Electronic liquid crystal state in the high-temperature superconductor YBa2Cu3O6.45

Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at millikelvin temperatures. We report the spontaneous onset of a one-dimensional, incommensurate modulation of the spin system in the high-transition-temperature superconductor YBa2Cu3O6.45 upon cooling below approximately 150 kelvin, whereas static magnetic order is absent above 2 kelvin. The evolution of this modulation with temperature and doping parallels that of the in-plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route toward electronic liquid crystals and that nematic order can coexist with high-temperature superconductivity in underdoped cuprates.

Tunable slow light device using quantum dot semiconductor laser

This investigation experimentally demonstrates a tunable slow light device using a quantum dot (QD) semiconductor laser. The QD semiconductor laser at 1.3 mum fabricated on a GaAs substrate is grown by molecular beam epitaxy. Tunable slow light can be achieved by adjusting the bias current and wavelength detuning. The slow light device operated under probe signal from 5 to 10 GHz is presented. Moreover, we also demonstrate that the tunable slow light device can be used in a subcarrier multiplexed system.

Constituents of the quasiparticle spectrum along the nodal direction of high-Tc cuprates

Applying the Kramers-Kronig consistent procedure, developed earlier, we investigate in detail the formation of the quasiparticle spectrum along the nodal direction of high-Tc cuprates. The heavily discussed "70 meV kink" on the renormalized dispersion exhibits a strong temperature and doping dependence when purified from structural effects such as bilayer splitting, diffraction replicas, etc. This dependence is well understood in terms of fermionic and bosonic constituents of the self-energy. The latter follows the evolution of the spin-fluctuation spectrum, emerging below some doping dependent temperature and sharpening below Tc, and is mainly responsible for the formation of the kink in question.

Doping dependence of bilayer resonant spin excitations in (Y, Ca)Ba2Cu3O6+x

Resonant magnetic modes with odd and even symmetries were studied by inelastic neutron scattering experiments in the bilayer high-Tc superconductor Y1-xCa+Ba2Cu3O6+y over a wide doping range. The threshold of the spin excitation continuum in the superconducting state, deduced from the energies and spectral weights of both modes, is compared with the superconducting d-wave gap, deduced from electronic Raman scattering in the B1g symmetry on the same samples. Above a critical doping level of delta approximately =0.19, both mode energies and the continuum threshold coincide. We find a simple scaling relationship between the characteristic energies and spectral weights of both modes, which indicates that the resonant modes are bound states in the superconducting energy gap, as predicted by the spin-exciton model of the resonant mode.

Magnetic order in the pseudogap phase of high-Tc superconductors

One of the leading issues in high-T(c) superconductors is the origin of the pseudogap phase in underdoped cuprates. Using polarized elastic neutron diffraction, we identify a novel magnetic order in the YB(2)Cu(3)O(6+) system. The observed magnetic order preserves translational symmetry of the lattice as proposed for orbital moments in the circulating current theory of the pseudogap state. To date, it is the first direct evidence of a hidden order parameter characterizing the pseudogap phase in high-T(c) cuprates.

Anomalous electronic Raman scattering in NaxCoO2.yH2O

Raman scattering experiments on NaxCoO2.yH2O single crystals show a broad electronic continuum with a pronounced peak around 100 cm(-1) and a cutoff at approximately 560 cm(-1) over a wide range of doping levels. The electronic Raman spectra in superconducting and nonsuperconducting samples are similar at room temperature, but evolve in markedly different ways with decreasing temperature. For superconducting samples, the low-energy spectral weight is depleted upon cooling below T* approximately 150 K, indicating the opening of a pseudogap that is not present in nonsuperconducting materials. Weak additional phonon modes observed below T* suggest that the pseudogap is associated with charge ordering.

Dynamic characteristics of long-wavelength quantum dot vertical-cavity surface-emitting lasers with light injection

This investigation experimentally demonstrates the dynamic characteristics of quantum dot vertical-cavity surface-emitting lasers (QD VCSEL) without and with light injection. The QD VCSEL is fully doped structure on GaAs substrate and operates in the 1.3 mum optical communication wavelength. The eye diagram, frequency response, and intermodulation distortion are presented. We also demonstrate that the frequency response enhancement by light injection technique allows us to improve the performance of subcarrier multiplexed system.

Kinks, nodal bilayer splitting, and interband scattering in YBa2Cu3O(6+x)

We apply the new-generation angle-resolved photoemission spectroscopy methodology to the most widely studied cuprate superconductor YBa2Cu3O(6+x). Considering the nodal direction, we found noticeable renormalization effects known as kinks both in the quasiparticle dispersion and scattering rate, the bilayer splitting, and evidence for strong interband scattering--all the characteristic features of the nodal quasiparticles detected earlier in Bi2Sr2CaCu2O(8+delta). The typical energy scale and the doping dependence of the kinks clearly point to their intimate relation with the spin-1 resonance seen in the neutron scattering experiments. Our findings strongly suggest a universality of the electron dynamics in the bilayer superconducting cuprates and a dominating role of the spin fluctuations in the formation of the quasiparticles along the nodal direction.

Effect of Zn and Ni impurities on the quasiparticle renormalization of superconducting Bi-2212

The Cu substitution by Zn and Ni impurities and its influence on the mass renormalization effects in angle-resolved photoelectron spectra (ARPES) of Bi2Sr2CaCu2O8-delta is addressed. We show that the nonmagnetic Zn atoms have a much stronger effect in both the nodal and antinodal parts of the Brillouin zone than magnetic Ni. The observed changes are consistent with the behavior of the spin resonance mode as seen by inelastic neutron scattering in YBCO. This strongly suggests that the "peak-dip-hump" and the kink in ARPES on the one side and neutron resonance on the other are closely related features.

Residency Selection Criteria: What Medical Students Perceive as Important

BACKGROUND

Little is known about medical students' perceptions of which criteria are important in residency selection. Student knowledge of this process may affect their education and their ability to obtain the residency of their choice.

OBJECTIVE

To determine the perceived importance among medical students of various selection criteria for residency.

DESIGN, SETTING AND PARTICIPANTS

Medical students at three institutions were asked to rate the importance of various residency selection criteria using a web-based survey instrument.

MAIN OUTCOME MEASURES

Sixteen residency selection criteria were included in the survey.

RESULTS

The overall response rate was 49.2%. Criteria perceived as extremely important by the majority of students were the interview (80.6%), grades in third and fourth year courses in their chosen specialty (73.3%), letters of recommendation excluding the Dean's letter (65.3%), and grades in third and fourth year clerkships (55.9%). USMLE Step 1 score (46.7%) was viewed as extremely important by many students. Moderately important: grades in fourth year electives not in their chosen specialty (57.3%), medical school's reputation (50.5%), number of honor grades (49.0%), USMLE Step 2 score (42.3%), and Dean's letter (41.1%). Mildly/not important: grades in the first and second years (56.8%), academic awards (55.2%), extracurricular activities (52.6%), research (50.9%), class rank (49.3%), and AOA (46.5%). Students in the clinical years of training were more likely to place importance on honors grades (p=0.04) and AOA (p=0.009) and were less likely to place importance on grades in fourth year electives not in their chosen specialty (p˂0.0001), scores on USMLE Step 1 (p=0.0003), USMLE Step 2 (p˂0.0001), and Dean's letter (p˂0.0001).

CONCLUSIONS

Misperceptions about which criteria are important in residency selection are common among medical students. Many overestimate the importance of subjective criteria while undervaluing objective criteria.

Nonmagnetic insulator state in Na1CoO2 and phase separation of na vacancies

Crystallographic, magnetic, and NMR properties of a Na1CoO2 single crystal with x approximately = 1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a nonmagnetic insulator, as expected for homogeneous planes of Co3+ ions with S = 0. In addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x approximately = 0.8 and the occurrence of a metal-insulator transition for x --> 1.

Magnetic ordering and spin waves in Na0.82CoO2

NaxCoO2, the parent compound of the recently synthesized superconductor Na(x)CoO(2):yH(2)O, exhibits bulk antiferromagnetic order below approximately 20 K for 0.75</=x</=0.9. We have performed neutron scattering experiments in which we observed Bragg reflections corresponding to A-type antiferromagnetic order in a Na0.82CoO2 single crystal and characterized the corresponding spin-wave dispersions. The spin waves exhibit a strongly energy-dependent linewidth. The in-plane and out-of-plane exchange constants resulting from a fit to a nearest-neighbor Heisenberg model are similar in magnitude, which is unexpected in view of the layered crystal structure of NaxCoO2. Possible implications of these observations are discussed.

Critical point and the nature of the pseudogap of single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta superconductors

We apply strong magnetic fields of H=28.5 to 43 T to suppress superconductivity (SC) in the cuprates Bi2Sr2-xLaxCuO6+delta (x=0.65, 0.40, 0.25, 0.15, and 0), and investigate the low temperature (T) normal state by 63Cu nuclear spin-lattice relaxation rate (1/T1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x approximately 0.05, which corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid that persists as the ground state when superconductivity is removed by the magnetic field. A comparison of the superconducting state with the H-induced normal state in the x=0.40 (Tc=32 K) sample indicates that there remains substantial part of the Fermi surface even in the fully developed PG state, which suggests that the PG and SC are coexisting matters.

Charge ordering and magnetopolarons in Na0.82CoO2

Using spectral ellipsometry, we measured the dielectric function of a Na(0.82(2))CoO2 crystal that exhibits bulk antiferromagnetism with T(N)=19.8 K. We identify two prominent transitions as a function of temperature. The first one at 280 K involves marked changes of the electronic and lattice responses that are indicative of charge ordering in the CoO2 layers. The second transition occurs around T(N)=19.8 K and reveals sizable spin-charge coupling. The data are discussed in terms of charge ordering and formation of magnetopolarons due to a charge-induced spin-state transition of adjacent Co3+ ions.

Oxygen superstructures throughout the phase diagram of (Y,Ca)Ba2Cu3O6+x

The doping dependence of short-range lattice superstructures in (Y,Ca)Ba2Cu3O6+x has been studied with high-energy x-ray scattering. We observe diffuse features with a well defined periodicity which depend on the oxygen concentration but not on the charge carrier concentration. In addition, we find that diffuse scattering is absent in underdoped YBa2Cu4O8, which does not sustain oxygen defects. Our combined data highlight that the diffuse scattering arises from short-range oxygen ordering and associated lattice distortions. Signatures of stripe ordering or fluctuations are not seen and therefore must be much weaker.

Two-dimensional geometry of spin excitations in the high-transition-temperature superconductor YBa2Cu3O6+x

The fundamental building block of the copper oxide superconductors is a Cu4O4 square plaquette. The plaquettes in most of these materials are slightly distorted to form a rectangular lattice, for which an influential theory predicts that high-transition-temperature (high-T(c)) superconductivity is nucleated in 'stripes' aligned along one of the axes. This theory received strong support from experiments that indicated a one-dimensional character for the magnetic excitations in the high-T(c) material YBa2Cu3O6.6 (ref. 4). Here we report neutron scattering data on 'untwinned' YBa2Cu3O6+x crystals, in which the orientation of the rectangular lattice is maintained throughout the entire volume. Contrary to the earlier claim, we demonstrate that the geometry of the magnetic fluctuations is two-dimensional. Rigid stripe arrays therefore appear to be ruled out over a wide range of doping levels in YBa2Cu3O6+x, but the data may be consistent with liquid-crystalline stripe order. The debate about stripes has therefore been reopened.

Manifestation of the magnetic resonance mode in the nodal quasiparticle lifetime of the superconducting cuprates

Studying the nodal quasiparticles in superconducting cuprates by photoemission with highly improved momentum resolution, we show that a new "kink" feature in the scattering rate is a key to uncover the nature of electron correlations in these compounds. Our data provide evidence that the main doping independent contribution to the scattering can be well understood in terms of the conventional Fermi liquid model, while the additional doping dependent contribution has a magnetic origin. This sheds doubt on applicability of a phonon-mediated pairing mechanism to high-temperature superconductors.

Natural frequency assessment of stability of root keeper magnetic devices

The aim of the study was to evaluate the potential for using natural frequency (NF) as an indicator for assessing the stability of a magnetic keeper device used in prosthodontic treatment. A three-dimensional finite element (FE) model of a root keeper-cement-dentine system was established for NF analysis. The model was first validated against a series of in vitro experiments. Then, NF values of the first vibrational mode of the FE model with various boundary conditions were calculated. The in vitro results showed that the measured NF values of the root keeper-incisor units decreased significantly (p<0.01) from 9.07 +/- 0.37 to 5.73 +/- 0.10 kHz when the units were embedded in simulated bony tissue. Results obtained from FE simulations demonstrated that the root keeper would fully loosen when the constant values of the spring elements were lower than 10(4) N-m(-1). Furthermore, a linear increase in the NF values of the model was observed from 6.16 to 15.52 kHz, when the constant was increased from 10(4) to 10(7) N-m(-1), and the values then reached a plateau. The results indicate that the NF value of a root keeper has the potential to be used for monitoring the stability of such a device.

In-Plane Spectral Weight Shift of Charge Carriers in YBa 2 Cu 3 O 6.9

The temperature-dependent redistribution of the spectral weight of the CuO2 plane-derived conduction band of the YBa2Cu3O6.9 high-temperature superconductor (superconducting transition temperature = 92.7 kelvin) was studied with wide-band (0.01- to 5.6-electron volt) spectroscopic ellipsometry. A superconductivity-induced transfer of the spectral weight involving a high-energy scale in excess of 1 electron volt was observed. Correspondingly, the charge carrier spectral weight was shown to decrease in the superconducting state. The ellipsometric data also provide detailed information about the evolution of the optical self-energy in the normal and superconducting states.

Intrinsic Josephson effects in the magnetic superconductor RuSr2GdCu2O8

We have measured interlayer current transport in small-sized RuSr2GdCu2O8 single crystals. We find a clear intrinsic Josephson effect showing that the material acts as a natural superconductor-insulator-ferromagnet-insulator-superconductor superlattice. Thus far, we detected no unconventional behavior due to the magnetism of the RuO2 layers.

Magnetism, charge order, and giant magnetoresistance in SrFeO(3-delta) single crystals

The electronic and magnetic properties of SrFeO(3-delta) single crystals with controlled oxygen content (0< or =delta< or =0.19) have been studied systematically by susceptibility, transport, and spectroscopic techniques. An intimate correlation between the spin-charge ordering and the electronic transport behavior is found. Giant negative as well as positive magnetoresistance are observed.

Two resonant magnetic modes in an overdoped high T(c) superconductor

A detailed inelastic neutron scattering study of the overdoped high temperature copper oxide superconductor Y(0.9)Ca(0.1)Ba(2)Cu(3)O(7) reveals two-distinct magnetic resonant modes in the superconducting state. The modes differ in their symmetry with respect to exchange between adjacent copper oxide layers. Counterparts of the mode with odd symmetry, but not the one with even symmetry, had been observed before at lower doping levels. The observation of the even mode resolves a long-standing puzzle, and the spectral weight ratio of both modes yields an estimate of the onset of particle-hole spin-flip excitations.