Coulomb Pseudogap Research Articles

Pseudogap metal induced by long-range Coulomb interactions

In correlated electron systems the metallic character of a material can be strongly suppressed near an integer concentration of conduction electrons as Coulomb interactions forbid the double occupancy of local atomic orbitals. While the Mott-Hubbard physics arising from such on-site interactions has been largely studied, several unexplained phenomena observed in correlated materials challenge this description and call for the development of new ideas. Here we explore a general route for obtaining correlated behavior that is decidedly different from the spin-related Mott-Hubbard mechanism and instead relies on the presence of unscreened, long-range Coulomb interactions. We find a pseudogap metal phase characterized by a divergent quasiparticle mass and the opening of a Coulomb pseudogap in the electronic spectrum. The destruction of the Fermi-liquid state occurs because the electrons move in a nearly frozen, disordered charge background, as collective charge rearrangements are drastically slowed down by the frustrating nature of long-range potentials on discrete lattices. The present pseudogap metal realizes an early conjecture by Efros, that a soft Coulomb gap should appear for quantum lattice electrons with strong unscreened interactions due to self-generated randomness.

Precursors to Exciton Condensation in Quantum Hall Bilayers.

Tunneling spectroscopy reveals evidence for interlayer electron-hole correlations in quantum Hall bilayer two-dimensional electron systems at layer separations near, but above, the transition to the incompressible exciton condensate at total Landau level filling ν_{T}=1. These correlations are manifested by a nonlinear suppression of the Coulomb pseudogap which inhibits low energy interlayer tunneling in weakly coupled bilayers. The pseudogap suppression is strongest at ν_{T}=1 and grows rapidly as the critical layer separation for exciton condensation is approached from above.

Thermodynamic evidence for valley-dependent density of states in bulk bismuth

Electron-like carriers in bismuth are described by the Dirac Hamiltonian, with a band mass becoming a thousandth of the bare electron mass along one crystalline axis. The existence of three anisotropic valleys offers electrons an additional degree of freedom, a subject of recent attention. Here, we map the Landau spectrum by angle-resolved magnetostriction, and quantify the carrier number in each valley: while the electron valleys keep identical spectra, they substantially differ in their density of states at the Fermi level. Thus, the electron fluid does not keep the rotational symmetry of the lattice at low temperature and high magnetic field, even in the absence of internal strain. This effect, reminiscent of the Coulomb pseudogap in localized electronic states, affects only electrons in the immediate vicinity of the Fermi level. It presents the most striking departure from the non-interacting picture of electrons in bulk bismuth.

Coulomb pseudogap in scattering-assisted tunneling of electrons between Landau-quantized two-dimensional electron gases

We investigate electron tunneling between two GaAs accumulation two-dimensional layers separated by an (AlGa)As barrier which is modulation-doped with donors. A Coulomb pseudogap suppresses electron tunneling at the Fermi level and is induced by a magnetic field applied perpendicular to the accumulation layers. By measuring the tunnel current as a function of magnetic field and bias voltage, we examine how the pseudogap is influenced by elastic scattering processes.

Breakdown of particle–hole symmetry in the lowest Landau level revealed by tunneling spectroscopy

Tunneling measurements on 2D electron gases at high magnetic field reveal a qualitative difference between the two spin sublevels of the lowest Landau level. While the tunneling current–voltage characteristic at filling factor ν = 1 / 2 is a single peak shifted from zero bias by a Coulomb pseudogap, the spectrum at ν = 3 / 2 shows a well-resolved double peak structure. This difference is present regardless of whether ν = 1 / 2 and ν = 3 / 2 occur at the same or different magnetic fields. No analogous effect is seen at ν = 5 / 2 and 7/2 in the first excited Landau level.

Resonant tunneling of electrons between two-dimensional systems of different densities in a quantizing magnetic field

The results of experimental investigation of the vertical electron transport in a GaAs/Al0.3Ga0.7As/GaAs single-barrier tunneling heterostructure with a doped barrier are presented. Two-dimensional accumulation layers appear on different sides of the barrier as a result of the ionization of Si donors in the barrier layer. The nonmonotonic shift of the current peak is found in the I–V curve of the tunneling diode in a magnetic field perpendicular to the planes of two-dimensional layers. Such a behavior is shown to be successfully explained in the model of appearing the Coulomb pseudogap and the pinning of the spin-split Landau levels at the Fermi levels of the contacts. In this explanation, it is necessary to assume that the Lande factor is independent of the filling factors of the Landau levels and is g* = 7.5 for both layers.

Fermionic Sherrington-Kirkpatrick models with Hubbard interaction: Magnetism and electronic structure

Models with range-free frustrated Ising spin- and Hubbard interaction are treated exactly by means of the discrete time slicing method. Critical and tricritical points, correlations, and the fermion propagator, are derived as a function of temperature T, chemical potential \mu, Hubbard coupling U, and spin glass energy J. The phase diagram is obtained. Replica symmetry breaking (RSB)-effects are evaluated up to four-step order (4RSB). The use of exact relations together with the 4RSB-solutions allow to model exact solutions by interpolation. For T=0, our numerical results provide strong evidence that the exact density of states in the spin glass pseudogap regime obeys \rho(E)=const |E-E_F| for energies close to the Fermi level. Rapid convergence of \rho'(E_F) under increasing order of RSB is observed. The leading term resembles the Efros-Shklovskii Coulomb pseudogap of localized disordered fermionic systems in 2D. Beyond half filling we obtain a quadratic dependence of the fermion filling factor on the chemical potential. We find a half filling transition between a phase for U>\mu, where the Fermi level lies inside the Hubbard gap, into a phase where \mu(>U) is located at the center of the upper spin glass pseudogap (SG-gap). For \mu>U the Hubbard gap combines with the lower one of two SG-gaps (phase I), while for \mu<U it joins the sole SG-gap of the half-filling regime (phase II). We predict scaling behaviour at the continuous half filling transition. Implications of the half-filling transition between the deeper insulating phase II and phase I for delocalization due to hopping processes in itinerant model extensions are discussed and metal-insulator transition scenarios described.

Coulomb pseudogap caused by partial localization of a three-dimensional electron system in the extreme quantum limit

Scanning tunneling spectroscopy is applied to a three-dimensional electron system in the extreme quantum limit (EQL). A parabolic pseudogap of a width of $\ifmmode\pm\else\textpm\fi{}2.5\mathrm{meV}$ is found at ${E}_{F}$ and interpreted as the Coulomb gap predicted by Efros and co-workers [J. Phys. C $8,$ L49 (1975); Sov. Phys. Semicond. $14,$ 487 (1980)] for localized particles. The fact that the depth of the gap is markedly above zero and fluctuates in space is interpreted as evidence for partial localization supporting our previous assumption that a distinct electron phase exists in the magnetic-field range between the transition to the EQL and the magnetic freeze-out.

Tunneling measurements of the Coulomb pseudogap in a two-dimensional electron system in a quantizing magnetic field

Tunneling measurements of the Coulomb pseudogap in a two-dimensional electron system in a quantizing magnetic field

Tunneling measurements of the Coulomb pseudogap in a two-dimensional electron system in a quantizing magnetic field

We study the Coulomb pseudogap for tunneling into the two-dimensional electron system of high-mobility (Al,Ga)As/GaAs heterojunctions subjected to a quantizing magnetic field at filling factor $\nu \leq 1$. Tunnel current-voltage characteristics show that for the double maximum observed in the tunnel resistance at $\nu \approx 1$ the pseudogap is linear in energy with a slope that depends on filling factor, magnetic field, and temperature. We give a qualitative account of the filling factor dependence of the pseudogap slope and we confirm the recently reported appearance of another relaxation time for tunneling at $\nu \approx 1$. For the tunnel resistance peaks at $\nu=1/3$ and 2/3 a completely different behaviour of the current-voltage curves is found and interpreted as manifestation of the fractional gap.

Tunnel'nye izmereniya kulonovskoi psevdoshcheli v dvumernoi elektronnoi sisteme v kvantuyushchem magnitnom pole

Tunnel'nye izmereniya kulonovskoi psevdoshcheli v dvumernoi elektronnoi sisteme v kvantuyushchem magnitnom pole

Suppression of superconductivity close to the metal-insulator transition in strongly disordered systems

By means of the self-consistent theory proposed earlier for a metal-insulator transition in strongly disordered systems, which takes into account interelectron interaction effects, the effects of the suppression of the superconducting-transition temperature T c , caused by the formation of a Coulomb pseudo-gap in the density of states, are studied in a wide interval of disorder values—from a weakly disordered metal to an Anderson insulator. It is shown that the proposed theory gives a satisfactory description of the experimental data for a number of systems that have been studied.