arXiv subjects

H. J. Kim

Publications and source records attributed to H. J. Kim.

At least 235 records · Page 13Linked to original sources

arXiv2001

Magnetic relaxation and critical current density of $MgB_2$ thin films

Magnetic relaxation and critical current density have been measured on a $MgB_2$ thin film in a wide region of temperature with the magnetic field up to 8 T. The irreversibility line has also been determined. It is found that the relaxation rate has a very weak temperature dependence below $1/2 T_c$ showing a clear residual relaxation rate at zero K, which cannot be easily explained as due to thermally activated flux creep. Furthermore the relaxation rate has a strong field dependence. The flux dynamics of thin films are very similar to that of high pressure synthesized bulks although the relaxation rate in thin film is systematically higher than that of a bulk sample. All the results here together with those from the bulk samples suggest that the flux dynamics may be dominated by the quantum effects, such as quantum fluctuation and tunneling.

cond-mat.supr-con↗

arXiv2001

Dendritic flux patterns in MgB2 films

Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and gradual flux penetration in an applied magnetic field, we observe an abrupt invasion of complex dendritic structures. When the applied field subsequently decreases, similar dendritic structures of the return flux penetrate the film. The static and dynamic properties of the dendrites are discussed.

cond-mat.supr-con↗

arXiv2001

Dendritic magnetic instability in superconducting MgB2 films

Magneto-opitcal studies of a c-oriented epitaxial MgB2 film show that below 10 K the global penetration of vortices is dominated by complex dendritic structures abruptly entering the film. We suggest that the observed behavior is due to a thermo-magnetic instability which is supported by vortex dynamics simulations. The instability is also responsible for large fluctuations in the magnetization curves in MgB2 at low temperatures.

cond-mat.supr-con↗

arXiv2001

Optical studies of charge dynamics in c-axis oriented superconducting MgB2 films

Temperature dependent optical conductivities and DC resistivity of c-axis oriented superconducting (Tc = 39.6 K) MgB2 films (~ 450 nm) have been measured. The normal state ab-plane optical conductivities can be described by the Drude model with a temperature independent Drude plasma frequency of ω_{p,D}=13,600 +/- 100 cm-1 or 1.68 +/- 0.01 eV. The normal state resistivity is fitted by the Bloch-Gruneisen formula with an electron-phonon coupling constant λ_{tr} = 0.13 +/- 0.02. The optical conductivity spectra below T_c of these films suggest that MgB2 is a multi-gap superconductor.

cond-mat.supr-con↗

arXiv2001

Anisotropic Superconductivity in Epitaxial MgB2 Films

High-quality epitaxial MgB2 thin films prepared by pulsed laser deposition with Tc = 39 K offer the opportunity to study the anisotropy and robustness of the superconducting state in magnetic fields. We measure the in-plane electrical resistivity of the films in magnetic fields to 60T and estimate the superconducting upper critical field Hc(0) = 24 +- 3 T for field oriented along the c-axis, and Hab(0) = 30 +- 2 T for field in the plane of the film. We find the zero-temperature coherence lengths xi_c(0) = 30 A and xi_ab(0) = 37 A to be shorter than the calculated electronic mean free path l = 100 +- 50 A, which places our films in the clean limit. The observation of such large upper critical fields from clean limit samples, coupled with the relatively small anisotropy, provides strong evidence of the viability of MgB2 as a technologically important superconductor.

cond-mat.supr-con↗

arXiv2001

High temporal stability of supercurrents in MgB(2) materials

Fine grained polycrystalline samples of MgB(2) superconductor were synthesized from the elements to contain < 5 % of impurity phases, according to X-ray powder diffractometry. The superconductive transition was sharp with a midpoint Tc = 38.5 K. The magnetization in the vortex state was studied as a function of applied field H, temperature T, and time t. From the equilibrium magnetization, the London penetration depth was obtained. The supercurrent density J(T,H,t) in the vortex state (derived from the irreversible magnetization) decreases approximately linearly with T, in contrast to the quasi-exponential falloff in high-Tc superconductors. The current is highly stable in time, with normalized decay rates S = -d ln(J)/d ln(t) well below those in high-Tc materials. These results are compared with those of other superconductors.

cond-mat.supr-con↗

arXiv2001

Critical Current Density of YBa{2}Cu{3}O{7-d} Low-Angle Grain Boundaries in Self-Field

A study has been perfomed on the superconducting critical current density J_{c} flowing across low angle grain boundaries in epitaxial thin films of YBa{2}Cu{3}O{7-d} (YBCO). The materials studied were dual grain boundary rings deposited on SrTiO{3} and containing 2, 3, 5 and 7 degree tilt boundaries. The current density in self-field was determined by magnetometric methods at temperatures from 5 K to T_c. We conclude that at the higher temperatures of coated conductor applications, there is limited potential for improving J_{c} by reducing the grain boundary angle below about 3 degrees.

cond-mat.supr-con↗

arXiv2000

Spatiotemporal Stochastic Resonance in Fully Frustrated Josephson Ladders

We consider a Josephson-junction ladder in an external magnetic field with half flux quantum per plaquette. When driven by external currents, periodic in time and staggered in space, such a fully frustrated system is found to display spatiotemporal stochastic resonance under the influence of thermal noise. Such resonance behavior is investigated both numerically and analytically, which reveals significant effects of anisotropy and yields rich physics.

cond-mat.supr-con↗

arXiv1999

A fast programmable trigger for isolated cluster counting in the BELLE experiment

We have developed a fast programmable trigger processor board based on a field programmable gate array and a complex programmable logic device for use in the BELLE experiment. The trigger board accommodates 144 ECL input signals, 2 NIM input signals, 24 ECL output signals, and the VME bus specification. An asynchronous trigger logic for counting isolated clusters is used. We have obtained trigger latency of 50 ns with a full access to input and output signals via a VME interface. The trigger logic can be modified at any time depending on the experimental conditions.

physics.ins-det↗

arXiv1999

Synchronization in a System of Globally Coupled Oscillators with Time Delay

We study the synchronization phenomena in a system of globally coupled oscillators with time delay in the coupling. The self-consistency equations for the order parameter are derived, which depend explicitly on the amount of delay. Analysis of these equations reveals that the system in general exhibits discontinuous transitions in addition to the usual continuous transition, between the incoherent state and a multitude of coherent states with different synchronization frequencies. In particular, the phase diagram is obtained on the plane of the coupling strength and the delay time, and ubiquity of multistability as well as suppression of the synchronization frequency is manifested. Numerical simulations are also performed to give consistent results.

cond-mat.stat-mech↗

arXiv1999

Test of CsI(Tl) crystals for the Dark Matter Search

Searches for weakly interacting massive particles(WIMP) can be based on the dete ction of nuclear recoil energy in CsI(Tl) crystals. We demonstrate that low energy gamma rays down to few keV is detected with CsI(Tl) crystal detector. A clear peak at 6 keV is observed using X-ray source. Good energy resolution and linearity have been achieved down to X-ray region. In addition, we also show that alpha particles and gamma rays can be clearly separated using the different time characteristics of the crystal.

physics.ins-det↗

arXiv1997

Evidence for nu_mu -> nu_e Neutrino Oscillations from LSND

A search for nu_mu -> nu_e oscillations has been conducted with the LSND apparatus at the Los Alamos Meson Physics Facility. Using nu_mu from pi^+ decay in flight, the nu_e appearance is detected via the charged-current reaction C(nu_e,e^-)X. Two independent analyses observe a total of 40 beam-on high-energy electron events (60 < E_e < 200 MeV) consistent with the above signature. This number is significantly above the 21.9 +- 2.1 events expected from the nu_e contamination in the beam and the beam-off background. If interpreted as an oscillation signal, the observed oscillation probability of (2.6 +- 1.0 +- 0.5) x 10^{-3} is consistent with the previously reported nu_mu_bar -> nu_e_bar oscillation evidence from LSND.

nucl-ex↗

arXiv1997

A letter of intent for an experiment to measure nu_mu to nu_e oscillations and nu_mu disappearance at the Fermilab Booster (BooNE)

This Letter of Intent to the Fermilab Program Advisory Committee describes a search for neutrino oscillations at the Fermilab booster. It is motivated by the LSND observation of nu_mu -> nu_e appearance. The booster neutrino experiment (BooNE) will be capable of observing both nu_mu -> nu_e appearance and nu_mu disappearance. In addition, the experiment will be able to measure Delta m^2 and sin^2(2theta) and search for CP violation in the lepton sector.

nucl-ex↗

arXiv1997

Evidence for nu_mu -> nu_e Oscillations from Pion Decay in Flight Neutrinos

A search for nu_mu -> nu_e oscillations has been conducted at the Los Alamos Meson Physics Facility using nu_mu from pi^+ decay in flight. An excess in the number of beam-related events from the C(nu_e,e^-)X inclusive reaction is observed. The excess is too large to be explained by normal nu_e contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of (2.6 +- 1.0 +- 0.5) x 10^{-3} is consistent with the previously reported nu_mu_bar -> nu_e_bar oscillation evidence from LSND.

nucl-ex↗

arXiv1997

Measurements of the reactions 12C + nu_e -> e- + 12N g.s. and 12C + nu_e -> e- + 12N

Charged current reactions of nu_e on 12C have been studied using a mu decay-at-rest nu_e beam from the Los Alamos Meson Physics Facility. More than 500 events from the exclusive reaction 12C + nu_e -> e- + 12N g.s. were measured in a large Liquid Scintillator Neutrino Detector (LSND). The observed energy dependence of the cross section and the angular distribution of the outgoing electron agree well with theoretical expectations. Measurements are also presented for inclusive transitions to 12N excited states, 12C + nu_e -> e- + 12N and compared with theoretical expectations. Results are consistent with a recent Continuum Random Phase Approximation (CRPA) calculation.

nucl-ex↗

arXiv1997

Measurements of the reactions 12C + nu_mue -> mu- + 12N g.s. and 12C + nu_mu -> mue- + 12N

Charged current scattering of nu_mu on 12C has been studied using a pi+ decay-in-flight nu_mu beam at the Los Alamos Meson Physics Facility. A sample of 56.8 +- 9.6 events satisfying criteria for the exclusive reaction 12C + nu_mu -> mu- + 12N_g.s. was obtained using the large Liquid Scintillator Neutrino Detector (LSND). The observed flux-averaged cross section, (6.6 +- 1.0 +- 1.0) x 10^-41 cm^2, agrees well with reliable theoretical expectations. A measurement was also obtained for the inclusive cross section to all accessible 12N states, 12C + nu_μ-> mu- + X. This flux-averaged cross section is (11.2 +- 0.3 +- 1.8)x 10^-40 cm^2, which is approximately half of that given by a recent Continuum Random Phase Approximation (CRPA) calculation.

nucl-ex↗

arXiv1996

Evidence for $\barν_μ\to\barν_e$ Oscillations from the LSND Experiment at LAMPF

A search for $\barν_μ\to \barν_{e}$ oscillations has been conducted at the Los Alamos Meson Physics Facility by using $\barν_μ$ from $μ^+$ decay at rest. The $\barν_e$ are detected via the reaction $\barν_e\,p \rightarrow e^{+}\,n$, correlated with a $γ$ from $np\rightarrow dγ$ ($2.2\,{\rm MeV}$). The use of tight cuts to identify $e^+$ events with correlated $γ$ rays yields 22 events with $e^+$ energy between 36 and $60\,{\rm MeV}$ and only $4.6 \pm 0.6$ background events. A fit to the $e^+$ events between 20 and $60\,{\rm MeV}$ yields a total excess of $51.8^{+18.7}_{-16.9} \pm 8.0$ events. If attributed to $\bar ν_μ\rightarrow \bar ν_e$ oscillations, this corresponds to an oscillation probability of ($0.31^{+0.11}_{-0.10} \pm 0.05$)\%.

nucl-ex↗

arXiv1996

The Liquid Scintillator Neutrino Detector and LAMPF Neutrino Source

A search for neutrino oscillations of the type nu_bar_mu to nu_bar_e has been conducted at the Los Alamos Meson Physics Facility using nu_bar_mu from muon decay at rest. Evidence for this transition has been reported previously. This paper discusses in detail the experimental setup, detector operation and neutrino source, including aspects relevant to oscillation searches in the muon decay-at-rest and pion decay in flight channels.

nucl-ex↗