Simulated structural and magnetic behavior of Mn-Ti intercalated dichalcogenide crystals.

We present the results of extensive Monte Carlo simulations of intercalated manganese-titanium (Mn-Ti) layered TiS2 crystals. The computational model involves mixtures of Mn and Ti in various percentages placed on a triangular lattice with fixed lattice sites and up to five layers. The range of concentrations of intercalated Mn studied was 5% ⩽ X Mn ⩽ 33% and for Ti, 0% ⩽ X Ti ⩽ 15%, where X A denotes the percentage of the total number of lattice sites occupied by species A. The species are allowed to interact spatially through a screened Coulomb potential and magnetically with external and RKKY field terms. Structurally, the pure Mn systems present as disordered at very low densities and evolve through a 2 × 2 structure (perfect at X Mn = 25%) up to a [Formula: see text] × [Formula: see text] lattice (perfect at X Mn = 33%), with variations of the two 'perfect' lattice structures depending on density. Changes in density for pure Mn systems as well as those intercalated with both Mn and Ti dramatically affects the system's structural and magnetic properties, and the magnetic behavior of various morphological features present in the system are discussed. The RKKY interaction is adjusted based on the intercalant compositions and is very sensitive to structural variations in the intercalant layers. The composition ranges studied here encompass and exceed those that are experimentally accessible, which helps place experimentally relevant densities in perspective.

Metal-insulator transition in variably doped (Bi(1-x)Sb(x))2Se3 nanosheets.

Topological insulators are novel quantum materials with metallic surface transport but insulating bulk behavior. Often, topological insulators are dominated by bulk contributions due to defect induced bulk carriers, making it difficult to isolate the more interesting surface transport characteristics. Here, we report the synthesis and characterization of nanosheets of a topological insulator Bi2Se3 with variable Sb-doping levels to control the electron carrier density and surface transport behavior. (Bi(1-x)Sb(x))2Se3 thin films of thickness less than 10 nm are prepared by epitaxial growth on mica substrates in a vapor transport setup. The introduction of Sb in Bi2Se3 effectively suppresses the room temperature electron density from ∼4 × 10(13) cm(-2) in pure Bi2Se3 (x = 0) to ∼2 × 10(12) cm(-2) in (Bi(1-x)Sb(x))2Se3 at x ∼ 0.15, while maintaining the metallic transport behavior. At x ≳ ∼0.20, a metal-insulator transition (MIT) is observed, indicating that the system has transformed into an insulator in which the metallic surface conduction is blocked. In agreement with the observed MIT, Raman spectroscopy reveals the emergence of vibrational modes arising from Sb-Sb and Sb-Se bonds at high Sb concentrations, confirming the appearance of the Sb2Se3 crystal structure in the sample. These results suggest that nanostructured chalcogenide films with controlled doping can be a tunable platform for fundamental studies and electronic applications of topological insulator systems.

Observation of infrared-active modes in Raman scattering from topological insulator nanoplates.

Two infrared (IR)-active vibrational modes, observed at 93 and 113 cm(-1) in Raman scattering, are evidence of an inversion symmetry breakdown in thin (~10 nm) nanoplates of topological insulator Bi(2)Te(3) as-grown on SiO(2). Both Raman and IR modes are preserved after typical device fabrication processes. In nanoplates transferred to another SiO(2) substrate via contact printing, however, the IR modes are absent, and the Raman spectra are similar to those from bulk samples. The differences between as-grown and transferred nanoplates may result from nanoplate-substrate interactions.

Reconstructed Fermi surface of underdoped Bi2Sr2CaCu2O(8+δ) cuprate superconductors.

The Fermi surface topologies of underdoped samples of the high-T(c) superconductor Bi2Sr2CaCu2O(8+δ) have been measured with angle resolved photoemission. By examining thermally excited states above the Fermi level, we show that the observed Fermi surfaces in the pseudogap phase are actually components of fully enclosed hole pockets. The spectral weight of these pockets is vanishingly small at the magnetic zone boundary, creating the illusion of Fermi "arcs." The area of the pockets as measured in this study is consistent with the doping level, and hence carrier density, of the samples measured. Furthermore, the shape and area of the pockets is well reproduced by phenomenological models of the pseudogap phase as a spin liquid.

Properties of hydrogen terminated diamond as a photocathode.

Electron emission from the negative electron affinity (NEA) surface of hydrogen terminated, boron doped diamond in the [100] orientation is investigated using angle resolved photoemission spectroscopy (ARPES). ARPES measurements using 16 eV synchrotron and 6 eV laser light are compared and found to show a catastrophic failure of the sudden approximation. While the high energy photoemission is found to yield little information regarding the NEA, low energy laser ARPES reveals for the first time that the NEA results from a novel Franck-Condon mechanism coupling electrons in the conduction band to the vacuum. The result opens the door to the development of a new class of NEA electron emitter based on this effect.

Quantum many-body calculation of mixed-parity pairing in the Sr2RuO4 superconductor induced by spin-orbit coupling.

The unusual superconducting state in Sr(2)RuO(4) has long been viewed as being analogous to a superfluid state in liquid (3)He. Nevertheless, calculations based on this odd-parity state are presently unable to completely reconcile the properties of Sr(2)RuO(4). Using a self-consistent quantum many-body scheme that employs realistic parameters, we are able to model several signature properties of the normal and superconducting states of Sr(2)RuO(4). We find that the dominant component of the model superconducting state is of even parity and closely related to superconducting state for the high-T(c) cuprates although a smaller odd-parity component is induced by spin-orbit coupling. This mixed pairing state gives a more complete representation of the complex phenomena measured in Sr(2)RuO(4).

High-energy kink observed in the electron dispersion of high-temperature cuprate superconductors.

Photoemission studies show the presence of a high-energy anomaly in the observed band dispersion for two families of cuprate superconductors, Bi2Sr2CaCu2O8+delta and La 2-x BaxCuO4. The anomaly, which occurs at a binding energy of approximately 340 meV, is found to be anisotropic and relatively weakly doping dependent. Scattering from short range or nearest neighbor spin excitations is found to supply an adequate description of the observed phenomena.

Orbital dependence of the fermi liquid state in Sr2RuO4.

We have used angle-resolved photoemission spectroscopy to determine the bulk electronic structure of Sr(2)RuO(4) above and below the Fermi liquid crossover near 25 K. Our measurements indicate that the properties of the system are highly orbital dependent. The quasi-2D gamma band displays Fermi liquid behavior while the remaining low energy bands show exotic properties consistent with quasi-1D behavior. In the Fermi liquid state below 25 K, the gamma band dominates the electronic properties, while at higher temperatures the quasi-1D beta and alpha bands become more important.

Quantum melting of the charge-density-wave state in 1T-TiSe2.

We report a Raman scattering study of low-temperature, pressure-induced melting of the charge-density-wave (CDW) phase of 1T-TiSe2. Our measurements reveal that the collapse of the CDW state occurs in three stages: (i) For P<5 kbar, the pressure dependence of the CDW amplitude mode energies and intensities are indicative of a "crystalline" CDW regime; (ii) for 525 kbar, the absence of amplitude modes reveals a metallic regime in which the CDW has melted.

Electron-hole coupling and the charge density wave transition in TiSe2.

Angle-resolved photoemission is employed to measure the band structure of TiSe2 in order to clarify the nature of the ( 2 x 2 x 2) charge density wave transition. The results show a very small indirect gap in the normal phase transforming into a larger indirect gap at a different location in the Brillouin zone. Fermi surface topology is irrelevant in this case. Instead, electron-hole coupling together with a novel indirect Jahn-Teller effect drives the transition.

Lobular neoplasia of the breast: higher risk for subsequent invasive cancer predicted by more extensive disease.

We have stratified the cancer risk implications of lobular pattern in situ neoplasias of the breast by separating marked examples of this histologic spectrum (lobular carcinoma in situ [LCIS]) from lesser examples (atypical lobular hyperplasia). The lesser-developed examples have been shown previously to have a lower relative risk (RR) of later invasive carcinoma of the breast (IBC). Forty-eight examples of LCIS were found in 10,542 otherwise benign breast biopsies, representing an incidence of 0.5%. Nine patients were excluded from follow-up because of bilateral mastectomy within 6 months of entry biopsy, IBC within 6 months of entry biopsy, or prior IBC. Follow-up of the remaining 39 patients was complete, averaged 18 years, and revealed an RR of subsequent IBC of 6.9 (P less than .00001). Average overall follow-up for LCIS patients was 19 years; it was 25 years for those alive and free of IBC at the time of their follow-up interview. Neither family history of IBC nor postmenopausal estrogen therapy further affected risk. The absolute risk of IBC after LCIS was 17% at 15 years (adjusted for withdrawals), and the RR was 8.0 in the first 15 years of follow-up compared with the general population. An analysis based on a time-dependent hazards model found that during the first 15 years following biopsy women with LCIS had 10.8 times the risk of breast cancer compared with biopsied women of comparable age who lacked proliferative disease. Some previously published articles reporting lobular neoplasia (LN) suggest that those series with the greatest incidences of LN (whether termed LN or LCIS) have the lowest RR of subsequent breast cancer. Those series with higher incidences of LN include less well-developed histologic patterns of LN (atypical lobular hyperplasia). We conclude that our study of LN and studies performed by others support the higher risk of IBC after histologically flagrant examples (LCIS, about nine times higher) and a relatively lower but definable risk after more histologically subtle examples (atypical lobular hyperplasia, four to five times lower). This relative cancer risk is probably not constant over more than 15 years; thus, cancer risk 15 to 25 years after initial diagnosis of LCIS is uncertain.

Comparison of measurement of effective renal plasma flow by single plasma sample and plasma disappearance slope/volume methods.

Numerous simplified methods for the estimation of ERPF have described, including the so-called slope/intercept (SI) methods, based on the analysis of the slope of certain segments of the 131I-OIH plasma disappearance curve and its y-axis intercept, and the single sample (SS) clearance method, based on theoretical volumes of OIH distribution at some fixed time after injection. Using ERPFs estimated from compartment analysis of the entire 60-min plasma disappearance curve, we have compared the errors of data calculated from use of eight SI methods made at various times along the disappearance curve with that from the optimum SS curve. The errors obtained from the SS method were approximately 50% less than those obtained from the SI methods. The errors of the SI methods are greater at both ends of the 60-min plasma curve than when samples are drawn near the mid-time. The SS method appears to be the method of choice for the estimation of ERPF using single injection techniques.