The science and technology of condensed matter physics - from atomic imaging to space research

Various areas of our ongoing condensed matter physics research which involve both fundamental physics and advanced technology are described. The research topics include studies of the vortex dynamics and pairing symmetry of high-temperature superconductors; development of precision clocks using high-Q superconducting microwave cavities; state-of-the-art measurements of the density and critical phenomena of liquid helium near phase transitions and under microgravity; as well as the physics and device applications of various magnetoresistive perovskites. The experimental scope encompasses techniques from atomic imaging to space research, and the important interplay of fundamental science and frontier technology in our research is also addressed

Recent Advances in High-Temperature Superconductivity

Recent experimental and theoretical developments in high-temperature superconductivity are reviewed, and the empirically asymmetric behavior between hole-doped and electron-doped cuprates is contrasted. A number of phenomena previously considered as essential for the formation of cuprate superconductivity, such as the pairing symmetry, pseudogap phenomenon, gapped incommensurate spin fluctuations and charged stripes, are found to be non-universal, and are likely the consequence of competing orders. It is suggested that the only ubiquitous properties among all cuprates are the strong electronic correlation and antiferromagnetic spin interaction in the CuO2 planes.Comment: 24 pages, 17 figures, 166 references. Review article, to appear in the Bulletin of Associations of Asia Pacific Physical Societies (AAPPS), Vol. 12. Contact author: Nai-Chang Yeh (e-mail: [email protected]

Non-universal pairing symmetry and pseudogap phenomena in hole- and electron-doped cuprate superconductors

Experimental studies of the pairing state of cuprate superconductors reveal asymmetric behaviors of the hole-doped (p-type) and electron-doped (n-type) cuprates. The pairing symmetry, pseudogap phenomenon, low-energy spin excitations and the spatial homogeneity of the superconducting order parameter appear to be non-universal among the cuprates, which may be attributed to competing orders. We propose that the non-universal pseudogap and nano-scale variations in the quasiparticle spectra may be the result of a charge nematic (CN) phase stabilized by disorder in highly two-dimensional (2D) p-type cuprates. The CN phase is accompanied by gapped spin excitations and competes with superconductivity (SC). In contrast, gapless spin excitations may be responsible for the absence of pseudogap and the presence of excess sub-gap spectral weight in the momentum-independent quasiparticle spectra of n-type cuprates. The physical implications and further verifications for these conjectures are discussed

Reply to “Comment on ‘Microwave vortex dissipation of superconducting Nd-Ce-Cu-O epitaxial films in high magnetic fields’”

We demonstrate with detailed analysis that the criticisms in the preceding Comment by Blackstead are largely due to insufficient understanding of the experimental issues associated with our system or the imposition of formalism that is inapplicable to our experiments. In particular, we distinguish the conventional formalism for “field-defined” surface resistance applicable to measurements on samples with filling factors i.e., the ratio of the sample volume to that of the microwave cavity approaching 1 from our “dissipation-defined” surface resistance derived from first principles for measurements on samples with very small filling factors

Collective modes and quasiparticle interference on the local density of states of cuprate superconductors

The energy, momentum, and temperature dependence of the quasiparticle local density of states (LDOS) of a two-dimensional d(x2)-(y2)-wave superconductor with random disorder is investigated using the first-order T-matrix approximation. The results suggest that collective modes such as spin-charge-density waves are relevant low-energy excitations of the cuprates that contribute to the observed LDOS modulations in recent scanning tunneling microscopy studies of Bi2Sr2CaCu2Ox

Spin-polarized tunneling spectroscopic studies of the intrinsic heterogeneity and pseudogap phenomena in colossal magnetoresistive manganite La_{0.7}Ca_{0.3}MnO_{3}

Spatially resolved tunneling spectroscopic studies of colossal magnetoresistive (CMR) manganite $\rm La_{0.7}Ca_{0.3}MnO_3$ (LCMO) epitaxial films on $\rm (LaAlO_3)_{0.3}(Sr_2AlTaO_6)_{0.7}$ substrate are investigated as functions of temperature, magnetic field and spin polarization by means of scanning tunneling spectroscopy. Systematic surveys of the tunneling spectra taken with Pt/Ir tips reveal spatial variations on the length scale of a few hundred nanometers in the ferromagnetic state, which may be attributed to the intrinsic heterogeneity of the manganites due to their tendency towards phase separation. The electronic heterogeneity is found to decrease either with increasing field at low temperatures or at temperatures above all magnetic ordering temperatures. On the other hand, spectra taken with Cr-coated tips are consistent with convoluted electronic properties of both LCMO and Cr. In particular, for temperatures below the magnetic ordering temperatures of both Cr and LCMO, the magnetic-field dependent tunneling spectra may be quantitatively explained by the scenario of spin-polarized tunneling in a spin-valve configuration. Moreover, a low-energy insulating energy gap $\sim 0.6$ eV commonly found in the tunneling conductance spectra of bulk metallic LCMO at $T \to 0$ may be attributed to a surface ferromagnetic insulating (FI) phase, as evidenced by its spin filtering effect at low temperatures and vanishing gap value above the Curie temperature. Additionally, temperature independent pseudogap (PG) phenomena existing primarily along the boundaries of magnetic domains are observed in the zero-field tunneling spectra. The PG becomes strongly suppressed by applied magnetic fields at low temperatures when the tunneling spectra of LCMO become highly homogeneous. These findings suggest that the occurrence PG is associated with the electronic heterogeneity of the manganites.Comment: 15 pages, 15 figures. Published in Physical Review B. Corresponding author: Nai-Chang Yeh (E-mail: [email protected]

Tabulation of Mie scattering calculation results for microwave radiative transfer modeling

In microwave radiative transfer model simulations, the Mie calculations usually consume the majority of the computer time necessary for the calculations (70 to 86 percent for frequencies ranging from 6.6 to 183 GHz). For a large array of atmospheric profiles, the repeated calculations of the Mie codes make the radiative transfer computations not only expensive, but sometimes impossible. It is desirable, therefore, to develop a set of Mie tables to replace the Mie codes for the designated ranges of temperature and frequency in the microwave radiative transfer calculation. Results of using the Mie tables in the transfer calculations show that the total CPU time (IBM 3081) used for the modeling simulation is reduced by a factor of 7 to 16, depending on the frequency. The tables are tested by computing the upwelling radiance of 144 atmospheric profiles generated by a 3-D cloud model (Tao, 1986). Results are compared with those using Mie quantities computed from the Mie codes. The bias and root-mean-square deviation (RMSD) of the model results using the Mie tables, in general, are less than 1 K except for 37 and 90 GHz. Overall, neither the bias nor RMSD is worse than 1.7 K for any frequency and any viewing angle

Spin-polarized quasiparticle transport in cuprate superconductors

The effects of spin-polarized quasiparticle transport in superconducting YBa2Cu3O7-delta (YBCO) epitaxial films are investigated by means of current injection into perovskite ferromagnet-insulator-superconductor (F-I-S) heterostructures. These effects are compared with the injection of simple quasiparticles into control samples of perovskite nonmagnetic metal-insulator-superconductor (N-I-S). Systematic studies of the critical current density (J(c)) as a function of the injection current density (J(inj)), temperature (T), and the thickness (d) of the superconductor reveal drastic differences between the F-I-S and N-I-S heterostructures, with strong suppression of J(c) and a rapidly increasing characteristic transport length near the superconducting transition temperature T-c only in the F-I-S samples. The temperature dependence of the efficiency (etaequivalent toDeltaJ(c)/J(inj); DeltaJ(c): the suppression of critical current due to finite J(inj)) in the F-I-S samples is also in sharp contrast to that in the N-I-S samples, suggesting significant redistribution of quasiparticles in F-I-S due to the longer lifetime of spin-polarized quasiparticles. Application of conventional theory for nonequilibrium superconductivity to these data further reveal that a substantial chemical potential shift mu(*) in F-I-S samples must be invoked to account for the experimental observation, whereas no discernible chemical potential shift exists in the N-I-S samples, suggesting strong effects of spin-polarized quasiparticles on cuprate superconductivity. The characteristic times estimated from our studies are suggestive of anisotropic spin relaxation processes, possibly with spin-orbit interaction dominating the c-axis spin transport and exchange interaction prevailing within the CuO2 planes. Several alternative scenarios attempted to account for the suppression of critical currents in F-I-S samples are also critically examined, and are found to be neither compatible with experimental data nor with the established theory of nonequilibrium superconductivity