Results 1 - 10 of 525
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[en] In this study, quantum hall event in multiple GaAs/Ga1-xAlAs quantum hall and Shubnikov-De Haas oscillations have been investigated
[tr]Kuyu genislikleri 50-145 A araliginda olan Hallcubugu geometrisinde hazirlanmis GaAs/Ga(1-x)AI,As coklu kuantum kuyulari (MQW) orneklerinde akim-gerilim (I-V) karakteristikleri, Kuantum Hall Olayi (QHE) ve Shubnikov-de Haas osilasyonlari olcumleri yapildi. Enine ve boyunu magnetorezistans olcumleri, (i) orgu sicakligi sabit tutularak (To=1,5K) elektrik alanin ( DC akim 1 mikroA ile 5miliA arasinda suruldu), ve (u) elektronlari isitmayacak kadar dusuk elektrik alani (50 mikroA akim) uygulanarak olcu sicakliginin (olco sicakligi 1,5-4,2K arasmda) fonksiyonu olarak yapddi. Magnetik alan (0-2,3T) orneklerin duzlemine dik olarak uygulandi. Verileri otomatik olarak toplamak ve analiz etmek icin bir yazilim gelictirildi. Enine magnetorezistans verilerinde gozlenen kuantize Hall platolarmdan gecirilen dogru yardimiyla, Hall tasiyici yogunlugu belirlendi ve h/e2 niceliginin degeri hesaplandi; h/e2 icin bulunan deger literaturde verilenler ile uyumludur. Boyuna magnetorezistans verilerinin magnetik alana gore ikinci turevinin negatifi hesaplanarak SdN osilasyonlari elde edildi. Bu yontemle elde edilen SdH osilasyonlari ile mevcut kuramsal ifadeler arasinda cok iyi uyum oldugu gozlendi. SdH osilasyonlari genliginin sicaklik ve magnetik alanla degisiminden iki boyutlu (2D) elektronlarin etkin kutlesi ve Dingle sicakligi belirlendi. Osilasyonlarin periyodu kullanilarak Fermi enerjisi ve 2D elektron yogunlugu hesaplandi. Kuramsal modellerin ongorduyu gibi, 2D elektronlarin etkin kutlesi kuantum kuyu genisligi azaldikca artmaktadir. SdH osilasyonlarmin genliyinin olcu sicakliyi ve uygulunan elektrik alana bagli degisimleri karsilastirilarak, elektron sicakligi hesaplandi
[en] κ-(BEDT-TTF)2I3 is an extreme case of an electronically two-dimensional (2D) organic metal with a superconducting transition at around 4 K. In magnetic fields up to 27 T the field, angular and the temperature dependence of de Haas-van Alphen (dHvA) and Shubnikov-de Haas (SdH) oscillation amplitudes were investigated. From the temperature dependence of the oscillation amplitudes the cyclotron effective mass m* was investigated using Lifshitz-Kosevich formula (LK) and the surprising result is, that m* appears to be magnetic field dependent as soon as the magnetic field is applied exactly perpendicular to the conducting planes so that the cyclotron orbit lies within those planes (i.e. Θ = 0 ), but field independent at Θ ≠ 0 . It is shown that the LK formula can satisfactorily be used for the interpretation of all data obtained at Θ ≠ 0 but not in the case where Θ = 0 . An explanation of the anomalous data at Θ = 0 can furthermore neither be given satisfactorily by the descriptions of magneto-quantum oscillations in 2D electronic systems nor by the occurrence of magnetic interaction effects (MI). Tentatively, the observed smaller oscillation amplitudes in κ-(BEDT-TTF)2I3 at Θ = 0 above 12 T and below 1 K compared to the standard LK theory will be discussed in terms of a loss in the number of fermions by the occurrence of quasiparticles with fractional statistics. (orig.)
[en] The transverse magnetoresistance of (TMTSF)2PF6 has been investigated in magnetic fields up to 32 T, at low temperatures down to 4.2 K. Shubnikov-de-Haas oscillations are directly observed on the magnetoresistance in fields higher than 18 T, but there is no evidence of a threshold field since the disappearance of oscillations can be explained by the normal field-dependent damping. However these oscillations vanish very quickly between 5 and 4,2 K. This unusual behaviour could be related to a phase-transition. Finally an exploration of the fundamental field versus theta is given (theta = angle between B and c*), showing the warping of the ''Fermi tubes'' along the c* axis
[en] The quantum magnetic oscillations of electrical (Shubnikov-de Haas effect) and thermal conductivities are studied for graphene which represents a distinctive example of planar systems with a linear, Dirac-like spectrum of quasiparticle excitations. We show that if utmost care is taken to separate electron and phonon contributions in the thermal conductivity, the oscillations of electron thermal conductivity κ(B) and Lorenz number, L(B) would be observable in the low-field (less than a few teslas) regime
[en] Motivated by experiments, we study the sign of the Coulomb drag voltage in a double layer system in a strong magnetic field. We show that the commonly used Fermi golden rule approach implicitly assumes a linear dependence of intralayer conductivity on density, and is thus inadequate in strong magnetic fields. Going beyond this approach, we show that the drag voltage commonly changes sign with density difference between the layers. We find that, in the quantum Hall regime, the Hall and longitudinal drag resistivities may be comparable. Our results are also relevant for pumping and acoustoelectric experiments
[en] We present measurements of the Shubnikov-de Haas effect on LuRh2Si2 in conjunction with electronic band structure calculations. These electronic structure investigations can help understand the peculiar temperature dependence of the Hall effect. In addition, LuRh2Si2 represents the non-magnetic reference compound to YbRh2Si2, a prototypical heavy-fermion system featuring an unconventional quantum critical point. In YbRh2Si2, an orbitally selective Mott transition occurs from a large Fermi surface configuration including f electrons to a small configuration with conduction electrons only. LuRh2Si2 resembles the small Fermi surface configuration of YbRh2Si2. We detect oscillations of the resistivity with a large number of frequencies and discuss the angular dependency. We compare the detected orbits with those predicted for the three sheets by band structure calculations and discuss implications for the interpretation of quantum oscillation measurements on YbRh2Si2.
[en] κ-(BEDT-TTF)2Cu[N(CN)2]Cl belongs to the family of charge transfer salts and shows many interesting phenomena at low temperatures. Especially the existence of a superconducting phase at temperatures up to 12.8 K bordering a magnetically ordered Mott-insulating state are of big interest in current research. This metal-insulator transition (MIT) can be tuned by applying hydrostatic pressures ≤ 0.4 kbar. Together with its highly anisotropic conductivity, this makes the compound a model example of a quasi-two-dimensional correlated electronic system. Here we report on a magnetotransport study of κ-(BEDT-TTF)2Cu[N(CN)2]Cl under pressure, in the vicinity of the MIT. A particular focus is put on tracing the behavior of Shubnikov - de Haas oscillations. While the oscillation frequencies only weakly change with pressure, the effective cyclotron masses exhibit a significant increase, accelerating towards the MIT. We compare the experimental data with those obtained on similar κ-type organic conductors and with theoretical predictions.
[en] Shubnikov-de Haas oscillations have been observed in Bi thin films fabricated by electrodeposition. The observed dominant oscillation periods of 0.17, 0.08, and 0.077 T-1, with the magnetic field along the trigonal, binary, and bisectrix axes of Bi, respectively, are in good agreement with those in bulk Bi single crystals
[en] Shubnikov-de Haas oscillations in NbSe3 and TaSe3 have been observed in the field range 0-170 kG using both ac and dc techniques. The Fermi-surface sections have been tracked over a substantial angular range and the observed anisotropies are consistent with elongated ellipsoids. In NbSe3 a single section with a minimum frequency of 0.28 MG dominates the oscillatory behavior and develops a strong harmonic content at high fields. Fits of the data to expressions including the spin splitting of the Landau levels gives a minimum g factor of 3.2 and a Dingle temperature of T/sub D/ = 2.3 K. The temperature dependence of the amplitude gives an effective mass of 0.24 for this section. The Hall effect has also been measured in NbSe3 and shows a sign change from positive to negative as the temperature is decreased below the 59-K transition in NbSe3. No significant change is observed near the 145-K transition. In TaSe3 at least 20 frequencies in the range 0.7-22 MG have been observed and these exhibit a distribution of fairly strong amplitudes with no single frequency dominating. The crystals of NbSe3 and TaSe3 are of comparable quality and resistance ratio so the results indicate that the Fermi surfaces have quite different parameters in the two metals