arXiv subjects

H. J. Lee

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

At least 37 records · Page 2Linked to original sources

arXiv2016

A CaMoO4 Crystal Low Temperature Detector for the AMoRE Neutrinoless Double Beta Decay Search

We report the development of a CaMoO4 crystal low temperature detector for the AMoRE neutrinoless double beta decay (0ν\b{eta}\b{eta}) search experiment. The prototype detector cell was composed of a 216 g CaMoO4 crystal and a metallic magnetic calorimeter. An over-ground measurement demonstrated FWHM resolution of 6-11 keV for full absorption gamma peaks. Pulse shape discrimination was clearly demonstrated in the phonon signals, and 7.6 σ of discrimination power was found for the α and \b{eta}/γ separation. The phonon signals showed rise-times of about 1 ms. It is expected that the relatively fast rise-time will increase the rejection efficiency of two-neutrino double beta decay pile-up events which can be one of the major background sources in 0ν\b{eta}\b{eta} searches.

physics.ins-det↗

arXiv2015

Technical Design Report for the AMoRE $0νββ$ Decay Search Experiment

The AMoRE (Advanced Mo-based Rare process Experiment) project is a series of experiments that use advanced cryogenic techniques to search for the neutrinoless double-beta decay of \mohundred. The work is being carried out by an international collaboration of researchers from eight countries. These searches involve high precision measurements of radiation-induced temperature changes and scintillation light produced in ultra-pure \Mo[100]-enriched and \Ca[48]-depleted calcium molybdate ($\mathrm{^{48depl}Ca^{100}MoO_4}$) crystals that are located in a deep underground laboratory in Korea. The \mohundred nuclide was chosen for this \zeronubb decay search because of its high $Q$-value and favorable nuclear matrix element. Tests have demonstrated that \camo crystals produce the brightest scintillation light among all of the molybdate crystals, both at room and at cryogenic temperatures. $\mathrm{^{48depl}Ca^{100}MoO_4}$ crystals are being operated at milli-Kelvin temperatures and read out via specially developed metallic-magnetic-calorimeter (MMC) temperature sensors that have excellent energy resolution and relatively fast response times. The excellent energy resolution provides good discrimination of signal from backgrounds, and the fast response time is important for minimizing the irreducible background caused by random coincidence of two-neutrino double-beta decay events of \mohundred nuclei. Comparisons of the scintillating-light and phonon yields and pulse shape discrimination of the phonon signals will be used to provide redundant rejection of alpha-ray-induced backgrounds. An effective Majorana neutrino mass sensitivity that reaches the expected range of the inverted neutrino mass hierarchy, i.e., 20-50 meV, could be achieved with a 200~kg array of $\mathrm{^{48depl}Ca^{100}MoO_4}$ crystals operating for three years.

physics.ins-det↗

arXiv2014

Melting of iron close to Earth's inner core boundary conditions and beyond

Several important geophysical features such as heat flux at the Core-Mantle Boundary or geodynamo production are intimately related with the temperature profile in the Earth's core. However, measuring the melting curve of iron at conditions corresponding to the Earth inner core boundary under pressure of 330 GPa has eluded scientists for several decades. Significant discrepancies in previously reported iron melting temperatures at high pressure have called into question the validity of dynamic measurements. We report measurements made with a novel approach using X-ray absorption spectroscopy using an X-ray free electron laser source coupled to a laser shock experiment. We determine the state of iron along the shock Hugoniot up to 420 GPa (+/- 50) and 10800 K (+/- 1390) and find an upper boundary for the melting curve of iron by detecting solid iron at 130 GPa and molten at 260, 380 and 420 GPa along the shock Hugoniot. Our result establishes unambiguous agreement between dynamic measurement and recent extrapolations from static data thus resolving the long-standing controversy over the reliability of using dynamic compression to study the melting of iron at conditions close to the Earth's inner core boundary and beyond.

physics.geo-ph↗

arXiv2014

Thermal Model and Optimization of a Large Crystal Detector using a Metallic Magnetic Calorimeter

We established a simple thermal model of the heat flow in a large crystal detector designed for a neutrinoless double beta decay experiment. The detector is composed of a CaMoO$_{4}$ crystal and a metallic magnetic calorimeter (MMC). The thermal connection between the absorber and the sensor consists of a gold film evaporated on the crystal surface and gold bonding wires attached to this film and the MMC sensor. The model describes athermal and thermal processes of heat flow to the gold film. A successive experiment based on optimization calculations of the area and thickness of the gold film showed a significant improvement in the size and rise-time of the measured signals.

physics.ins-det↗

arXiv2012

Non-modal analysis of the diocotron instability. Plane geometry

The comprehensive investigation of the temporal evolution of the diocotron instability of the plane electron strip on the linear stage of its development is performed. By using the Kelvin's method of the shearing modes we elucidate the role of the initial perturbations of the electron density, which is connected with problem of the continuous spectrum. The linear non-modal evolution process, detected by the solution of the initial value problem, leads towards convergence to the phase-locking configuration of the mutually growing normal modes.

physics.plasm-ph↗

arXiv2011

Diffusive instability of a Townsend discharge

The role of the electron diffusion on the stability of a Townsend discharge is investigated. It is obtained, that electron diffusion modifies the condition of the steady self-sustenance of the discharge, and make discharge unstable.

physics.plasm-ph↗

arXiv2011

Fermi Surface Reconstruction in CeTe2 Induced by Charge Density Wave: ARPES Study

Electronic structures of a charge-density-wave CDW system CeTe_2-xSb_x (x=0, 0.05) have been investigated by employing angle-resolved photoemission spectroscopy (ARPES). The observed Fermi surface (FS) agrees very well with the calculated FS for the undistorted CeTe_2 both in shapes and sizes. The metallic states crossing the Fermi level(E_F) are observed in ARPES. The carriers near E_F have mainly the Te(1) 5p character, with the negligible contribution from Ce 4f states to the CDW formation. The supercell (shadow) bands and the corresponding very weak FS's are found to arise from band-folding due to the interaction with Ce-Te(2) layers. This work shows that the origin of the CDW formation in CeTe_2 is the FS nesting and that the CDW modulation vector is along Gamma-X (Q_CDW ~ X)

cond-mat.str-el↗

arXiv2009

Tunnel magnetoresistance due to Coulomb blockade effects in quasi-one dimensional polymer nanofibers

We report on the low temperature tunnel magnetoresistance (MR) in quasi one-dimensional (1D) nanofibers made of conjugated polymers. The MR voltage bias dependence reveals an enhancement (at low biases) and the oscillatory behavior at temperatures below 10 K. The low temperature isotropic MR behavior has been attributed to the charging effects in the polymer nanofiber which considered as an array of small conducting regions separated by nano barriers. These effects at low temperatures lead to the single electron tunneling represented by the Coulomb blockade regime as well as to an enhancement and oscillation of the tunnel MR.

cond-mat.stat-mech↗

arXiv2009

Hot electron relaxation in a heavy fermion system with tuned doping

Femtosecond time-resolved optical spectroscopy was used to systematically study photoexcited carrier relaxation dynamics in the intermediate-valence heavy fermion system Yb1-xLuxAl3 (0 < x < 1). Given the demonstrated sensitivity of this experimental technique to the presence of the low energy gaps in the charge excitation spectrum, the aim of this work was to study the effect of dilution of the Kondo lattice on its low energy electronic structure. The results imply that in Yb1-xLuxAl3 the hybridization gap, resulting from hybridization of local moments and conduction electrons, persists up to 30% doping. Interestingly, below some characteristic, doping dependent temperature T*(x) the relaxation time divergence, governed by the relaxation bottleneck due to the presence of the indirect hybridization gap, is truncated. This observation is attributed to the competing ballistic transport of hot electrons out of the probed volume at low temperatures. The derived theoretical model accounts for both the functional form of relaxation dynamics below T*(x), as well as the doping dependence of the low temperature relaxation rate in Yb1-xLuxAl3.

cond-mat.str-el↗

arXiv2006

Time-Resolved Quasiparticle Dynamics in the Spin-Density-Wave State

Time-resolved photoinduced reflectivity is measured in the spin-density-wave (SDW) phase using itinerant antiferromagnets UMGa$_{5}$ (M=Ni, Pt). For UNiGa$_{5}$ [$T_{N}$=85 K, $Q$=($π$,$π$,$π$)], the relaxation time $τ$ shows a sharp increase at $T_{N}$ consistent with the opening of a SDW gap. For UPtGa$_{5}$ [$T_{N}$=26 K, $Q$=(0,0,$π$)], no change in $τ$ is observed at $T_{N}$ or at the lowest temperatures. We attribute this to the absence of the SDW gap at the Fermi level, due to a different modulation vector $Q$, which leads to a gapless quasiparticle spectrum. Our results challenge the conventional wisdom that a SDW phase necessarily implies a SDW gap at the Fermi level.

cond-mat.str-el↗

arXiv2006

Quasiparticle Relaxation Across a Spin Gap in the Itinerant Antiferromagnet UNiGa5

Ultrafast time-resolved photoinduced reflectivity is measured for the itinerant antiferromagnet UNiGa$_{5}$ ($T_{N} \approx$85 K) from room temperature to 10 K. The relaxation time $τ$ shows a sharp increase at $T_{N}$ consistent with the opening of a spin gap. In addition, the temperature dependence of $τ$ below $T_{N}$ is consistent with the opening of a spin gap leading to a quasiparticle recombination bottleneck as revealed by the Rothwarf-Taylor model. This contrasts with canonical heavy fermions such as CeCoIn$_{5}$ where the recombination bottleneck arises from the hybridization gap.

cond-mat.str-el↗

arXiv2006

Enhanced Optical Gap in Bi-layered Manganites La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ near $x=0.4$

We have systematically investigated the optical conductivity spectra of La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ ($0.3 \leqslant x \leqslant 0.5$). We find that just above the magnetic ordering temperatures, the optical gap shows an enhancement up to $\sim 0.3$ eV near $x=0.4$. Based on a $x$-dependent comparison of the nesting vector of the hypothetical Fermi surface and the superlattice wave-vector, we suggest that the peculiar $x$-dependence of the optical gap can be understood in terms of charge and lattice correlation enhanced by the charge density wave instability in nested Fermi surface.

cond-mat.str-el↗

arXiv2005

Coulomb-blockade transport in quasi-one dimensional polymer nanofibers

We report the low temperature current-voltage (I-V) characteristics studies in quasi-one dimensional conducting polymer nanofibers. We find a threshold voltage Vt below which little current flows at temperatures below 30-40 K. For V > Vt current scales as (V/Vt - 1)^zeta, where zeta ~ 1.8-2.1 at high biases. Differential conductance oscillations are found whose magnitude increases as temperature decreases below 10 K. We attribute the observed low temperature I-V behavior to Coulomb blockade effects with a crossover to Luttinger liquid-like behavior at high temperature. We demonstrate that at low temperatures such a doped conjugated polymer fiber can be considered as an array of small conducting regions separated by nanoscale barriers, where the Coulomb blockade tunneling is the dominant transport mechanism.

cond-mat.soft↗

arXiv2004

One-dimensional transport in polymer nanofibers

We report our transport studies in quasi one-dimensional (1D) conductors - helical polyacetylene fibers doped with iodine and the data analysis for other polymer single fibers and tubes. We found that at 30 K < T < 300 K the conductance and the current-voltage characteristics follow the power law: G(T) ~ T^alpha with alpha ~ 2.2-7.2 and I(V) ~ V^betta with betta ~ 2-5.7. Both G(T) and I(V) show the features characteristic of 1D systems such as Luttinger liquid or Wigner crystal. The relationship between our results and theories for tunneling in 1D systems is discussed.

cond-mat.soft↗

arXiv2003

Is quantum entanglement invariant in special relativity?

Quantum entanglements are of fundamental importance in quantum physics ranging from the quantum information processing to the physics of black hole. Here, we show that the quantum entanglement is not invariant in special relativity. This suggests that nearly all aspects of quantum information processing would be affected significantly when relativistic effects are considered because present schemes are based on the general assumption that entanglement is invariant. There should be additional protocols to compensate the variances of entanglement in in quantum information processing. Furthermore, extending our results to general relativity may provide clues to the fate of the information contained in an entangled Hawking pair inside and outside the event horizon as black hole evaporates.

quant-ph↗

arXiv2002

Complicated nature of the gap in MgB2: magnetic field-dependent optical studies

We investigated the magnetic field (H)-dependent optical conductivity spectra of MgB2 thin film in the far-infrared region. The H-dependences can be explained by the increase of normal metallic regions embedded in the superconducting background. The area fraction of the normal metallic region increases rather quickly at low field, but slowly at high field. It follows neither H- nor H^{1/2}-dependences. The results suggest the complicated nature of the superconducting gap in MgB2.

cond-mat.str-el↗

arXiv2002

Spin/Orbital Pattern-Dependent Polaron Absorption in Nd(1-x)Sr(x)MnO3

We investigated optical properties of Nd(1-x)Sr(x)MnO3 (x= 0.40, 0.50, 0.55, and 0.65) single crystals. In the spin/orbital disordered state, their conductivity spectra look quite similar, and the strength of the mid-infrared absorption peak is proportional to x(1-x) consistent with the polaron picture. As temperature lowers, the Nd(1-x)Sr(x)MnO3 samples enter into various spin/orbital ordered states, whose optical responses are quite different. These optical responses can be explained by the spin/orbital ordering pattern-dependent polaron hopping.

cond-mat.str-el↗

arXiv2002

Optical Evidence of Multiphase Coexistence in Single Crystalline (La,Pr,Ca)MnO3

We investigated temperature (T)- and magnetic field-dependent optical conductivity spectra (\s\w) of a La_5/8-yPr_yCa_3/8MnO_3 (y~0.35) single crystal, showing intriguing phase coexistence at low T. At T_C < T < T_CO, a dominant charge-ordered phase produces a large optical gap energy of ~0.4 eV. At T < T_C, at least two absorption bands newly emerge below 0.4 eV. Analyses of (\s\w) indicate that the new bands should be attributed to a ferromagnetic metallic and a charge-disordered phase that coexist with the charge-ordered phase. This optical study clearly shows that La_5/8-yPrCa_3/8MnO_3 (y~0.35) is composed of multiphases that might have different lattice strains.

cond-mat.str-el↗