Pulsed magnetic fields of up to 55T are used to investigate the transport properties of the topological insulator Bi2Se3 in the extreme quantum limit. For samples with a bulk carrier density of n = 2.9 × 10 16 cm −3 , the lowest Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For fields well beyond this limit, Shubnikov-de Haas oscillations arising from quantization of the 2D surface state are observed, with the ν =1 Landau level attained by a field of ∼ 35T. These measurements reveal the presence of additional oscillations which occur at fields corresponding to simple rational fractions of the integer Landau indices.The recent prediction and discovery that Bi 2 Se 3 and Bi 2 Te 3 are three-dimensional topological insulators (TI) [1][2][3][4][5] has sparked a flurry of investigations. In a magnetic field, their relativistic dispersion causes the energy spectrum to be quantized so that E ν ∝ √ Bν, where B is the magnetic field and ν = 1, 2, 3, .., is the energy level, known as the Landau level (LL). The progression of energy levels E ν has been recently observed in scanningtunneling microscopy (STM) on Bi 2 Se 3 [6,7]. Other experiments on bulk samples and nanoribbons have reported universal conductance fluctuations attributed to the surface state [8,9]. However, all of the unambiguous measurements for the existence of the surface state have been made by surface sensitive probes. In this letter, we study samples of (Bi 1−x Sb x ) 2 Se 3 in which the Fermi surface of the Dirac fermions is small enough that pulsed fields of up to 55 T can access their quantum limit -the limit in which only a few of the lowest LLs are occupied. This is achieved by depleting the carrier density of the bulk and thereby reducing the size of the Dirac Fermi surface [10,11]). We demonstrate for the first time, a system in which not only the transport properties of Dirac fermions can be studied, but studied in the 2D quantum limit where novel correlation effects are most likely to arise [12,13].Generally speaking, transport measurements in Bi 2 Se 3 and Bi 2 Te 3 are plagued by bulk conducting channels from either the conduction band or by impurity bands introduced by foreign dopants [14,15]. The materials challenge is therefore finding a way to cleanly eliminate the bulk conductivity so that the properties of the surface can be observed. However, the carrier densities reported to date in studies of TI remain only as low [2][3][4][5] and 33×10 16 cm −3 (red and orange curves on Figure 1 (a)) were obtained by slow cooling a binary melt with different Bi:Se ratio. The principal origin of these relatively high carrier densities is Se deficiency and antisite defects. Samples with n≤ 11 × 10 16 cm −3 were obtained by slow cooling a ternary melt containing progressively more Sb. Although Sb is isovalent with Bi, Sb substitution apparently acts to control the defect density in the bulk crystals, reducing the bulk carrier density. Before measurements were performed, samples were cleaved on both sides with a scalpel blade i...

2Close to optimal doping, the copper oxide superconductors show 'strange metal' behavior 1,2 , suggestive of strong fluctuations associated with a quantum critical point [3][4][5][6] . Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting 'dome'. The underdoped region of the temperature-doping phase diagram from which superconductivity emerges is referred to as the 'pseudogap' 7-13 because evidence exists for partial gapping of the conduction electrons, but so far there is no compelling thermodynamic evidence as to whether the pseudogap is a distinct phase or a continuous evolution of physical properties on cooling. Here we report that the pseudogap in YBa 2 Cu 3 O 6+δ is a distinct phase, bounded by a line of phase transitions. The doping dependence of this line is such that it terminates at zero temperature inside the superconducting dome. From this we conclude that quantum criticality drives the strange metallic behavior and therefore superconductivity in the cuprates.Resonant ultrasound spectroscopy (RUS) measures the frequencies f n and widths Γ n of the vibrational normal modes of a crystal acting as a free mechanical resonator. The frequencies of the normal modes are determined by density and geometry of the crystal as well as its elastic properties. The elastic component of the temperature evolution of these frequencies, ∆f n (T ), depends on a linear combination of all elastic moduli and reflects changes in the thermodynamic state of the system such as those associated with a phase transi- (Figure 4(a,b)). Causality requires that the maxima in energy absorption are accompanied by elastic stiffening over the same temperature range. This stiffening is observed in addition to the distinct break in slope at T * (Figure 2(b)).The potential for RUS to determine the broken symmetry in the pseudogap phase was limited in this study by the precision with which crystal shape could be controlled, an issue that may be resolvable as sample preparation techniques improve. The pseudogap phase 5 transition is located by our RUS measurements with ±3K uncertainty, improving on the ±30K uncertainty in onset of neutron spin-flip scattering. This clearly separates the onset of magnetic order 8-11 at T * from the onset T K of the Kerr rotation signal 27 and charge order 28 at lower temperature (Figure 3). In our measurements we observe an increase in energy absorption over a broad region near T K (Figure 2(c)), however we do not observe an accompanying thermodynamic signature there. Our observed evolution of the pseudogap phase boundary from underdoped to overdoped establishes the presence of a quantum critical point inside the superconducting dome, suggesting a quantum-critical origin for both the strange metallic behavior and the mechanism of superconducting pairing.

No abstract

The anomalous metallic state in the high-temperature superconducting cuprates is masked by superconductivity near a quantum critical point. Applying high magnetic fields to suppress superconductivity has enabled detailed studies of the normal state, yet the direct effect of strong magnetic fields on the metallic state is poorly understood. We report the high-field magnetoresistance of thin-film La Sr CuO cuprate in the vicinity of the critical doping, 0.161 ≤ ≤ 0.190. We find that the metallic state exposed by suppressing superconductivity is characterized by magnetoresistance that is linear in magnetic fields up to 80 tesla. The magnitude of the linear-in-field resistivity mirrors the magnitude and doping evolution of the well-known linear-in-temperature resistivity that has been associated with quantum criticality in high-temperature superconductors.

Specific heat of a material is a measure of heat necessary to raise the temperature of a given amount of material, typically a gram or a mol, by 1 Kelvin. Near absolute zero, this bulk thermodynamic quantity is a sensitive probe of the low energy excitations of a complex quantum

Establishing the appropriate theoretical framework for unconventional superconductivity in the iron-based materials requires correct understanding of both the electron correlation strength and the role of Fermi surfaces. This fundamental issue becomes especially relevant with the discovery of the iron chalcogenide superconductors. Here, we use angle-resolved photoemission spectroscopy to measure three representative iron chalcogenides, FeTe0.56Se0.44, monolayer FeSe grown on SrTiO3 and K0.76Fe1.72Se2. We show that these superconductors are all strongly correlated, with an orbital-selective strong renormalization in the dxy bands despite having drastically different Fermi surface topologies. Furthermore, raising temperature brings all three compounds from a metallic state to a phase where the dxy orbital loses all spectral weight while other orbitals remain itinerant. These observations establish that iron chalcogenides display universal orbital-selective strong correlations that are insensitive to the Fermi surface topology, and are close to an orbital-selective Mott phase, hence placing strong constraints for theoretical understanding of iron-based superconductors.

We report quantum oscillation measurements that enable the direct observation of the Fermi surface of the low temperature ground state of BaFe2As2. From these measurements we characterize the low energy excitations, revealing that the Fermi surface is reconstructed in the antiferromagnetic state, but leaving itinerant electrons in its wake. The present measurements are consistent with a conventional band folding picture of the antiferromagnetic ground state, placing important limits on the topology and size of the Fermi surface.PACS numbers: 74.25. Fy, 74.25.Ha, 72.80.Ng The nature of superconductivity in Fe-pnictide family of compounds has thus far eluded a universally accepted explanation. Part of the problem is understanding the fundamental quasiparticle dynamics of the parent compounds which show evidence for electron itineracy on the one hand [1,2,3,4], and local magnetism on the other [5,6,7,8]. Recent ARPES measurements have suggested a novel exchange mechanism driving the magnetism [5,9] due to an apparent band splitting at the transition temperature T SDW (135K in BaFe 2 As 2 ) while other ARPES measurements have suggested Fermi surface nesting [4,10] by mapping the shape of the observed Fermi pockets to an inferred nesting instability. In addition, while neutron data has suggested the complete suppression of magnetic order in F-doped CeFeAsO before the material becomes superconducting[6], muon spectroscopy has detected magnetic fluctuations inside the superconducting dome [11] in F-doped SmFeAsO. ARPES has also observed the persistence of nesting instabilities in the superconducting state of K-doped BaFe 2 As 2 [10]. These observations have left open such questions as to what the role of disorder and magnetism is in shaping the superconducting mechanism, whether the superconducitvity emerges from the normal state Fermi surface or the reconstructed state, or even what the microscopic nature of the magnetism is in the parent compounds [12]. Resolving these issues requires that the low energy quasiparticle excitations are revealed. This is especially true because knowledge of the itinerant nature of the low temperature ground state places significant constraints on the magnetism associated with the order. In the present paper we report quantum oscillation (QO) measurements consistent with a nesting mechanism that folds bands of the non-magnetic state in a conventional manner. As predicted by recent theoretical investigations, we find that the SDW instability does not fully gap the Fermi surface [13].In the measurements reported here on BaFe 2 As 2 we use two separate techniques, torque magnetometry and a radio frequency contactless conductivity technique using a tunnel diode oscillator (TDO), both of which have been used recently to observe oscillations in the closely related compounds LaFePO[1] and SrFe 2 As 2 .[2] We observe three small pockets comprising 1.7%, 0.7% and 0.3%, of the paramagnetic Brillouin zone (that associated with the tetragonal state) and produce band structure calculations of a re...

In this work, we study the AxFe2−ySe2 (A=K, Rb) superconductors using angle-resolved photoemission spectroscopy. In the low temperature state, we observe an orbital-dependent renormalization for the bands near the Fermi level in which the dxy bands are heavily renormliazed compared to the dxz/dyz bands. Upon increasing temperature to above 150K, the system evolves into a state in which the dxy bands have diminished spectral weight while the dxz/dyz bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature induced crossover from a metallic state at low temperature to an orbital-selective Mott phase (OSMP) at high temperatures. Furthermore, the fact that the superconducting state of AxFe2−ySe2 is near the boundary of such an OSMP constraints the system to have sufficiently strong on-site Coulomb interactions and Hund's coupling, and hence highlight the non-trivial role of electron correlation in this family of iron superconductors.