In Kondo insulator samarium hexaboride SmB6, strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 5 Kelvin, a behavior recently demonstrated to arise from the surface conductance. However, questions remain whether and where a topological surface state exists. Quantum oscillations have not been observed to map the Fermi surface. We solve the problem by resolving the Landau Level quantization and Fermi surface topology using torque magnetometry. The observed Fermi surface suggests a two dimensional surface state on the (101) plane. Furthermore, the tracking of the Landau Levels in the infinite magnetic field limit points to -1/2, which indicates a 2D Dirac electronic state.The recent development of topological insulators is a triumph of single electron band theory [1][2][3][4][5][6][7][8] . It is interesting to understand whether similar exotic states of matter can arise once strong electronic interaction comes into play. Kondo insulators, a strongly-correlated heavyfermion system, offer a good playground for the exploration of this question. In a Kondo insulator 9,10 , the hybridization between itinerant electrons and localized orbitals opens a gap and makes the material insulating. Once the sample temperature is cold enough, the electronic structure in the strongly correlated system can be mapped to a rather simple electronic state that resembles a normal topological insulator 11 . As a result, in the ground state of the Kondo insulator there exists a bulk insulating state and a conductive surface state. In samarium hexaboride (SmB 6 ), the existence of the surface state has been suggested by recent experimental observations of the surface conductance as well as a map of the hybridization gap 12-14 . However, a direct observation of the Fermi surface has not yet been achieved by transport measurements in Kondo insulators. In this letter we report the observation of quantum oscillations in Kondo insulator SmB 6 using torque magnetometry. The observed Fermi surface is shown to be two-dimensional (2D) and arises from the crystalline (101) surface, and the Landau Level index plot shows a Berry phase contributed -1/2 factor in the infinite field limit, which indicates that this Fermi surface encloses Dirac points, a characteristic property of topological insulators.The direct observation of quantum oscillations is an essential step in understanding the electronic state of the bulk and surfaces of Kondo insulator. Wolgast et al. have argued strongly that the great robustness and certain other properties of the low T surface conductivity of SmB 6 are best understood as a consequence of having TI surface states 12 . Nonetheless there is yet no direct evidence for this interpretation of the surface conduction. Such evidence should come from microscopic measurements of the electronic structure, as has been accomplished for the weakly correlated TI materials, such as Bi 2 Se 3 , Bi 2 Te 3 , and graphene [15][16][17][18][19...

The recent conjecture of a topologically-protected surface state in SmB 6 and the verification of robust surface conduction below 4 K have prompted a large effort to understand the surface states.Conventional Hall transport measurements allow current to flow on all surfaces of a topological insulator, so such measurements are influenced by contributions from multiple surfaces of varying transport character. Instead, we study magnetotransport of SmB 6 using a Corbino geometry, which can directly measure the conductivity of a single, independent surface. Both (011) and (001) crystal surfaces show a strong negative magnetoresistance at all magnetic field angles measured.The (011) surface has a carrier mobility of 122 cm 2 /V·sec with a carrier density of 2.5×10 13 cm −2 , which are significantly smaller than indicated by Hall transport studies. This mobility value can explain a failure so far to observe Shubnikov-de Haas oscillations. Analysis of the angle-dependence of conductivity on the (011) surface suggests a combination of a field-dependent enhancement of the carrier density and a suppression of Kondo scattering from native oxide layer magnetic moments as the likely origin of the negative magnetoresistance. Our results also reveal a hysteretic behavior whose magnitude depends on the magnetic field sweep rate and temperature. Although this feature becomes smaller when the field sweep is slower, does not disappear or saturate during our slowest sweep-rate measurements, which is much slower than a typical magnetotransport trace.These observations cannot be explained by quantum interference corrections such as weak antilocalization, but are more likely due to an extrinsic magnetic effect such as the magnetocaloric effect or glassy ordering.

Topological insulator Bi2Se3 has shown a number of interesting physical properties. Doping Bi2Se3 with copper (Cu) or strontium (Sr) has been demonstrated to make the material superconducting and potentially even a topological superconductor. The recent discovery of superconducting niobium (Nb) doped Bi2Se3 reveals an exciting new physical phenomenon, the coexistence of superconductivity and magnetic ordering, as well as signatures of an odd-parity p-wave superconducting order. To understand this new phenomenon, a detailed knowledge of the electronic structure is needed. To date, the quantum oscillations needed to reveal this structure have not been reported in Nb-doped Bi2Se3. In this rapid communication, we present the first observation of quantum oscillations in the magnetization (the de Haas-van Alphen effect) of Nb-doped Bi2Se3. In the fully superconducting crystal, two distinct orbits are observed, in sharp contrast to Bi2Se3, Cu-doped Bi2Se3, and Sr-doped Bi2Se3. The multiple frequencies observed in our quantum oscillations, combined with our electrical transport studies, indicate the multi-orbit nature of the electronic state of Nb-doped Bi2Se3.

Strong evidence for charge-density correlation in the underdoped phase of the cuprate YBa 2 Cu 3 O y was obtained by NMR and resonant X-ray scattering. The fluctuations were found to be enhanced in strong magnetic fields. Recently, 3D charge-density-wave (CDW) formation with long-range order (LRO) was observed by X-ray diffraction in H > 15 T. To elucidate how the CDW transition impacts the pair condensate, we have used torque magnetization to 45 T and thermal conductivity κ xx to construct the magnetic phase diagram in untwinned crystals with hole density p = 0.11. We show that the 3D CDW transitions appear as sharp features in the susceptibility and κ xx at the fields H K and H p , which define phase boundaries in agreement with spectroscopic techniques. From measurements of the melting field H m (T ) of the vortex solid, we obtain evidence for two vortex solid states below 8 K. At 0.5 K, the pair condensate appears to adjust to the 3D CDW by a sharp transition at 24 T between two vortex solids with very different shear moduli. At even higher H (41 T), the second vortex solid melts to a vortex liquid which survives to fields well above 41 T. de Haas-van Alphen oscillations appear at fields 24-28 T, below the lower bound for the upper critical field H c2 .cuprate superconductivity | high-field phase diagram | vortex liquid | torque magnetometry | thermal conductivity T he existence of static charge order in underdoped YBa 2 Cu 3 O y (YBCO) in an intense magnetic field was reported by Wu et al.(1) using NMR. Subsequently, Ghiringhelli et al. (2) uncovered temperature-dependent charge-density-wave (CDW) correlations in zero magnetic field by resonant X-ray scattering (RXS). The RXS signal onsets near 180 K and peaks at the superconducting critical temperature T c before falling. Several groups (3-5) recently showed that the RXS intensity is enhanced in finite field H. At low temperature T, several experiments have uncovered field-induced transitions. These include a transition at 18 T from ultrasonic measurements (6), features in the high-field thermal conductivity (7), and the onset of line splitting in the NMR spectra starting at the charge-ordering field H ch = 10-15 T and saturating at around 18-20 T (depending on the hole density p) (8). Recent X-ray diffraction experiments (9-11) show that the transition to a 3D CDW with long-range order (LRO) onsets near H = 15 T.The field-induced CDW state raises several intriguing questions regarding its relation to superconductivity. Does CDW formation with LRO suppress the superconducting condensate? Where is the true upper critical field H c2 ? How does it affect the stability of the vortex solid? Although NMR and X-ray diffraction are incisive probes of charge modulation and CDW formation, they do not couple directly to the fundamental excitations of the pair condensate (quasi-particles and vortices), so they are less sensitive to the pairing correlations which reflect superconductivity. By contrast, the diamagnetic magnetization M d (which dominates the observed mag...

Carbon quantum dots (CQDs) are novel carbon nanomaterials and are attracting increasing interest due to their good characteristics such as hydrophilicity, chemical stability, quantum yield, small particle sizes, and low cytotoxicity. Herein, we used CQDs as stabilizers and exfoliation agents to exfoliate graphite to graphene in an aqueous medium for the first time. The functions of CQDs are to reduce the surface tension of water to match that of graphite and to make weak interactions (π-π conjugation, hydrophobic force, and the Coulomb attraction) with the graphite surface. Different characterization methods were used to evaluate the presence of layers (<5 layers) of graphene sheets with fewer defects and low oxidation. In the future, CQDs can also be good candidates to exfoliate other two-dimensional materials, such as WS2, BN, MoS2, and g-C3N4, to form two-dimensional heterostructures for a range of possible applications.