arXiv subjects

H. Lee

Publications and source records attributed to H. Lee.

At least 91 records · Page 5Linked to original sources

arXiv2012

Field-Tuned Superconductor-Insulator Transition in BaPb1-xBixO3

BaPb$_{1-x}$Bi$_x$O$_3$ is found to exhibit a field-tuned superconductor to insulator transition for Bi compositions 0.24 $\leq x \leq$ 0.29. The magnetoresistance of optimally doped samples manifests a temperature-independent crossing point and scaling of the form $ρ(T,H)=ρ_c F(|H-H_{c}|T^{-1/zν})$, where $H_c$ is the field determined by the temperature-independent crossing point, and $zν$ = 0.69 $\pm$ 0.03. High resolution transmission electron microscopy measurements reveal a complex intergrown nanostructure comprising tetragonal and orthorhombic polymorphs. Data are analyzed in terms of both a classical effective medium theory and a field-tuned quantum phase transition, neither of which provides a completely satisfactory explanation for this remarkable phenomenology.

cond-mat.supr-con↗

arXiv2012

Demonstration of Communication using Neutrinos

Beams of neutrinos have been proposed as a vehicle for communications under unusual circumstances, such as direct point-to-point global communication, communication with submarines, secure communications and interstellar communication. We report on the performance of a low-rate communications link established using the NuMI beam line and the MINERvA detector at Fermilab. The link achieved a decoded data rate of 0.1 bits/sec with a bit error rate of 1% over a distance of 1.035 km, including 240 m of earth.

hep-ex↗

arXiv2012

Pressure effects on the heavy-fermion antiferromagnet CeAuSb2

The f-electron compound CeAuSb2, which crystallizes in the ZrCuSi2-type tetragonal structure, orders antiferromagnetically between 5 and 6.8 K, where the antiferromagnetic transition temperature T_N depends on the occupancy of the Au site. Here we report the electrical resistivity and heat capacity of a high-quality crystal CeAuSb2 with T_N of 6.8 K, the highest for this compound. The magnetic transition temperature is initially suppressed with pressure, but is intercepted by a new magnetic state above 2.1 GPa. The new phase shows a dome shape with pressure and coexists with another phase at pressures higher than 4.7 GPa. The electrical resistivity shows a T^2 Fermi liquids behavior in the complex magnetic state, and the residual resistivity and the T^2 resistivity coefficient increases with pressure, suggesting the possibility of a magnetic quantum critical point at a higher pressure.

cond-mat.str-el↗

arXiv2012

Arachne - A web-based event viewer for MINERvA

Neutrino interaction events in the MINERvA detector are visually represented with a web-based tool called Arachne. Data are retrieved from a central server via AJAX, and client-side JavaScript draws images into the user's browser window using the draft HTML 5 standard. These technologies allow neutrino interactions to be viewed by anyone with a web browser, allowing for easy hand-scanning of particle interactions. Arachne has been used in MINERvA to evaluate neutrino data in a prototype detector, to tune reconstruction algorithms, and for public outreach and education.

hep-ex↗

arXiv2011

Graded index and randomly oriented core-shell silicon nanowires with broadband and wide angle antireflection for photovoltaic cell applications

Antireflection with broadband and wide angle properties is important for a wide range of applications on photovoltaic cells and display. The SiOx shell layer provides a natural antireflection from air to the Si core absorption layer. In this work, we have demonstrated the random core-shell silicon nanowires with both broadband (from 400nm to 900nm) and wide angle (from normal incidence to 60\degree) antireflection characteristics within AM1.5 solar spectrum. The graded index structure from the randomly oriented core-shell (Air/SiOx/Si) nanowires may provide a potential avenue to realize a broadband and wide angle antireflection layer.

physics.optics↗

arXiv2010

Strong Interactions of Single Atoms and Photons near a Dielectric Boundary

Modern research in optical physics has achieved quantum control of strong interactions between a single atom and one photon within the setting of cavity quantum electrodynamics (cQED). However, to move beyond current proof-of-principle experiments involving one or two conventional optical cavities to more complex scalable systems that employ N >> 1 microscopic resonators requires the localization of individual atoms on distance scales < 100 nm from a resonator's surface. In this regime an atom can be strongly coupled to a single intracavity photon while at the same time experiencing significant radiative interactions with the dielectric boundaries of the resonator. Here, we report an initial step into this new regime of cQED by way of real-time detection and high-bandwidth feedback to select and monitor single Cesium atoms localized ~100 nm from the surface of a micro-toroidal optical resonator. We employ strong radiative interactions of atom and cavity field to probe atomic motion through the evanescent field of the resonator. Direct temporal and spectral measurements reveal both the significant role of Casimir-Polder attraction and the manifestly quantum nature of the atom-cavity dynamics. Our work sets the stage for trapping atoms near micro- and nano-scopic optical resonators for applications in quantum information science, including the creation of scalable quantum networks composed of many atom-cavity systems that coherently interact via coherent exchanges of single photons.

quant-ph↗

arXiv2010

Crystal field effects and quadrupole fluctuations in Ce$_3$Au$_3$Sb$_4$ detected by Sb NQR

We report $^{121,123}$Sb NQR studies on single crystals of the narrow gap semiconductor Ce3Au3Sb4. The temperature dependence of the nuclear quadrupole frequency ($ν_Q$), as well as the magnetic susceptibility, is well explained by crystal electric field effects. The nuclear spin-lattice relaxation rate ($T_1^{-1}$) of both $^{121}$Sb and $^{123}$Sb increases rapidly with decreasing temperature. The ratio of $T_1^{-1}$ for the two Sb isotopes is constant at high temperature but it decreases at low temperatures, indicating the important role of quadrupole fluctuations of the Ce ions. We propose that quadrupole fluctuations could be the origin of the large specific heat at low temperature via phonon-quadrupole moment coupling.

cond-mat.str-el↗

arXiv2010

Strong Gravitational Lensing by the Super-massive cD Galaxy in Abell 3827

We have discovered strong gravitational lensing features in the core of the nearby cluster Abell 3827 by analyzing Gemini South GMOS images. The most prominent strong lensing feature is a highly-magnified, ring-shaped configuration of four images around the central cD galaxy. GMOS spectroscopic analysis puts this source at z~0.2. Located ~20" away from the central galaxy is a secondary tangential arc feature which has been identified as a background galaxy with z~0.4. We have modeled the gravitational potential of the cluster core, taking into account the mass from the cluster, the BCG and other galaxies. We derive a total mass of (2.7 +- 0.4) x 10^13 Msun within 37 h^-1 kpc. This mass is an order of magnitude larger than that derived from X-ray observations. The total mass derived from lensing data suggests that the BCG in this cluster is perhaps the most massive galaxy in the nearby Universe.

astro-ph.CO↗

arXiv2010

An Aromatic Inventory of the Local Volume

Using infrared photometry from the Spitzer Space Telescope, we perform the first inventory of aromatic feature emission (AFE, but also commonly referred to as PAH emission) for a statistically complete sample of star-forming galaxies in the local volume. The photometric methodology involved is calibrated and demonstrated to recover the aromatic fraction of the IRAC 8 micron flux with a standard deviation of 6% for a training set of 40 SINGS galaxies (ranging from stellar to dust dominated) with both suitable mid-infrared Spitzer IRS spectra and equivalent photometry. A potential factor of two improvement could be realized with suitable 5.5 and 10 micron photometry, such as what may be provided in the future by JWST. The resulting technique is then applied to mid-infrared photometry for the 258 galaxies from the Local Volume Legacy (LVL) survey, a large sample dominated in number by low-luminosity dwarf galaxies for which obtaining comparable mid-infrared spectroscopy is not feasible. We find the total LVL luminosity due to five strong aromatic features in the 8 micron complex to be 2.47E10 solar luminosities with a mean volume density of 8.8E6 solar luminosities per cubic Megaparsec. Twenty-four of the LVL galaxies, corresponding to a luminosity cut at M = -18.22 in the B band, account for 90% of the aromatic luminosity. Using oxygen abundances compiled from the literature for 129 of the 258 LVL galaxies, we find a correlation between metallicity and the aromatic to total infrared emission ratio but not the aromatic to total 8 micron dust emission ratio. A possible explanation is that metallicity plays a role in the abundance of aromatic molecules relative to the total dust content, but other factors such as star formation and/or the local radiation field affect the excitation of those molecules.

astro-ph.CO↗

arXiv2009

Field-induced quantum critical point in the pressure-induced superconductor CeRhIn5

When subjected to pressure, the prototypical heavy-fermion antiferromagnet CeRhIn5 becomes superconducting, forming a broad dome of superconductivity centered around 2.35 GPa (=P2) with maximal Tc of 2.3 K. Above the superconducting dome, the normal state shows strange metallic behaviours, including a divergence in the specific heat and a sub-T-linear electrical resistivity. The discovery of a field-induced magnetic phase that coexists with superconductivity for a range of pressures P < P2 has been interpreted as evidence for a quantum phase transition, which could explain the non-Fermi liquid behavior observed in the normal state. Here we report electrical resistivity measurements of CeRhIn5 under magnetic field at P2, where the resistivity is sub-T-linear for temperatures above Tc (or T_FL) and a T^2-coefficient A found below T_FL diverges as Hc2 is approached. These results are similar to the field-induced quantum critical compound CeCoIn5 and confirm the presence of a quantum critical point in the pressure-induced superconductor CeRhIn5.

cond-mat.str-el↗

arXiv2009

NMR investigation of superconductivity and antiferromagnetism in CaFe$_2$As$_2$ under pressure

We report $^{75}$As NMR measurements in CaFe$_2$As$_2$, made under applied pressures up to 0.83 GPa produced by a standard clamp pressure cell. Our data reveal phase segregation of paramagnetic (PM) and antiferromagnetic (AFM) phases over a range of pressures, with the AFM phase more than 90 % dominant at low temperatures. In situ RF susceptibility measurements indicate the presence of superconductivity. $^{75}$As spin-lattice relaxation experiments indicate that the $^{75}$As nuclei sample the superconductivity while in the magnetically-ordered environment.

cond-mat.supr-con↗

arXiv2009

Isotropic Quantum Scattering and Unconventional Superconductivity

Superconductivity without phonons has been proposed for strongly correlated electron materials that are tuned close to a zero-temperature magnetic instability of itinerant charge carriers. Near this boundary, quantum fluctuations of magnetic degrees of freedom assume the role of phonons in conventional superconductors, creating an attractive interaction that glues electrons into superconducting pairs. Here we show that superconductivity can arise from a very different spectrum of fluctuations associated with a local or Kondo-breakdown quantum-critical point that is revealed in isotropic scattering of charge carriers and a sub-linear temperature-dependent electrical resistivity. At this critical point, accessed by applying pressure to the strongly correlated, local-moment antiferromagnet CeRhIn5, magnetic and charge fluctuations coexist and produce electronic scattering that is maximal at the optimal pressure for superconductivity. This previously unanticipated source of pairing glue opens possibilities for understanding and discovering new unconventional forms of superconductivity.

cond-mat.str-el↗

arXiv2007

The pulsation modes of the pre-white dwarf PG 1159-035

PG 1159-035, a pre-white dwarf with T_eff=140,000 K, is the prototype of both two classes: the PG1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. In this work, we used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods and identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M = 0.59 +/- 0.02 solar masses for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at r_c = 0.83 +/- 0.05 stellar radius. From the multiplet splitting, we calculated the rotational period P_rot = 1.3920 +/- 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for l=1 modes and less than 50% for l=2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.

astro-ph↗

arXiv2006

Optimal Estimates for the Electric Field in Two-Dimensions

The purpose of this paper is to set out optimal gradient estimates for solutions to the isotropic conductivity problem in the presence of adjacent conductivity inclusions as the distance between the inclusions goes to zero and their conductivities degenerate. This difficult question arises in the study of composite media. Frequently in composites, the inclusions are very closely spaced and may even touch. It is quite important from a practical point of view to know whether the electric field (the gradient of the potential) can be arbitrarily large as the inclusions get closer to each other or to the boundary of the background medium. In this paper, we establish both upper and lower bounds on the electric field in the case where two circular conductivity inclusions are very close but not touching. We also obtain such bounds when a circular inclusion is very close to the boundary of a circular domain which contains the inclusion. The novelty of these estimates, which improve and make complete our earlier results published in Math. Ann., is that they give an optimal information about the blow-up of the electric field as the conductivities of the inclusions degenerate.

math.AP↗

arXiv2006

Asymptotic Analysis of High-Contrast Phononic Crystals and a Criterion for the Band-Gap Opening

We investigate the band-gap structure of the frequency spectrum for elastic waves in a high-contrast, two-component periodic elastic medium. We consider two-dimensional phononic crystals consisting of a background medium which is perforated by an array of holes periodic along each of the two orthogonal coordinate axes. In this paper we establish a full asymptotic formula for dispersion relations of phononic band structures as the contrast of the shear modulus and that of the density become large. The main ingredients are integral equation formulations of the solutions to the harmonic oscillatory linear elastic equation and several theorems concerning the characteristic values of meromorphic operator-valued functions in the complex plane such as Generalized Rouché's theorem. We establish a connection between the band structures and the Dirichlet eigenvalue problem on the elementary hole. We also provide a criterion for exhibiting gaps in the band structure which shows that smaller the density of the matrix is, wider the band-gap is, provided that the criterion is fulfilled. This phenomenon was reported by Economou and Sigalas who observed that periodic elastic composites whose matrix has lower density and higher shear modulus compared to those of inclusions yield better open gaps. Our analysis in this paper agrees with this experimental finding.

math.AP↗

arXiv2005

Severe Fermi Surface Reconstruction at a Metamagnetic-Transition in Ca$_{2-x}$Sr$_x$RuO$_4$ (for $0.2 \leq x \leq 0.5$)

We report an electrical transport study in Ca$_{2-x}$Sr$_{x}$RuO$_4$ single crystals at high magnetic fields ($B$). For $x =0.2$, the Hall constant $R_{xy}$ decreases sharply at an anisotropic metamagnetic (MM) transition reaching its value for Sr$_2$RuO$_4$ at high fields. A sharp decrease in the $A$ coefficient of the resistivity $T^2$-term and a change in the structure of the angular magnetoresistance oscillations (AMRO) for $B$ rotating in the planes, confirms the reconstruction of the Fermi surface (FS). Our observations and LDA calculations indicate a strong dependence of the FS on the Ca concentration and suggest the coexistence of itinerant and localized electronic states in single layered ruthenates.

cond-mat.str-el↗

arXiv2004

Magnetic field-tuned quantum critical point in CeAuSb_2

Transport, magnetic and thermal properties at high magnetic fields (H) and low temperatures (T) of the heavy fermion compound CeAuSb_2 are reported. At H=0 this layered system exhibits antiferromagnetic order below T_N = 6 K. Applying B along the inter-plane direction, leads to a continuous suppression of T_N and a quantum critical point at H_c ~ 5.4 T. Although it exhibits Fermi liquid behavior within the Neel phase, in the paramagnetic state the fluctuations associated with H_c give rise to unconventional behavior in the resistivity (sub-linear in T) and to a TlnT dependence in the magnetic contribution to the specific heat. For H > H_c and low T the electrical resistivity exhibits an unusual T^3-dependence.

cond-mat.str-el↗

arXiv2004

Microelectromagnets for the manipulation of biological systems

Microelectromagnet devices, a ring trap and a matrix, were developed for the microscopic control of biological systems. The ring trap is a circular Au wire with an insulator on top. The matrix has two arrays of straight Au wires, one array perpendicular to the other, that are separated and topped by insulating layers. Microelectromagnets can produce strong magnetic fields to stably manipulate magnetically tagged biological systems in a fluid. Moreover, by controlling the currents flowing through the wires, a microelectromagnet matrix can move a peak in the magnetic field magnitude continuously over the surface of the device, generate multiple peaks simultaneously and control them independently. These capabilities of a matrix can be used to trap, continuously transport, assemble, separate and sort biological samples on micrometer length scales. Combining microelectromagnets with microfluidic systems, chip-based experimental systems can be realized for novel applications in biological and biomedical studies.

q-bio.QM↗