Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of co
mpounds, there exists no information for structurally simple HgBa$_2$CuO$_{4+delta}$. Here we report neutron scattering results for HgBa$_2$CuO$_{4+delta}$ (superconducting transition temperature T$_c$ $sim$ 71 K, pseudogap temperature T* $sim$ 305 K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of the cuprates: the X-shaped hourglass response and the resonance mode in the superconducting state. Instead, the response is Y-shaped, gapped, and significantly enhanced below T*, and hence a prominent signature of the pseudogap state.
We report in-plane resistivity ($rho$) and transverse magnetoresistance (MR) measurements in underdoped HgBa$_2$CuO$_{4+delta}$ (Hg1201). Contrary to the longstanding view that Kohlers rule is strongly violated in underdoped cuprates, we find that it
is in fact satisfied in the pseudogap phase of Hg1201. The transverse MR shows a quadratic field dependence, $deltarho/rho_o=a H^{2}$, with $a(T)propto T^{-4}$. In combination with the observed $rhopropto T^2$ dependence, this is consistent with a single Fermi-liquid quasiparticle scattering rate. We show that this behavior is universal, yet typically masked in cuprates with lower structural symmetry or strong disorder effects.
There exists increasing evidence that the phase diagram of the high-transition temperature (Tc) cuprate superconductors is controlled by a quantum critical point. One distinct theoretical proposal is that, with decreasing hole-carrier concentration,
a transition occurs to an ordered state with two circulating orbital currents per CuO2 square. Below the pseudogap temperature T* (T* > Tc), the theory predicts a discrete order parameter and two weakly-dispersive magnetic excitations in structurally simple compounds that should be measurable by neutron scattering. Indeed, novel magnetic order and one such excitation were recently observed. Here, we demonstrate for tetragonal HgBa2CuO4+d the existence of a second excitation with local character, consistent with the theory. The excitations mix with conventional antiferromagnetic fluctuations, which points toward a unifying picture of magnetism in the cuprates that will likely require a multi-band description.
We report on an all-electrical measurement of the spin Hall effect in epitaxial Fe/In_{x}Ga_{1-x}As heterostructures with n-type channel doping (Si) and highly doped Schottky tunnel barriers. A transverse spin current generated by an ordinary charge
current flowing in the In_{x}Ga_{1-x}As is detected by measuring the spin accumulation at the edges of the channel. The spin accumulation is identified through the observation of a Hanle effect in the Hall voltage measured by pairs of ferromagnetic contacts. We investigate the bias and temperature dependence of the resulting Hanle signal and determine the skew and side-jump contributions to the total spin Hall conductivity.
