Optimally doped silver selenide and silver telluride exhibit linear positive magnetoresistance over decades in magnetic field and on a scale comparable to the colossal magnetoresistance compounds. We use hydrostatic pressure to smoothly alter the band structure of Ag-rich and Ag-deficient samples of semiconducting Ag 26d Te of fixed stoichiometry and disorder. We find that the magnetoresistance spikes and the linear field dependence emerges when the bands cross and the Hall coefficient changes sign. DOI: 10.1103/PhysRevLett.88.066602 PACS numbers: 72.20.My, 72.15.Gd, 72.80.Jc The magnetoresistive response of a material can open a window into the dispersion and dynamics of the charge carriers, and in opportune cases can be exploited for technological use. The magnetoresistance at small fields is usually quadratic because of the vector nature of the magnetic field, and is expected to saturate when the applied field becomes large [1]. Under special circumstances, the resistivity can grow linearly with applied field [2]. High-field linear magnetoresistance can be found in polycrystalline materials with open orbits in the Fermi surface [3], in inhomogeneous materials where the tensor components of the resistivity can be mixed [4], and in the extreme quantum limit where one Landau level dominates (e.g., bismuth) [5 -7].A few years ago, positive linear magnetoresistance was observed from magnetic fields of mT to T in the silver chalcogenides, Ag 2 Se and Ag 2 Te [8]. Stoichiometric material remains indifferent to the application of a magnetic field [9], but small amounts of excess silver or excess Se/Te lead to changes in the resistivity of many hundreds of percent in fields of a few T. This large magnetic response is comparable in absolute magnitude to that observed in manganese perovskites, the so-called colossal magnetoresistance materials, but occurs here in intrinsically nonmagnetic materials that can be fabricated both in bulk and as thin films [8,[10][11][12][13].The unusually large range of linearity observed in some samples of Ag 21d Se and Ag 21d Te, the failure of the magnetoresistance to saturate even when the product of the cyclotron frequency and the scattering time, v c t, greatly exceeds one, and the robust absolute scale of the response combine to make the silver chalcogenides especially inviting materials to illuminate the mechanisms of linear magnetoresistance. In particular, Abrikosov has proposed that an essential ingredient for "quantum linear magnetoresistance" in both the small and large field limits is a semiconducting gap that approaches zero, with an energy dispersion that becomes linear in momentum [2,14].In this Letter, we use hydrostatic pressure to tune the band gap of both p-type and n-type samples of silver telluride of fixed stoichiometry and disorder. Under pressure, hole-dominated transport can be transformed into electron-dominated transport, and barely metallic n-type samples can be converted into good metals. We find for p-type material that both the linear magnetoresistance a...

High fidelity pressure measurements in the zero temperature limit provide a unique opportunity to study the behavior of strongly interacting, itinerant electrons with coupled spin and charge degrees of freedom. Approaching the exactitude that has become the hallmark of experiments on classical critical phenomena, we characterize the quantum critical behavior of the model, elemental antiferromagnet chromium, lightly doped with vanadium. We resolve the sharp doubling of the Hall coefficient at the quantum critical point and trace the dominating effects of quantum fluctuations up to surprisingly high temperatures.PACS numbers: 75.40. Cx, 75.30.Fv, 71.27.+a, 42.50.Lc Phase transitions at the absolute zero of temperature are a result of Heisenberg's uncertainty principle rather than of a thermal exploration of states. Their ubiquity in materials of large technological interest, including transition metal oxides and sulfides, metal hydrides, superconducting cuprates, and colossal magnetoresistance manganites, combined with the intellectual challenge presented by many strongly interacting quantum degrees of freedom, places quantum phase transitions at the core of modern condensed matter physics. Quantum fluctuations inextricably intertwine the static and dynamical response of the material changing state, introducing new critical exponents, new scaling laws, and new relationships between the spin and charge degrees of freedom [1,2,3].Varying the quantum fluctuations required for zerotemperature phase transitions is more difficult than changing temperature, with the result that the understanding of quantum phase transitions is far less detailed than that of their classical analogues. Typically, fluctuations are varied by scanning the composition of an alloy, such as the doped cuprates that host hightemperature superconductivity. The result is that a new sample, each with unique disorder, must be fabricated for every zero-point fluctuation rate sampled. Tuning the transition with an external magnetic field, a quantity that is easily and precisely regulated, is a cleaner technique and provides correspondingly greater detail for both insulators [4] and metals [5]. However, magnetic fields also break time-reversal symmetry, which is particularly significant for quantum phase transitions because the dynamics responsible for zero-point fluctuations are altered profoundly. It is important, therefore, to examine a quantum phase transition for a simple material with high precision without applying a symmetry-breaking field. In response to this challenge, we have performed a high-resolution hydrostatic pressure study of a model quantum phase transition: elemental chromium diluted with its neighbor vanadium, small amounts of which can smoothly suppress Cr's spin-density-wave transition to T = 0. The Cr-V single crystals permit tuning with high fidelity and we are able to characterize precisely the signatures of vanishing magnetic order in a system sufficiently simple to promise theoretical tractability.Cr is the archetypical met...

We report the structural heterogeneity of recrystallized linear low-density polyethylene (LLDPE) films (25, 50, and 100 nm in thickness) in the direction normal to the surface, based on in situ grazing incidence small-angle X-ray scattering (GISAXS) and X-ray diffraction (GID) measurements. The GID results have clarified the presence of the edge-on lamellae at the surfaces and in the interior of the LLDPE films prepared on Si substrates as thin as 25 nm in thickness. However, the degree of the crystallinity for the 25 nm thick film was almost half of those for the 50 and 100 nm thick films, while the melting temperature (T m ) for all the films remained unchanged relative to the bulk (T m = 117 °C). Moreover, the GISAXS results for the 25 nm thick film indicate the structural heterogeneity in the direction normal to the surface: (i) At the polymer/air interface, the presence of the disordered edge-on lamellae which lack well-defined long periods even at T ≪ T m ; (ii) At the polymer/substrate interface, the persistence of a substrate-bound edge-on lamellar layer even at T ≫ T m ; (iii) In between the two interfacial layers, the existence of the well-ordered edge-on lamellae with the long periods. These heterogeneous structures can be explained as a consequence of the nucleation initiated at the topmost surface of the substrate-bound lamellar layer.

We have probed the temperature and magnetic-field dependence of the thermopower and resistance of a p-type silver chalcogenide, Ag2−δTe. The application of a magnetic field causes not only a large magnetoresistance but also a giant magnetothermopower effect. The maximum change of thermopower is as high as 470 μV/K in a 7 T magnetic field. Both the magnetoresistance and the magnetothermopower show a pronounced peak and nearly linear behavior near the sign change of the thermopower. Bandcrossing and quantum confinement due to disorder appear to play key roles in the heightened response to field.

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