Field- and concentration-tuned scaling of a quantum phase transition in a magnetically doped semiconductor

Helgren, E.; Zeng, Li; Burch, Kenneth S.; Basov, D. N.; Hellman, F.

doi:10.1103/physrevb.73.155201

Cited by 12 publications

(7 citation statements)

References 22 publications

(36 reference statements)

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“…Transport measurements in a-Gd x Si 1−x films revealed both insulating and metallic properties for x Ͻ 0.13 and x Ͼ 0.14, respectively. 19 We chose this material because its resistivity ͑ϳ2.5 m⍀ cm at low T͒ is high compared to more standard metallic spin glasses, which makes it more suitable for transport noise experiments.…”

Section: Experimental Techniquesmentioning

confidence: 99%

Tracking spin-glass barriers versus field and temperature ina-Gd0.19Si0.81

et al. 2009

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Detailed signatures of the states of the low-temperature spin-glass phase in a-GdSi films were followed using resistance fluctuations. Two-state switches between different spin-glass configurations were unambiguously identified by the field dependences of the occupation ratios of the states. The temperature dependences of the rates and occupation ratios were tracked in order to determine thermodynamic parameters of the switchers and the temperature dependences of barriers. A reasonable distribution of energy and entropy differences between states was found. Most importantly, the kinetic barriers were found to show no sign of divergence with lowering temperature, contrary to expectations based on prior theoretical and experimental works.

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Section: Experimental Techniquesmentioning

confidence: 99%

Tracking spin-glass barriers versus field and temperature ina-Gd0.19Si0.81

et al. 2009

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“…22 Similar magnetoconductance has in fact also been observed in amorphous metal semiconductor alloys, such as a-GdSi. 23 From the earliest reports the transport behavior of conducting polymer samples that fall on the insulating side of the MIT have been modeled using variable range hopping (VRH), 24 though the fits of the data over which this VRH occurs persist often to room temperature, which is quite different than insulating samples of metal semiconductor alloys which only exhibit VRH behavior up to approximately 20K and certainly much higher in temperature than doped crystalline semiconductor systems where VRH is observed up to even lower temperatures, depending on how far the sample is tuned into the insulating phase.…”

Section: Introductionmentioning

confidence: 99%

Electrodynamics of the conducting polymer polyaniline on the insulating side of the metal-insulator transition

Helgren

Penney²,

Diefenbach

et al. 2017

Phys. Rev. B

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Conducting polymer samples of Polyaniline (PANI) exhibit a dramatic change in their conductivity as function of protonation level, analogous with the changes in the transport properties of semiconductors upon doping. In this work PANI samples were prepared by protonating with varying concentrations of hydrochloric acid. The complex terahertz frequency-dependent conductivity and the dc conductivity of these samples were measured and analyzed in the framework of the disorder-driven, metal-insulator quantum phase transition. The samples were determined to all fall on the insulating phase of this phase transition. The frequency dependence of both the real and imaginary terahertz conductivity are found to be consistent with theories that include electronic correlation effects.

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“…Previous studies on Gd-doped e-beam co-evaporated amorphous Si have shown strong interactions between the magnetic moments and carriers leading to very large negative MR (e.g. 10 5 at 1 K), anomalous magneto-optical properties and spin-glass freezing [ Refs 7,8,9,10]. The magnitude of this temperature-dependent negative MR below a characteristic temperature (T * ) was suggested to depend on electron screening related to the metallicity of the amorphous matrix [8].…”

Section: Introductionmentioning

confidence: 99%

Microstructure, magnetotransport, and magnetic properties of Gd-doped amorphous carbon

et al. 2007

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The magnetic rare earth element gadolinium (Gd) was doped into thin films of amorphous carbon (hydrogenated a-C:H, or hydrogen-free a-C) using magnetron co-sputtering. The Gd acted as a magnetic as well as an electrical dopant, resulting in an enormous negative magnetoresistance below a temperature (T ′ ). Hydrogen was introduced to control the amorphous carbon bonding structure.High-resolution electron microscopy, ion-beam analysis and Raman spectroscopy were used to characterize the influence of Gd doping on the a-Gd x C 1−x (:H y ) film morphology, composition, density and bonding. The films were largely amorphous and homogeneous up to x=22.0 at.%. As the Gd doping increased, the sp 2 -bonded carbon atoms evolved from carbon chains to 6-member graphitic rings. Incorporation of H opened up the graphitic rings and stabilized a sp 2 -rich carbonchain random network. The transport properties not only depended on Gd doping, but were also very sensitive to the sp 2 ordering. Magnetic properties, such as the spin-glass freezing temperature and susceptibility, scaled with the Gd concentration.Amorphous carbon (a-C) thin films are amorphous semiconductors with a tunable band gap. Unlike other group IV amorphous semiconductors, such as a-Si or a-Ge, which have a tetrahedral sp 3 -bonded random-network matrix, a typical a-C film has a mixture of sp 2 and sp 3 bonding. Changing the sp 2 /sp 3 ratio tunes the band gap of a-C between that of diamond (100% sp 3 , band gap E g = 5.4eV ) and graphite (100% sp 2 , semimetal with E g = 0). Studies of a-C, diamond-like materials and carbon nanotubes have long focused on the superior mechanical properties. However, recent developments in novel electronic and spintronic materials have drawn attention to the electrical, optical and magneto-electronic properties [1]. Room-temperature positive magnetoresistance (MR∼60%) was found in some magnetically-doped carbon thin films such as Co x C 1−x [2] and Ni x C 1−x [3], with granular magnetic particles in an amorphous carbon matrix. Room-temperature positive MR was also reported for non-magnetically-doped (B-doped) polycrystalline diamond thin films (MR∼100%) [4, 5] and undoped a-C/n-Si (MR∼12%) two-layer structures [6]. These results suggest potential technological applications of carbon-based thin films for electronics and spintronics applications.In this work, we have co-sputtered a-C and the magnetic rare earth element Gd (a-Gd x C 1−x (:H y )) with a wide range of x, and studied the magnetic and magneto-transport properties. Gd has a half-filled f electron shell and is trivalent in solids, which gives rise to a large local moment with J =S =7/2 and three valence electrons. Previous studies on Gd-doped e-beam co-evaporated amorphous Si have shown strong interactions between the magnetic moments and carriers leading to very large negative MR (e.g. 10 5 at 1 K), anomalous magneto-optical properties and spin-glass freezing [Refs 7,8,9,10]. The magnitude of this temperature-dependent negative MR below a characteristic temperature (T * ) w...

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Field- and concentration-tuned scaling of a quantum phase transition in a magnetically doped semiconductor

Cited by 12 publications

References 22 publications

Tracking spin-glass barriers versus field and temperature ina-Gd0.19Si0.81

Tracking spin-glass barriers versus field and temperature ina-Gd0.19Si0.81

Electrodynamics of the conducting polymer polyaniline on the insulating side of the metal-insulator transition

Microstructure, magnetotransport, and magnetic properties of Gd-doped amorphous carbon

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