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Despite their name, the bulk electrical conductivity of most topological insulators is relatively high, masking many of the important characteristics of its protected, surface conducting states. Counter-doping reduces the bulk conductivity of Bi 2 Se 3 significantly, allowing these surface states and their properties to be clearly identified. The newly discovered three-dimensional strong topological insulators (STIs) exhibit topologically protected Dirac surface states 1 , 2 . Although the STI surface state has been studied spectroscopically, for example, by photoemission 3 , 4 , 5 and scanned probes 6 , 7 , 8 , 9 , 10 , transport experiments 11 , 12 , 13 , 14 , 15 , 16 , 17 have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (∼ 10 nm), low-doped Bi 2 Se 3 (≈10 17  cm −3 ) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region 18 , 19 , 20 . A theory of charge disorder in a Dirac band 19 , 20 , 21 explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e 2 / h per surface) and the residual (puddle) carrier density (0.4×10 12 to 4×10 12  cm −2 ). From the measured carrier mobilities 320–1,500 cm 2  V −1  s −1 , the charged impurity densities 0.5×10 13 to 2.3×10 13  cm −2 are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (1×10 13 to 3×10 13  cm −2 ), identifying dopants as the charged impurities.

In the high-temperature cuprate superconductors, the pervasiveness of anomalous electronic transport properties suggests that violation of conventional Fermi liquid behavior is closely tied to superconductivity. In other classes of unconventional superconductors, atypical transport is well correlated with proximity to a quantum critical point, but the relative importance of quantum criticality in the cuprates remains uncertain. Here, we identify quantum critical scaling in the electron-doped cuprate material La2-xCexCuO4 with a line of quantum critical points that surrounds the superconducting phase as a function of magnetic field and charge doping. This zero-temperature phase boundary, which delineates a metallic Fermi liquid regime from an extended non-Fermi liquid ground state, closely follows the upper critical field of the overdoped superconducting phase and gives rise to an expanse of distinct non-Fermi liquid behavior at finite temperatures. Together with signatures of two distinct flavors of quantum fluctuations, these facts suggest that quantum criticality plays a significant role in shaping the anomalous properties of the cuprate phase diagram.

Some types of batteries contain both a transition metal reducible metal, such as the cathode material Ag2VP2O8. During operation, both Ag and V ions are reduced, and the Ag atoms can form wires to enhance the internal conductivity. Kirshenbaum et al. probe the discharge of a battery at different rates and track the formation of Ag atoms using in situ energy-dispersive x-ray diffraction (see the Perspective by Dudney and Li). They show how the discharge rate affects whether the Ag or V is preferentially reduced and also the distribution of the Ag atoms, and then correlate this to the loss of battery capacity at higher discharge rates. Science, this issue p. 149; see also p. 131 The functional capacity of a battery is observed to decrease, often quite dramatically, as discharge rate demands increase. These capacity losses have been attributed to limited ion access and low electrical conductivity, resulting in incomplete electrode use. A strategy to improve electronic conductivity is the design of bimetallic materials that generate a silver matrix in situ during cathode reduction. Ex situ x-ray absorption spectroscopy coupled with in situ energy-dispersive x-ray diffraction measurements on intact lithium/silver vanadium diphosphate (Li/Ag2VP2O8) electrochemical cells demonstrate that the metal center preferentially reduced and its location in the bimetallic cathode are rate-dependent, affecting cell impedance. This work illustrates that spatial imaging as a function of discharge rate can provide needed insights toward improving realizable capacity of bimetallic cathode systems.

Quantum-mechanical fluctuations between competing phases induce exotic collective excitations that exhibit anomalous behavior in transport and thermodynamic properties, and are often intimately linked to the appearance of unconventional Cooper pairing. High-temperature superconductivity, however, makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe 1/3 Co 1/3 Ni 1/3 ) 2 As 2 , indicative of quantum criticality at zero temperature and applied magnetic field. Beyond a linear-in-temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find a scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system unhindered by a pairing instability. Extensive theoretical and experimental efforts have been devoted to the effect of quantum criticality in our understanding of the physics of high-temperature superconductors and strongly correlated electron materials, yet it remains a puzzle in condensed matter physics. The authors report observations of a quantum criticality by investigating the non-Fermi liquid thermodynamics and transport behaviour in a non-superconducting iron pnictide.

Some types of batteries contain both a transition metal reducible metal, such as the cathode material Ag 2 VP 2 O 8 . During operation, both Ag and V ions are reduced, and the Ag atoms can form wires to enhance the internal conductivity. Kirshenbaum et al. probe the discharge of a battery at different rates and track the formation of Ag atoms using in situ energy-dispersive x-ray diffraction (see the Perspective by Dudney and Li). They show how the discharge rate affects whether the Ag or V is preferentially reduced and also the distribution of the Ag atoms, and then correlate this to the loss of battery capacity at higher discharge rates. Science , this issue p. 149 ; see also p. 131 The change in battery capacity with discharge rate is related to internal structural changes using in situ imaging. [Also see Perspective by Dudney and Li ] The functional capacity of a battery is observed to decrease, often quite dramatically, as discharge rate demands increase. These capacity losses have been attributed to limited ion access and low electrical conductivity, resulting in incomplete electrode use. A strategy to improve electronic conductivity is the design of bimetallic materials that generate a silver matrix in situ during cathode reduction. Ex situ x-ray absorption spectroscopy coupled with in situ energy-dispersive x-ray diffraction measurements on intact lithium/silver vanadium diphosphate (Li/Ag 2 VP 2 O 8 ) electrochemical cells demonstrate that the metal center preferentially reduced and its location in the bimetallic cathode are rate-dependent, affecting cell impedance. This work illustrates that spatial imaging as a function of discharge rate can provide needed insights toward improving realizable capacity of bimetallic cathode systems.

The functional capacity of a battery is observed to decrease, often quite dramatically, as discharge rate demands increase. These capacity losses have been attributed to limited ion access and low electrical conductivity, resulting in incomplete electrode use. A strategy to improve electronic conductivity is the design of bimetallic materials that generate a silver matrix in situ during cathode reduction. Ex situ x-ray absorption spectroscopy coupled with in situ energy-dispersive x-ray diffraction measurements on intact lithium/silver vanadium diphosphate (Li/Ag2VP2O8) electrochemical cells demonstrate that the metal center preferentially reduced and its location in the bimetallic cathode are rate-dependent, affecting cell impedance. This work illustrates that spatial imaging as a function of discharge rate can provide needed insights toward improving realizable capacity of bimetallic cathode systems.

The newly discovered three-dimensional strong topological insulators (STIs) exhibit topologically protected Dirac surface states. Although the STI surface state has been studied spectroscopically, for example, by photoemission and scanned probes, transport experiments have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin ( 10nm), low-doped Bi sub(2)Se sub(3) ( approximately 10 super(17)cm super(-3)) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with a linear Hall resistivity and a well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region. A theory of charge disorder in a Dirac band explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e super(2)/h per surface) and the residual (puddle) carrier density (0.410 super(12) to 410 super(12)cm super(-2)). From the measured carrier mobilities 320-1,500cm super(2)V super(-1)s super(-1), the charged impurity densities 0.510 super(13) to 2.310 super(13)cm super(-2) are inferred. They are of a similar magnitude to the measured doping levels at zero gate voltage (110 super(13) to 310 super(13)cm super(-2)), identifying dopants as the charged impurities.

I report on superconductivity in undoped SrFe2As 2 and find that it is caused by lattice strain in the as-grown crystals that can be removed or returned with annealing or pressure, respectively. To study the magnetic/structural transition I measure the evolution of these transitions in solid solutions of the [Ca, Sr, Ba]Fe2As2 series and determine that the Neél temperature is independent of the size of the antiferromagnetically ordered moment. I present the first reported phase diagrams for Ni- and Pt-substitution in SrFe2As2, showing that the simple charge-counting picture of chemical substitution cannot completely describe the onset and offset of the superconducting phase. Finally, I use the transport scattering rate to explain the variation in Tc seen in transition metal substituted 122s. I will show that pair breaking can explain the variation in the optimum transition temperature, and that the rate of suppression of Tc with scattering will show that the pairing symmetry of the iron-based superconductors is a sign-changing, multiband s-wave order parameter that must include both inter- and intraband scattering.