Optical Hall conductivity σxy(ω) is measured from the Faraday rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz frequency regime. The Faraday rotation angle (∼ fine structure constant ∼ mrad) is found to significantly deviate from the Drude-like behavior to exhibit a plateau-like structure around the Landau-level filling ν = 2. The result, which fits with the behavior expected from the carrier localization effect in the ac regime, indicates that the plateau structure, although not quantized, still exists in the terahertz regime.The quantum Hall effect (QHE), a highlight in the twodimensional electron gas (2DEG) system in strong magnetic fields [1][2][3][4], still harbors, despite its long history, a wealth of important physics. While static properties of the integer QHE have been well understood, we are still some way from a full understanding of dynamical responses in the QHE in the ac or even optical regime. In the static case, the states localized due to disorder with the localization length smaller than the sample size or the inelastic scattering length are crucial in realizing the quantum plateaus for the dc Hall current in a dc electric field [5][6][7][8][9][10][11]. On the other hand, the conventional wisdom for the dynamical response would be that an ac field will delocalize wave functions to make QHE disappear.For relatively low frequencies, the breakdown of QHE in ac fields has a long history of investigation [12]. One issue was whether the delocalization occurs for lowfrequencies (∼ 10 MHz), but the results were not conclusive. Subsequently, experimental study was extended to the microwave regime in the 1980s, where the delocalization as seen in the Hall conductivity σ xy was shown to be absent in the microwave (i.e., gigahertz) regime [13], while the gigahertz responses of the longitudinal conductivity σ xx [9,14,15] were explained with the scaling theory of localization [16]. Thus, a fundamental problem remains as to whether and how QHE is affected in the much higher, terahertz (closer to the optical) frequency regime (ω ∼ 10 12 Hz ∼ 10 −2 eV/h). This is an essential question, since the frequency is exactly the energy scale of interest (i.e., the cyclotron energyhω c ∼ 10 −2 eV for a magnetic field ∼ 10 T, which is the spacing between Landau levels, a prerequisite for QHE).Theoretically, the accurate quantization in QHE is firmly established as a topological (Chern) number [17] in the static case. However, such a picture may not be extended to the ac regime where the topological 'protection' no longer exists. Recently, Morimoto et al. [18] have theoretically examined the ac response of the disordered QHE systems based on the exact diagonalization method, and showed that a plateau-like behavior still exists in σ xy even in the terahertz energy range. This has motivated us to experimentally examine QHE by going beyond the microwave regime, which has so far remained a challenge. An essential experimental ingredient that enables the measurement is a recent development in t...

Two-dimensional (2D) superconductivity was studied by magnetotransport measurements on single-atomic-layer Pb films on a cleaved GaAs(110) surface. The superconducting transition temperature shows only a weak dependence on the parallel magnetic field up to 14 T, which is higher than the Pauli paramagnetic limit. Furthermore, the perpendicular-magnetic-field dependence of the sheet resistance is almost independent of the presence of the parallel field component. These results are explained in terms of an inhomogeneous superconducting state predicted for 2D metals with a large Rashba spin splitting.PACS numbers: 73.20.At,Superconductivity in ultrathin films has been studied for a long time. In Ref.[1], superconductivity was observed even for a few-monolayer thickness in quenchcondensed films of Bi and Pb deposited on a glazed alumina substrate coated with amorphous Ge. Very recently, it has been revealed that superconductivity can occur in single atomic layers of Pb and In grown epitaxially on a Si(111) substrate [2,3]. A single-atomic-layer metal film on an insulating substrate is an interesting system for studies of superconductivity, not only because it is a complete two-dimensional (2D) system but also because of the broken spatial inversion symmetry. The asymmetry of the confining potential in the direction perpendicular to the 2D plane, combined with atomic spin-orbit coupling, is expected to cause the Rashba effect, which lifts the spin degeneracy of the 2D electronic states [4,5]. Actually, angle-resolved photoelectron spectroscopy measurements showed a large Rashba spin splitting of the order of 100 meV on the surfaces of heavy elements, such as Au [6], W [7], and Bi [8], and those of lighter elements, such as Si and Ge, covered with a monolayer of heavy elements, such as Bi [9][10][11].In this Letter, we report magnetotransport measurements on superconducting monolayer Pb films produced by quench condensation onto a cleaved GaAs(110) surface. Here, we focus on the effect of the magnetic field applied parallel to the surface. While the perpendicular component H ⊥ of the magnetic field strongly affects the orbital motion of electrons in the 2D plane, the parallel component H is expected to couple only to the spin degree of freedom. We show that the reduction of the superconducting transition temperature T c is very small even in strong parallel magnetic fields (H ⊥ = 0), which are much larger than the Pauli paramagnetic limit. Furthermore, the H ⊥ dependence of the sheet resistance at low temperature is found to be almost independent of the presence of H . These results are explained by assuming an inhomogeneous superconducting state predicted for Rashba spin-split 2D systems.In order to measure the sheet resistance R sq of ultrathin films, we apply the experimental procedure developed for studies on adsorbate-induced surface inversion layers on InAs [13][14][15] and InSb [16]. In this work, we used a nondoped insulating GaAs single-crystal substrate so as not to create conduction channels in the substrate...

Direct demagnetization has been made for two-dimensional solid 3He in both the paramagnetic and the antiferromagnetic phases. The lowest temperature is about 10 microK, judging from the observed magnetization for the paramagnetic solid 3He. The magnetization of the antiferromagnetic solid 3He shows a gradual increase to about 10 microK for the 4/7 phase adsorbed on both one layer of 4He and two layers of HD preplated graphite. This strongly suggests that the triangular antiferromagnet with the higher order multiple exchange has a quantum spin liquid ground state with nearly zero or extremely small spin gap less than 10 microK.

Strain dependence of hole effective mass (m∗) in the strained Ge channel was systematically studied, and monotonic m∗ reduction by more than 20% was clearly observed when the strain increased from 0.8% up to 2.8%. The scattering mechanism, which strongly depended on the modulation-doping structure as well as strains, was also investigated based on the Dingle ratio evaluation, and the interface roughness scattering was found to be effectively suppressed by adopting the inverted structure even for the largely strained channels.

Magnetotransport measurements have been performed on two-dimensional electron gases formed at InAs(110) surfaces covered with a submonolayer of Fe. Hysteresis in the magnetoresistance, a difference in remanent magnetoresistance between zero-field-cooling procedures and field-cooling procedures, and logarithmic time-dependent relaxation after magnetic field sweep are clearly observed at 1.7 K for a coverage of 0.42 monolayer. These features are associated with spin-glass ordering in the Fe film.PACS numbers: 75.70. Ak, 73.25.+i, 75.50.Lk Spin glasses are magnetic systems with randomly competing interactions. They have attracted great interest during the last few decades [1,2,3]. Spin-glass models and related methods have also been useful in other areas of science such as simulation of protein folding [4] and optimization problems in computer science [5]. Most of the attempts to understand spin glasses have been concerned with the behavior in three dimensions. It is generally believed that, in two dimensions, the spin-glass ordering does not occur at nonzero temperature. The lower critical dimension of spin-glass ordering has been shown to be d l > 2 for Heisenberg spins [6] and XY spins [7], and Ising spins with Gaussian distribution of disorder [8,9]. Although the situation for the Ising model with bimodal (±J) disorder was controversial [10,11], recent theoretical investigations do not support the existence of the spin-glass phase for T > 0 [12,13,14]. On the other hand, numerical calculations have demonstrated that the spin-glass-like ordering temperature can be nonzero for a two-dimensional (2D) Ising model with random nearestneighbor interactions and ferromagnetic second-neighbor interactions [15,16]. Experimentally, spin-glass behavior was found in thin films [17,18,19,20] and layered compounds [21,22,23]. However, no observation has been reported for a single layer system with strict two dimensionality.Submonolayer films of magnetic materials adsorbed on nonmagnetic substrates are promising candidates for 2D spin-glass systems. A random distribution of adsorbates can be obtained by deposition at low substrate temperatures, where surface diffusion is minimal and island growth is limited. In the case that the sign of the interaction depends on the relative position of adatoms, a competition between ferromagnetic and antiferromagnetic interactions is expected. As the substrate, narrow band-gap III-V semiconductors have a remarkable property. It is well known that a two-dimensional electron gas (2DEG) can be easily formed on the surface of InAs and InSb. Photoelectron spectroscopy measurements have shown that the position of the Fermi level lies above the conduction-band minimum at cleaved (110) surfaces with various kinds of adsorbed materials [24,25]. Recently the present authors have performed magnetotransport measurements on inversion layers formed on cleaved surfaces of p-type InAs [26,27] and InSb [28] covered with submonolayers of Ag or alkali metals. The observed coverage dependence of the Hall mobi...