The existence of the magnetic-field-induced liquid-to-solid phase transition in an extreme quantumlimit 2D electron plasma is established for electrons at a high-quality GaAs/GaAlAs heterojunction by detection of a gapless magnetophonon excitation branch with radio-frequency spectroscopy. The phase diagram, determined for Wigner-Seitz to Bohr radius ratio 1.6 <») limit. PACS numbers: 73.40.Kp, 64.70.-p, 73.20.MfA zero-temperature electron plasma confined to a plane shows unusually varied physical properties as a function of density ris and perpendicular magnetic field H, In zero field and high density, the zero-point energy arising from the Pauli exclusion principle dominates, resulting in a liquid state with Fermi energy Ef^^h^/ma^, which corresponds to the energy of confinement in the Wigner-Seitz radius, a, defined by Ka^^X/ris. As the density is reduced the Coulomb interaction Vc^e^lea in the medium of dielectric constant e gains in importance, ultimately ordering the electrons into a crystal. Wigner * predicted this quantum phase transition long ago, but it has never been seen for want of a suitable physical system.In a magnetic field, however, each single-particle state is already confined by the Lorentz force to an area --(po/H'^lKli occupied by a flux quantum ^o^hc/e Un'^ihc/eH)^^^ is the zero-point cyclotron radius] and takes on discrete energy values (n+ j)hcoc where h(Oc -h^/ml^ -heH/mc is the cyclotron energy. Each of these Landau cyclotron oscillators has an average share of v=2(////a)^ electrons. Increasing the field decreases this filling factor v, successive Landau levels empty, and, in the presence of moderate disorder, one observes the integrally quantized Hall resistance.^ When V < 1 there is spatial freedom for all the electrons for the same zero-point energy. This greatly favors Coulomb-induced correlation. The many-body ground state is a correlated liquid giving rise to the fractional quantum Hall effect^ until, below a critical filling factor estimated between y and j,^ a new ground state with long-range crystallike correlation is expected. To date, this transition, too, has eluded unambiguous observation.The present experiment gives positive evidence of this magnetically induced Wigner solid. Advances in the technology of modulation-dopedGaAs/GaAlAs heterojunctions have made available very high-quality 2D quantum electron systems. Some time ago,^ it was already suggested that the abnormal behavior observed in the dc transport for v < j could be a result of a magnetically induced Wigner solid. But to be affirmative required a much more specific type of experiment. Our experiment tests for a defining property of a solid-its rigidity to shear. An unbounded charged 2D liquid has only compressional (plasmon) excitations in zero field whose frequency-wave-vector relation is (ol^lKe^risqlem. A perpendicular magnetic field i...

The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time [1][2][3][4][5][6][7][8][9][10]. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) [11,12] further strengthens these efforts. With a crystal structure similar to the infinite-layer cupratestransition metal oxide layers separated by a rare-earth spacer layerformal valence counting suggests that these materials have monovalent Ni 1+ cations with the same 3d electron count as Cu 2+ in the cuprates. Here, we use x-ray spectroscopy in concert with density functional theory to show that the electronic structure of RNiO2 (R = La, Nd), while similar to the cuprates, includes significant distinctions. Unlike cuprates with insulating spacer layers between the CuO2 planes, the rare-earth spacer layer in the infinite-layer nickelate supports a weaklyinteracting three-dimensional 5d metallic state. This three-dimensional metallic state hybridizes with a quasi-two-dimensional, strongly correlated state with 3dx 2 -y 2 symmetry in the NiO2 layers. Thus, the infinite-layer nickelate can be regarded as a sibling of the rare earth intermetallics [13-15], well-known for heavy Fermion behavior, where the NiO2 correlated layers play an analogous role to the 4f states in rare-earth heavy Fermion compounds. This unique Kondo-or Anderson-lattice-like "oxide-intermetallic" replaces the Mott insulator as the reference state from which superconductivity emerges upon doping.While the mechanism of superconductivity in the cuprates remains a subject of intense research, early on it was suggested that the conditions required for realizing high Tc superconductivity are rooted in the physics of a two-dimensional electron system subject to strong local repulsion [16,17]. This describes the Mott (charge-transfer) insulators in the stoichiometric parent compounds, characterized by spin ½ Heisenberg antiferromagnetism, from which superconductivity emerges upon doping. A long-standing question regards whether these "cuprate-Mott" conditions can be realized in other oxides; and extensive efforts to synthesize and engineer nickel oxides (nickelates) have promised such a realization [1-10]. Infinite-layer NdNiO2 became the first such nickelate superconductor following the recent discovery of superconductivity in Srdoped samples [11]. The undoped parent compound, produced by removing the apical oxygen atoms from the perovskite nickelate NdNiO3 using a metal hydride-based soft chemistry reduction process [10,[18][19][20], appears to be a close sibling of the cuprates-it is isostructural to the infinitelayer cuprates with monovalent Ni 1+ cations and possesses the same 3d 9 electron count as that of Cu 2+ cations in undoped cuprates. Yet, as we will reveal, the electronic structure of the undoped RNiO2 (R = La and Nd) remains distinct from the Mott, or charge-transfer, compounds of undoped cuprates, and even...

We have performed Ce L3-edge x-ray absorption spectroscopy (XAS) measurements on CeO1−xFxBiS2, in which the superconductivity of the BiS2 layer and the ferromagnetism of the CeO1−xFx layer are induced by the F-doping, in order to investigate the impact of the F-doping on the local electronic and lattice structures. The Ce L3-edge XAS spectrum of CeOBiS2 exhibits coexistence of 4f 1 (Ce 3+ ) and 4f 0 (Ce 4+ ) state transitions revealing Ce mixed valency in this system. The spectral weight of the 4f 0 state decreases with the F-doping and completely disappears for x > 0.4 where the system shows the superconductivity and the ferromagnetism. The results suggest that suppression of Ce-S-Bi coupling channel by the F-doping appears to drive the system from the valence fluctuation regime to the Kondo-like regime, leading to the coexistence of the superconducting BiS2 layer and the ferromagnetic CeO1−xFx layer.

We have used Bi and Ce L3-edges extended x-ray absorption fine structure measurements to study local structure of CeO(1-x)F(x)BiS2 system as a function of F-substitution. The local structure of both BiS2 active layer and CeO1-xFx spacer layer changes systematically. The in-plane Bi-S1 distance decreases (ΔRmax ∼ 0.08 Å) and the out-of-plane Bi-S2 distance increases (ΔRmax ∼ 0.12 Å) with increasing F-content. On the other hand, the Ce-O/F distance increases (ΔRmax ∼ 0.2 Å) with a concomitant decrease of the Ce-S2 distance (ΔRmax ∼ 0.15 Å). Interestingly, the Bi-S1 distance is characterized by a large disorder that increases with F-content. The results provide useful information on the local atomic displacements in CeO(1-x)F(x)BiS2, that should be important for the understanding of the coexistence of superconductivity and low temperature ferromagnetism in this system.