20 ms) field magnetization, temperature dependent AC - susceptibility, and the static low magnetic field measurements carried out on 10 nm sized Pr0.5Ca0.5MnO3 nanoparticles (PCMO10). The saturation field for the magnetization of PCMO10 (~ 250 kOe) is found to be reduced in comparison with that of bulk PCMO (~300 kOe). With increasing temperature, the critical magnetic field required to 'melt' the residual charge-ordered phase decays exponentially while the field transition range broadens, which is indicative of a Martensite-like transition. The AC - susceptibility data indicate the presence of a frequency-dependent freezing temperature, satisfying the conventional Vogel-Fulcher and power laws, pointing to the existence of a spin-glass-like disordered magnetic phase. The present results lead to a better understanding of manganite physics and might prove helpful for practical applications.

We report on exchange bias effects in 10 nm particles of Pr0.5Ca0.5MnO3 which appear as a result of competing interactions between the ferromagnetic (FM)/anti-ferromagnetic (AFM) phases. The fascinating new observation is the demonstration of the temperature dependence of oscillatory exchange bias (OEB) and is tunable as a function of cooling field strength below the SG phase, may be attributable to the presence of charge/spin density wave (CDW/SDW) in the AFM core of PCMO10. The pronounced training effect is noticed at 5 K from the variation of the EB field as a function of number of field cycles (n) upon the field cooling (FC) process. For n > 1, power-law behavior describes the experimental data well; however, the breakdown of spin configuration model is noticed at n \geq 1.

Exchange bias (EB) and the training effects (TE) in an antiferromagnetically coupled La0.7Sr0.3MnO3 / SrRuO3 superlattices were studied in the temperature range 1.8 - 150 K. Strong antiferromagnetic (AFM) interlayer coupling is evidenced from AC - susceptibility measurements. Below 100 K, vertical magnetization shifts are present due to the two remanent states corresponding to the two ferromagnetic (FM) layers at FM and AFM coupling condition. After field cooling (FC), significant decrease in the exchange bias field (HEB) is observed when cycling the system through several consecutive hysteresis loops. Quantitative analysis for the variation of HEB vs. number of field cycles (n) indicates an excellent agreement between the theory, based on triggered relaxation phenomena, and our experimental observations. Nevertheless, the crucial fitting parameter K indicates smooth training effect upon repeated field cycling, in accordance with our observation., Comment: Accepted Europhysics Letters

The transport properties of La$_{1.89}$Ce$_{0.11}$CuO$_{4}$(LCCO) and La$_{1.89}$Ce$_{0.11}$(Cu$_{0.99}$Co$_{0.01}$)O$_{4}$ (LCCO:Co) superconducting thin films are investigated. When the external field $\bf H$ is applied along the crystallographic c-axis, a double sign reversal of the Hall voltage in the mixed state of LCCO:Co thin films is observed whereas a single sign reversal is detected in LCCO. A double sign reversal of the Hall signal in LCCO can be recovered if the magnetic field is tilted away from the plane of the film. We find that the transition from one to two of the Hall sign reversal coincides with the change in the pinning from strong to weak. This temperature/field induced transition is caused either by the magnetic impurities in LCCO:Co or by the coupling between the pancake vortices and the in-plane Josephson vortices in LCCO. These results are in agreement with early theoretical and numerical predictions., Comment: 6 pages, 4 figures, the proceedings of VORTEX VII in Physica C

We report the first direct observation of the influence of high magnetic fields on the Jahn-Teller (JT) transition in TbVO4. Contrary to spectroscopic and magnetic methods, X-ray diffraction directly measures the JT distortion; the splitting between the (311)/(131) and (202)/(022) pairs of Bragg reflections is proportional to the order parameter. Our experimental results are compared to mean field calculations, taking into account all possible orientations of the grains relative to the applied field, and qualitative agreement is obtained., Comment: 11 pages, 4 figures, submitted to Phys. Rev. Lett

The contribution of superconducting fluctuations to the conductivity, or paraconductivity is studied in the underdoped regime of $La_{2-x}Sr_xCuO_4$ cuprates. A perpendicular magnetic field up to 50 T is applied to suppress the superconductivity and obtain the normal state resistivity which is then used to calculate the paraconductivity. Surprisingly enough, it is consistent with a two-dimensional Aslamazov-Larkin (AL) regime of Gaussian fluctuations close to the critical temperature. At higher temperature, the paraconductivity shows a power-law decrease in temperature (as $T^{-\alpha}$) as was previously shown for underdoped $YBa_2Cu_3O_{7-\delta}$ and $Bi_2Sr_2CaCu_2O_{8+\delta}$ samples. Our observations are not consistent with the existence of Kosterlitz-Thouless fluctuations. This tends to indicate that the superconducting pair amplitude is not already defined above $T_C$ in the pseudogap state., Comment: 10 pages, 3 figures, 1 bib file

We report on in-situ resistivity measurements on GaMnAs during post-growth annealing in different atmospheres. A drop in the resistivity is observed when the GaMnAs is exposed to oxygen, which indicates that the passivation of Mn interstitials (Mn_I) at the free surface occurs through oxidation. The presence of oxygen can therefore be an important annealing condition for the optimization of GaMnAs thin films, all the more since the oxidation appears to be limited to the sample surface. Annealing in an oxygen-free atmosphere leads to an increase in the resistivity indicating a second annealing mechanism besides the out-diffusion of Mn_I. According to our magnetization and Hall effect data, this mechanism reduces the amount of magnetically and electrically active Mn atoms., Comment: 3 pages, 3 figures

We report experimental studies of crystals of Mn12 molecular magnetic clusters in pulsed magnetic fields with sweep rates up to 4x10^3 T/s. The steps in the magnetization curve are observed at fields that are shifted with respect to the resonant field values. The shift systematically increases as the rate of the field sweep goes up. These data are consistent with the theory of the collective dipolar relaxation in molecular magnets., Comment: 4 pages, 4 figures

Magnetoresistance (MR) of Bi-2212 single crystals with T$_{c}$ $\approx 87-92 K$ is studied in pulsed magnetic fields up to 50T along the c-axis in a wide temperature range. The negative out-of-plane and the positive in-plane MRs are measured in the normal state. Both MRs have similar magnitudes, exceeding any orbital contribution by two orders in magnitude. These are explained as a result of the magnetic pair-breaking of preformed pairs. Resistive upper critical fields H$_{c2}$(T) determined from the in-- and out-of-plane MRs are about the same. They show non-BCS temperature dependences compatible with the Bose-Einstein condensation field of preformed charged bosons.

A large positive magnetoresistivity (up to tens of percents) is observed in both underdoped and overdoped superconducting La2-xSrxCuO4 epitaxial thin films at temperatures far above the superconducting critical temperature Tc. For the underdoped samples, this magnetoresistance far above Tc cannot be described by the Kohler rule and we believe it is to be attributed to the influence of superconducting fluctuations. In the underdoped regime, the large magnetoresistance is only present when at low temperatures superconductivity occurs. The strong magnetoresistivity, which persists even at temperatures far above Tc, can be related to the pairs forming eventually the superconducting state below Tc. Our observations support the idea of a close relation between the pseudogap and the superconducting gap and provide new indications for the presence of pairs above Tc.

In type II hard superconductors the irreversible magnetization shows an impressive variety of different magnetic field dependencies. In this paper we will try to describe the M(H) relation at fixed temperature To by a numerical model in which we incorporate two features: the magnetic and electric field dependency of the local critical current density j(To,H,E). The electric field is determined by the magnetic field sweep rate; in effect E ~ dB/dt. We confront the model with experimental pulsed field magnetization measurements (PFMM) on a fast melt processed YBa2Cu3O7 sample., Comment: 4 pages, PDF-file

Recently, the normal state resistivity of high temperature superconductors (in particular in La2-xSrxCuO4 single crystals) has been studied extensively in the region below Tc by suppressing the superconducting state in high magnetic fields. In the present work we report on the normal state resistance of underdoped La2-xSrxCuO4 thin films under epitaxial strain, measured far below Tc by applying pulsed fields up to 60 T. We will compare the transport measurements on these high temperature superconductors with transport data reported for the Sr2.5Ca11.5Cu24O41 spin ladder compound. This comparison leads to an interpretation of the data in terms of the recently proposed 1D quantum transport model and the charge-stripe models., Comment: 5 pages, PDF file

The understanding of flux jumps in the high temperature superconductors is of importance since the occurrence of these jumps may limit the perspectives of the practical use of these materials. In this work we present the experimental study of the role of heavy ion irradiation in stabilizing the HTSC against flux jumps by comparing un-irradiated and 7.5 10^10 Kr-ion/cm2 irradiated (YxTm1-x)Ba2Cu3O7 single crystals. Using pulsed field magnetization measurements, we have applied a broad range of field sweep rates from 0.1T/s up to 1800 T/s to investigate the behavior of the flux jumps. The observed flux jumps, which may be attributed to thermal instabilities, are incomplete and have different amplitudes. The flux jumps strongly depend on the magnetic field, on the magneto-thermal history of the sample, on the magnetic field sweep rate, on the critical current density jc, on the temperature and on the thermal contact with the bath in which the sample is immersed., Comment: 5 pages, PDF-file

The normal state resistivity of high temperature superconductors can be probed in the region below Tc by suppressing the superconducting state in high magnetic fields. Here we present the normal state properties of YBa2Cu3Ox thin films in the underdoped regime and the normal state resistance of La2-xSrxCuO4 thin films under epitaxial strain, measured below Tc by applying pulsed fields up to 60 T. A universal rho(T) behaviour is reported. We interpret these data in terms of the recently proposed 1D quantum transport model with the 1D paths corresponding to the charge stripes., Comment: 5 pages, PDF and PS, including figures, presented at MOS99 and accepted for publication in J. of Low Temp. Phys

We have studied the influence of columnar defects, created by heavy-ion (Kr) irradiation with doses up to 6 10^11 Kr-ions/cm2, on the superconducting critical parameters of single crystalline (YxTm1-x)Ba2Cu3O7. Magnetisation measurements in pulsed fields up to 50 T in the temperature range 4.2 - 90 K revealed that: (i) in fields up to T the critical current Jc(H,T) is considerably enhanced and (ii) down to temperatures T ~ 40 K the irreversibility field Hirr(T) is strongly increased. The field range and magnitude of the Jc(H,T) and Hirr(T) enhancement increase with increasing irradiation dose. To interpret these observations, an effective matching field was defined. Moreover, introducing columnar defects also changes the pinning force fp qualitatively. Due to stronger pinning of flux lines by the amorphous defects, the superconducting critical parameters largely exceed those associated with the defect structures in the unirradiated as-grown material: Jc,irrad(77 K, 5 T) ^3 10* Jc,ref(77 K, 5 T)., Comment: 11 pages, all PDF, contribution to Physica C

The temperature dependence of the resistivity r(T) of the novel Sr2.5Ca11.5Cu24O41 ladder compound under hydrostatic pressure of up to 8 GPa has been explained by assuming that the relevant length scale for electrical transport is given by the magnetic correlation length related to the opening of a spin-gap in a 1D even-chain spin-ladder (1D-SL). The pressure dependence of the gap was extracted by applying this model to r(T) data obtained at different pressures. The r(T) dependence of the underdoped cuprate YBa2Cu4O8 demonstrates a remarkable scaling with the r(T) of the 1D-SL compound Sr2.5Ca11.5Cu24O41. This scaling implies that underdoped cuprates at Tc < T < T* are in the 1D regime and their pseudo-gap below T* is the spin-gap in the even-chain 1D-SL formed at T < T* in these materials., Comment: 10 pages, all PDF, contribution to Europhys.Lett."