Resistivity of two-dimensional systems in a magnetic field at the filling factor ν = 1/2

Experimental data on the diagonal resistivity ρ _xx of GaAs/AlGaAs heterostructures in a magnetic field at the filling factor ν = 1/2 have been compared with the existing theoretical predictions [B. I. Halperin et al., Phys. Rev. B 47, 7312 (1993) and F. Evers et al., Phys. Rev. B 60, 8951 (1999)]. The experimental results have been found to follow the relation ρ _xx (1/2) ∝ n ^?2 d ^?1.64, which disagrees with the predictions.     

Numerical calculation of the neutral fermion gap at the ν=5/2 fractional quantum Hall state

We present the first numerical computation of the neutral fermion gap, Δ(F), in the ν=5/2 quantum Hall state, which is analogous to the energy gap for a Bogoliubov-de Gennes quasiparticle in a superconductor. We find Δ(F)≈0.027e(2)/ε?(0), comparable to the charge gap. We also deduce an effective Fermi velocity v(F) for neutral fermions from the low-energy spectra for odd numbers of electrons, and thereby obtain a correlation length ξ(F)=v(F)/Δ(F)≈1.3?(0). We comment on implications for experiments, topological quantum information processing, and electronic mechanisms of superconductivity.     

Quantized ν = 5/2 state in a two-subband quantum hall system

The evolution of the fractional quantum Hall state at filling 5/2 is studied in density tunable two-dimensional electron systems formed in wide wells in which it is possible to induce a transition from single- to two-subband occupancy. In 80 and 60 nm wells, the quantum Hall state at 5/2 filling of the lowest subband is observed even when the second subband is occupied. In a 50 nm well, the 5/2 state vanishes upon second subband population. We attribute this distinct behavior to the width dependence of the capacitive energy for intersubband charge transfer and of the overlap of the subband probability densities.     

Observation of acoustic edge magnetoplasmons near the filling factor ν = 1

Investigation of acoustic edge magnetoplasma excitations in micrometer two-dimensional electron disks has been carried out. It has been shown that additional acoustic edge magnetoplasma modes associated with the existence of incompressible strips in the two-dimensional electron system caused by Zeeman spin splitting can appear in the system at temperatures below the Zeeman energy. The magnetic dispersion of the first “spin” branch of acoustic edge magnetoplasmons has been studied. It has been shown that this mode vanishes as the filling factor ν = 1 is approached. The dependence of the relative amplitude of acoustic edge magnetoplasmons on the filling factor has also been investigated.     

Anomalous charge tunneling in fractional quantum Hall edge states at a filling factor ν=5/2

We explain effective charge anomalies recently observed for fractional quantum Hall edge states at ν=5/2 [M. Dolev, Y. Gross, Y.?C. Chung, M. Heiblum, V. Umansky, and D. Mahalu, Phys. Rev. B 81, 161303(R) (2010)]. The experimental data of differential conductance and excess noise are fitted, using the anti-Pfaffian model, by properly taking into account renormalizations of the Luttinger parameters induced by the coupling of the system with an intrinsic 1/f noise. We demonstrate that a peculiar agglomerate excitation with charge e/2, double the expected e/4 charge, dominates the transport properties at low energies.     

Tilt-induced anisotropic to isotropic phase transition at ν = 5/2

A modest in-plane magnetic field B(∥) is sufficient to destroy the fractional quantized Hall states at ν = 5/2 and 7/2 and replace them with anisotropic compressible phases. Remarkably, we find that at larger B(∥) these anisotropic phases can themselves be replaced by isotropic compressible phases reminiscent of the composite fermion fluid at ν = 1/2. We present strong evidence that this transition is a consequence of the mixing of Landau levels from different electric subbands. We also report surprising dependences of the energy gaps at ν = 5/2 and 7/3 on the width of the confinement potential.     

Evolution of 4f heavy fermion state from 5f heavy fermion state in U1−xCexPd2Al3 system

We report the effect of Ce substitution for U in the heavy-electron antiferromagnetic superconductor UPd₂Al₃. CePd₂Al₃ is an established non-superconducting heavy-electron system which undergoes antiferromagnetic ordering below 2.8 K. Thus studies on U_1−xCe_xPd₂Al₃ system provide a unique opportunity to observe the evolution from a 4f heavy-electron state to a 5f heavy-electron state. We have measured the resistivity and magnetic susceptibility from 1.5 to 300 K and the heat capacity from 2 to 20 K for several U_1−xCe_xPd₂Al₃ samples. Our studies show that the antiferromagnetic (AF) ordering temperature (T_N) of U_1−xCe_xPd₂Al₃ does not decrease monotonically from T_N = 14 K for UPd₂Al₃ to T_N = 2.8 K for CePd₂Al₃ but rather shows a local maximum of 5 K near x = 0.4.     

Atomic Fermi gas in the trimerized kagomé lattice at 2/3 filling

We study low temperature properties of a spinless interacting Fermi gas in the trimerized kagomé lattice. The case of two fermions per trimer is described by a quantum spin 1/2 model on the triangular lattice with couplings depending on the bond directions. Using exact diagonalizations we show that the system exhibits nonstandard properties of a quantum spin-liquid crystal, combining a planar antiferromagnetic order with an exceptionally large number of low-energy excitations.     

Ground state energy calculations of the ν=1/2 and the ν=5/2 FQHE system

We reconsider energy calculations of the spin polarized ν = 1/2 Chern-Simons theory. We show that one has to be careful in the definition of the Chern-Simons path integral in order to avoid an IR divergent magnetic ground state energy in RPA as in [J. Dietel et al, Eur. Phys. J. B 5, 439 (1998)]. We correct the path integral and get a well behaved magnetic energy by considering the energy of the maximal divergent graphs as well as the Hartree-Fock graphs. Furthermore, we consider the ν = 1/2 and the ν = 5/2 system with spin degrees of freedom. In doing this we formulate a Chern-Simons theory of the ν = 5/2 system by transforming the interaction operator to the next lower Landau level. We calculate the Coulomb energy of the spin polarized as well as the spin unpolarized ν = 1/2 and the ν = 5/2 system as a function of the interaction strength in RPA. These energies are in good agreement with numerical simulations of interacting electrons in the first as well as in the second Landau level. Furthermore, we calculate the compressibility, the effective mass and the excitations of the spin polarized ν = 2 + 1/ systems where is an even number. Received 13 June 2000     

Competing topological orders in the ν=12/5 quantum Hall state

We provide numerical evidence that a p(x)-ip(y) paired Bonderson-Slingerland (BS) non-Abelian hierarchy state is a strong candidate for the observed ν=12/5 quantum Hall plateau. We confirm the existence of a gapped incompressible ν=12/5 quantum Hall state with shift S=2 on the sphere, matching that of the BS state. The exact ground state of the Coulomb interaction at S=2 is shown to have a large overlap with the BS trial wave function. Larger overlaps are obtained with BS-type wave functions that are hierarchical descendants of general p(x)-ip(y) weakly paired states at ν=5/2. We perform a finite-size scaling analysis of the ground-state energies for ν=12/5 states at shifts corresponding to the BS (S=2) and 3-clustered Read-Rezayi (S=-2) universality classes. This analysis reveals very tight competition between these two non-Abelian topological orders.     

NMR probing of the spin polarization of the ν=5/2 quantum Hall state

Resistively detected nuclear magnetic resonance is used to measure the Knight shift of the 75As nuclei and determine the electron spin polarization of the fractional quantum Hall states of the second Landau level. We show that the 5/2 state is fully polarized within experimental error, thus confirming a fundamental assumption of the Moore-Read theory. We measure the electron heating under radio frequency excitation and show that we are able to detect NMR at electron temperatures down to 30 mK.     

Breaking of particle-hole symmetry by Landau level mixing in the ν=5/2 quantized Hall state

We perform numerical studies to determine if the fractional quantum Hall state observed at a filling factor of ν=5/2 is the Moore-Read wave function or its particle-hole conjugate, the so-called anti-Pfaffian. Using a truncated Hilbert space approach we find that, for realistic interactions, including Landau-level mixing, the ground state remains fully polarized and the anti-Pfaffian is strongly favored.     

High-resolution rovibrational analysis of vibrational states of A2 symmetry of the dideuterated methane CH2D2: the levels ν 5 and ν 7 + ν 9

The infrared spectrum of the CH₂D₂ molecule has been measured in the region 900–1500?cm^?1 on a Bomem DA002 Fourier transform spectrometer with a resolution of 0.0024?cm^?1 (FWHM, unapodized). Transitions belonging to the hot bands ν ₇?+?ν₉?ν ₇, ν₇?+?ν₉? ν ₉, ν₅?+?ν₇?ν₅, and ν₅?+?ν₉?ν₅ were extracted from the recorded spectra to determine the rovibrational energies of the A₂ symmetry vibrational states (v ₇?=?v ₉?=?1) at 2329.698?cm^?1 and (v ₅ ?=?1) at 1331.409?cm^?1. Vibrational energies as well as rotational and centrifugal distortion parameters of the (v ₇?=?v ₉=1) and (v ₅?=?1) states were determined that reproduce the experimental rovibrational energy levels of the (v ₇?=?v ₉?=?1) and (v ₅?=?1) vibrational states with a d _rms deviation of 0.0017 and 0.0006?cm^?1, respectively. The results are discussed in relation to the equilibrium structure of methane, which is redetermined here from the experimental data, and in relation to its potential hypersurface and anharmonic vibrational dynamics.     

Measurement of the electric form factor of the neutron at Q2 = 0.3-0.8 (GeV/ c) 2 ⋆

The electric form factor of the neutron, G_E,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(¯e, e¯n)p reaction. Three data points have been extracted at squared four-momentum transfers Q ² = 0.3, 0.6 and 0.8 (GeV/c)². Corrections for nuclear binding effects have been applied.This revised version was published online in March 2005. In the previous version, the email address of one author was inadvertently assigned to multiple authors.     

Anomalous robustness of the ν=5/2 fractional quantum Hall state near a sharp phase boundary

We report magnetotransport measurements in wide GaAs quantum wells with a tunable density to probe the stability of the fractional quantum Hall effect at a filling factor of ν=5/2 in the vicinity of the crossing between Landau levels (LLs) belonging to the different (symmetric and antisymmetric) electric subbands. When the Fermi energy (E(F)) lies in the excited-state LL of the symmetric subband, the 5/2 quantum Hall state is surprisingly stable and gets even stronger near this crossing, and then suddenly disappears and turns into a metallic state once E(F) moves to the ground-state LL of the antisymmetric subband. The sharpness of this disappearance suggests a first-order transition.     

Divergence of the grüneisen ratio at quantum critical points in heavy fermion metals

We present low-temperature volume thermal expansion, beta, and specific heat, C, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si0.95Ge0.05)(2) which are located very near to quantum critical points. For both systems, beta shows a more singular temperature dependence than C, and thus the Grüneisen ratio Gamma proportional to beta/C diverges as T-->0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin fluctuations. By contrast, the observed singularity in YbRh2(Si0.95Ge0.05)(2) cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.     

Magnetoresistance in the variable-range hopping conduction regime in diluted arrays of quantum dots with the filling factor 2 < η < 3

Magnetoresistance mechanisms in an array of quantum dots with hopping conduction, which is determined by electronic states with the orbital angular momentum l = 1, and filling factor 2 < ν < 3 have been considered. The magnetoresistance mechanism associated with the existence of the nodal planes of the wavefunctions of such electrons has been analyzed in detail. The dependence of this mechanism on both the shape of quantum dots and the dimension of the array has been examined including the spin-orbit interaction and effects associated with the interference of tunneling paths. Also it has been shown that a change in the energy of the orbital motion of the electron with l = 1 in the magnetic field leads to an additional mechanism of positive magnetoresistance proportional to the square of the field.