Bulk Fermi Surface of Charge-Neutral Excitations in SmB_{6} or Not: A Heat-Transport Study.

Recently, there have been increasingly hot debates on whether a bulk Fermi surface of charge-neutral excitations exists in the topological Kondo insulator SmB_{6}. To unambiguously resolve this issue, we perform the low-temperature thermal conductivity measurements of a high-quality SmB_{6} single crystal down to 0.1 K and up to 14.5 T. Our experiments show that the residual linear term of thermal conductivity at the zero field is zero, within the experimental accuracy. Furthermore, the thermal conductivity is insensitive to the magnetic field up to 14.5 T. These results demonstrate the absence of fermionic charge-neutral excitations in bulk SmB_{6}, such as scalar Majorana fermions or spinons and, thus, exclude the existence of a bulk Fermi surface suggested by a recent quantum oscillation study of SmB_{6}. This puts a strong constraint on the explanation of the quantum oscillations observed in SmB_{6}.

Quantum transport evidence for the three-dimensional Dirac semimetal phase in Cd3As2.

We report the quantum transport properties of Cd3As2 single crystals in a magnetic field. A large linear quantum magnetoresistance is observed near room temperature. With decreasing temperature, the Shubnikov-de Haas oscillations appear in both the longitudinal resistance R(xx) and the transverse Hall resistance R(xy). From the strong oscillatory component ΔR(xx), a linear dependence of the Landau index n on 1/B is obtained, and it gives an n-axis intercept between 1/2 and 5/8. This clearly reveals a nontrivial π Berry's phase, which is a distinguished feature of Dirac fermions. Our quantum transport results provide bulk evidence for the existence of a three-dimensional Dirac semimetal phase in Cd3As2.

Anomalous reduction of the Lorenz ratio at the quantum critical point in YbAgGe.

We report measurements of the electrical and thermal transport on the hexagonal heavy-fermion metal YbAgGe for temperatures T ≥ 40 mK and in magnetic fields H∥ab up to 14 T. This distorted kagome-lattice system displays a series of magnetic states and a quantum critical point at H(c) = 4.5 T. The Lorenz ratio L(T)/L0 displays a marked reduction only close to H(c). A T-linear contribution below 120 mK, present at all different fields, allows us to extrapolate the Lorenz ratio towards T = 0. At the critical field this yields L/L0 = 0.92±0.03, suggesting a violation of the Wiedemann-Franz law due to strong inelastic scattering.

Quantum criticality and nodal superconductivity in the FeAs-based superconductor KFe2As2.

The in-plane resistivity rho and thermal conductivity kappa of the FeAs-based superconductor KFe2As2 single crystal were measured down to 50 mK. We observe non-Fermi-liquid behavior rho(T) approximately T{1.5} at H{c{2}}=5 T, and the development of a Fermi liquid state with rho(T) approximately T{2} when further increasing the field. This suggests a field-induced quantum critical point, occurring at the superconducting upper critical field H{c{2}}. In zero field, there is a large residual linear term kappa{0}/T, and the field dependence of kappa_{0}/T mimics that in d-wave cuprate superconductors. This indicates that the superconducting gaps in KFe2As2 have nodes, likely d-wave symmetry. Such a nodal superconductivity is attributed to the antiferromagnetic spin fluctuations near the quantum critical point.

Evidence for competing magnetic and superconducting phases in superconducting Eu 1-x Sr x Fe 2-y Co y As 2 single crystals.

In single crystals of Eu(1-x)Sr(x)Fe(2-y)Co(y)As(2), Co doping suppresses spin-density wave (SDW) ordering and induces a superconducting transition. A resistivity reentrance due to the antiferromagnetic ordering of Eu(2+) spins is observed, indicating the competition between antiferromagnetism (AFM) and superconductivity (SC). It is striking that the resistivity reentrance can be completely suppressed by a small magnetic field due to a field-induced metamagnetic transition from AFM to ferromagnetism (FM). The resistivity reentrance can also be suppressed by the substitution of Eu(2+) ions with nonmagnetic Ba(2+)/Sr(2+) to completely destroy the AFM ordering. These results indicate that the AFM order appears destructive to SC, while FM can coexist with the superconductivity. Further we find that magnon excitation exists in AFM ordering and can be suppressed by an applied field. Coexistence of SC from the FeAs layer and the inner field produced by the ferromagnetic Eu(2+) layer suggest a possible p-wave component in the superconducting order parameter.

Primary role of the barely occupied states in the charge density wave formation of NbSe2.

NbSe2 is a prototypical charge-density-wave (CDW) material, whose mechanism remains mysterious so far. With angle resolved photoemission spectroscopy, we recovered the long-lost nesting condition over a large broken-honeycomb region in the Brillouin zone, which consists of six saddle band point regions with high density of states (DOS), and large regions away from Fermi surfaces with negligible DOS at the Fermi energy. We show that the major contributions to the CDW formation come from these barely occupied states rather than the saddle band points. Our findings not only resolve a long-standing puzzle, but also overthrow the conventional wisdom that CDW is dominated by regions with high DOS.

Evolution of the electronic structure of 1T-Cu(x)TiSe(2).

The electronic structure of a new charge-density-wave system or superconductor, 1T-Cu(x)TiSe(2), has been studied by photoemission spectroscopy. A correlated semiconductor band structure is revealed for the undoped case, which resolves a long-standing controversy in the system. With Cu doping, the charge-density wave is suppressed by the raising of the chemical potential, while the superconductivity is enhanced by the enhancement of the density of states, and possibly suppressed at higher doping by the strong scattering.

Strength and microstructure of IPS Empress 2 glass-ceramic after different treatments.

PURPOSE: This investigation was designed to determine whether heat pressing and/or simulated heat treatments affect the flexure strength and microstructure of the lithium disilicate glass-ceramic of the IPS Empress 2 system. MATERIALS AND METHODS: Four groups of the lithium disilicate glass-ceramic were prepared as follows: group 1 = as-received material; group 2 = heat-pressed material; group 3 = heat-pressed and stimulated initial heat-treated material; and group 4 = heat-pressed and simulated heat-treated material with full firings for a final restoration. Three-point bending tests and scanning electron microscopy (SEM) analysis were conducted. RESULTS: The flexure strength of group 2 was significantly higher than that of group 1. However, there were no significant differences in strength among groups 2, 3, and 4, or between groups 1 and 4. The SEM micrographs of the lithium disilicate glass-ceramic showed a closely packed, multidirectionally interlocking microstructure of numerous lithium disilicate crystals protruding from the glass matrix. The crystals in the glass matrix of the heat-pressed materials (groups 2, 3, and 4) were a little more homogeneous and about 2 times bigger than those of the as-received material (group 1). These changes of the microstructure were greatest between groups 1 and 2. However, there were no marked differences among groups 2, 3, and 4. CONCLUSION: Although there were significant increases in the strength and some changes of the microstructure after the heat-pressing operation, the combination of heat pressing and simulated subsequent heat treatments did not produce an increase of strength of IPS Empress 2 glass-ceramic.

The esthetics of the smile: a review of some recent studies.

PURPOSE: This article reviews recent research on the esthetics of the smile, covering the attractiveness of the smile, the effect of aging on the smile, oral condition and the smile, personality and smile, and smile exercises. MATERIAL AND METHODS: The subjects were Koreans with normal occlusion. Photographs of a full smile were taken and the esthetic quality of the subjects' smiles was estimated. Smile scores were correlated with oral condition, personality, the practice of smile exercises, and elements of the smile, such as the position of the lip in a smile. The personality of the subjects was assessed by means of a Sixteen Personality Factor Questionnaire. Gibson's smile exercises were used to investigate the effect of smile exercise. RESULTS: In an attractive smile, the full shape of the maxillary anterior teeth was shown between the upper and lower lip, the upper lip curved upward or was straight, the maxillary anterior incisal curve was parallel to the lower lip, and teeth were displayed to the first molar. The amount of maxillary incisal exposure gradually decreased with age, accompanied by a gradual increase in mandibular incisal exposure. Personality traits such as warmth, calmness, extroversion, and low anxiety were closely related to an attractive smile. Smile exercises were an effective means of improving the esthetic level of the smile if patients exercised continuously. CONCLUSION: An attractive smile is important for esthetic treatment. The lip position, oral condition, personality traits, and smile exercise affect the esthetics of the smile.

Effects of veneering and glazing on the strength of heat-pressed ceramics.

A newly developed press-type all-ceramic crown system, the IPS-Empress system (Ivoclar), has recently been introduced. Two methods may be used to obtain the desired shade: surface staining and glazing; veneer technique. The purpose of this study was to determine whether these two methods affected flexure strength of Empress glass ceramic. Eight groups of test bars were pressed. In groups 1 and 2, one surface was stained and glazed. The bars were placed face down (1) or up (2) for testing. For comparison, group 3 was heat-treated only (simulating stain and glaze firing). In groups 4-7, one surface of the bars was either veneered with porcelain on the bottom (4) or top surface (5) and then subsequently glazed (6 and 7). Group 8 was just heat-treated (simulating veneer and glaze firings). The results showed that there were no significant differences in strength between groups 2, 5 and 7 compared to the reference groups 3 and 8 (159 +/- 28 and 175 +/- 32 MPa, respectively), indicating that, from the mechanical point of view, the two surface techniques can be equally used on Empress ceramic. If the porcelain veneer supported the ceramic (4), the strength was significantly decreased. The highest mean value was obtained in group 1 (220 +/- 34 MPa).

Heat-pressed ceramics: technology and strength.

The flexural strength of a new heat-pressed ceramic material (IPS-Empress) was measured before and after pressing and/or simulated firing treatments (eg, veneering, surface coloring, glazing). Heat pressing the material significantly improved its flexure strength whereas heat treating the material alone did not. Additional firings (heat treatments) after heat pressing further increased material strength. The final strength values ranged between 160 and 180 MPa and were comparable to some other all-ceramic systems. No clinical implications were drawn from these data.