We study the transport properties of the Kondo insulator SmB 6 with a specialized configuration designed to distinguish bulk-dominated conduction from surface-dominated conduction. We find that as the material is cooled below 4 K, it exhibits a crossover from bulk to surface conduction with a fully insulating bulk. We take the robustness and magnitude of the surface conductivity, as is manifest in the literature of SmB 6 , to be strong evidence for the topological insulator metallic surface states recently predicted for this material.

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...

We present ac measurements of the diagonal conductivity o xx , in the integer quantum Hall regime for frequency /between 0.2 and 14 GHz and temperature r>:50 mK. Re(cr xx ) was obtained from the measured attenuation of a coplanar transmission line on the sample surface. For/S; 1 GHz, a xx peaks between IQHE minima broaden as / increases, roughly as (AB)

We have fabricated a two-dimensional array of Josephson junctions within 100 nm of a twodimensional electron gas (2DEG) in a GaAs ͞AlGaAs heterostructure. The screening provided by the 2DEG causes the array to show superconducting behavior despite a large junction resistance. Varying the resistance per square of the 2DEG changes the dissipation in the electrodynamic environment of the array independently of any other parameters in the system. As the resistance increases, the current-voltage characteristics of the array change from superconducting to insulating in character. [S0031-9007(97)02822-6] PACS numbers: 74.50. + r, 05.30. -d, 74.25.Fy, 74.40. + k A variety of diverse physical systems, including granular [1] or homogeneous [2] thin films, two-dimensional (2D) Josephson-junction arrays [3], and high temperature superconductors [4] undergo a superconductor-insulator (S-I) phase transition as a characteristic resistance of the system in its normal state increases through a critical value on the order of the resistance quantum R Q h͞4e 2 ഠ 6.45 kV. The transition is quantum mechanical in nature: the increasing normal state resistance is associated with an increase in quantum fluctuations of the superconducting phase. Eventually, these fluctuations destroy global phase coherence and lead to an insulating state. It has been suggested [5-8] that the S-I transition in these systems could be driven by changes in dissipation; however, there appears to be no unambiguous supporting evidence. In thin films disorder plays a strong role, and recent theoretical work treating these systems as charge-2e bosons moving in a random 2D potential [9] has been met with substantial experimental verification [2,10]. Furthermore, the physical origin of the dissipation is unclear. In the case of Josephson junction arrays, although quasiparticle tunneling [5] at energies large compared to the superconducting gap produces dissipation characterized by the normal state resistance R N , it is unlikely that the relevant energy scales are so large [3,11]. At lower energies, quasiparticle dissipation is negligible since the subgap resistance is much larger than R N . In high temperature superconductors, it has been proposed [8] that an interpenetrating fluid of normal electrons produces the dissipation. However, radiation damage inflicted to increase the normal state resistance probably also increases the disorder and reduces the density of superconducting electrons.In this Letter, we describe the unambiguous observation of a dissipation-driven S-I transition. The sample was a specially designed and fabricated 40 3 40 Josephson junction array for which we can continuously vary the dissipation associated with the local electrodynamic environment independently of any other relevant parameters. To provide the variable dissipation, we fabricated the array on a GaAs͞Al 0.3 Ga 0.7 As heterostructure in which a 2D electron gas (2DEG) is located approximately 100 nm from the surface (see Fig. 1). The heterostructure was grown on a GaAs substrate usi...

A single-phase synthesis of thiolate monolayer-protected gold clusters (MPCs) is described. This method avoids the problem of persistent ionic contamination from residual phase-transfer catalyst while retaining the versatility associated with the commonly used two-phase Brust synthesis. MPCs having alkyl, diphenylacetylene, ether, amide, or ester functionalities were prepared by this single-phase synthesis and characterized via 1H NMR, FT-IR, and UV−vis absorption spectroscopy, TGA, STM, and TEM. Comparisons of products synthesized by this method and by existing methods are made for a subset of MPCs. Results are considered in the context of using MPC thin films as chemically sensitive interface layers in chemiresistor vapor sensors.

We study two kinds of quantum interference effects in transport -the Aharonov-Bohm effect and the weak-localization effect -in quasi-one-dimensional wires and rings to address issues concerning the phase-coherence length, spin-orbit scattering, and the Aux cancellation mechanism which is predicted to be present when the elastic mean free path exceeds the sample width. Our devices are fabricated on GaAs/Al"Gal "As and pseudomorphic Ga"Inl As/Al" In, As heterostructure materials and the experiments carried out at 0.4 -20 K temperatures. In the GaAs/Al"Ga~, As devices which exhibit no significant spin-orbit scattering, we were able to extract a phase-coherence length I& from the amplitude of the Aharonov-Bohm magnetoresistance oscillations in different sized rings. We find it to be in agreement with l& deduced from the weak-localization data in parallel wires when the one-dimensional weaklocalization theory including thegux cancellation mechttnism is used to fit the data. We therefore unambiguously establish that the same l& governs the behavior of the two quantum interference phenomena of Aharonov-Bohm oscillations and weak localization, and that the Aux cancellation is in force. In the pseudomorphic Ga"In, "As/Al"In, "As heterostructure devices which exhibit strong spin-orbit interaction effects, l& exceeds the spin-orbit-scattering length at low temperatures.The amplitude of Aharonov-Bohm oscillations can only be explained by introducing reduction factors due to spin-orbit scattering.

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.

The inverted resistance method was used in this study to extend the bulk resistivity ofSmB6to a regime where the surface conduction overwhelms the bulk. Remarkably, regardless of the large off-stoichiometric growth conditions (inducing disorder by samarium vacancies, boron interstitials, etc.), the bulk resistivity shows an intrinsic thermally activated behavior that changes ∼7–10 orders of magnitude, suggesting thatSmB6is an ideal insulator that is immune to disorder.