Band-Gap Tuning and Linear Magnetoresistance in the Silver Chalcogenides

Abstract

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...

Model for a macroscopically disordered conductor with an exactly linear high-field magnetoresistance

“…The present results agree qualitatively with the experiments of Lee et al,6 which also show that the TMR peaks at pressures where the Hall coefficient changes sign. But this agreement should be viewed cautiously.…”

“…7 The second proposed mechanism 8 is that this nonsaturating TMR arises from macroscopic sample inhomogeneities. Such inhomogeneities could produce large spatial fluctuations in the conductivity tensor and hence a large TMR, especially at large H. This explanation seems plausible because the chalcogenides probably have a granular microstructure, 6 and hence a spatially varying conductivity.…”

“…4,5 In these materials, over the temperature range from 4 to 300 K, the resistivity increases approximately linearly with H up to fields, applied perpendicular to the direction of current flow, as high as 60 T. Moreover, the TMR is especially large and most clearly linear at pressures where the Hall resistivity changes sign. 6 Because of this linearity, these materials may be useful as magnetic field sensors even at megagauss fields.But beyond the possible applications, the origin of the effect remains mysterious. According to conventional theories, such narrow gap semiconductors should have a saturating TMR.…”

Epitaxial growth of Bi2Se3 topological insulator thin films on Si (111)

“…5(b). This has been identified as the quantum linear Hall effect, 28,29 where the large linearity is predominantly caused by coupling of the spin to the field. The thinner films also have a trace of linearity at 30 can be observed at low temperature and high magnetic field.…”

Nonsaturating magnetoresistance of inhomogeneous conductors: Comparison of experiment and simulation

“…They exhibit negligible physical magnetoresistance, 11 as predicted from conventional theories. By contrast, minute amounts of excess Ag or Te/ Se-at levels as small as 1 part in 10 000-lead to a huge and linear magnetoresistance over a broad temperature range, with no sign of saturation up to 60 T. [12][13][14][15][16][17] In particular, the unusual linearity extends deep into the low-field regime with H Ӷ 1/ , where is the typical mobility of the material. Such behavior shows no resemblance to conventional semiconductors, where the magnetoresistance grows quadratically with field and reaches saturation at fields typically of order 1 T. Therefore, it has been argued that the observed magnetoresistance must be caused by the inhomogeneous distribution of excess or deficient silver ions.…”