Néel transition and sublattice magnetization of pure and doped La2CuO4

Mott gap collapse in lightly hole-doped Sr2-xKxIrO4

The evolution of Sr₂IrO₄ upon carrier doping has been a subject of intense interest, due to its similarities to the parent cuprates, yet the intrinsic behaviour of Sr₂IrO₄ upon hole doping remains enigmatic. Here, we synthesize and investigate hole-doped Sr_2-xK_xIrO₄ utilizing a combination of reactive oxide molecular-beam epitaxy, substitutional diffusion and in-situ angle-resolved photoemission spectroscopy. Upon hole doping, we observe the formation of a coherent, two-band Fermi surface, consisting of both hole pockets centred at (π, 0) and electron pockets centred at (π/2, π/2). In particular, the strong similarities between the Fermi surface topology and quasiparticle band structure of hole- and electron-doped Sr₂IrO₄ are striking given the different internal structure of doped electrons versus holes.

Witnessing the formation and relaxation of dressed quasi-particles in a strongly correlated electron system

The non-equilibrium approach to correlated electron systems is often based on the paradigm that different degrees of freedom interact on different timescales. In this context, photo-excitation is treated as an impulsive injection of electronic energy that is transferred to other degrees of freedom only at later times. Here, by studying the ultrafast dynamics of quasi-particles in an archetypal strongly correlated charge-transfer insulator (La2CuO(4+δ)), we show that the interaction between electrons and bosons manifests itself directly in the photo-excitation processes of a correlated material. With the aid of a general theoretical framework (Hubbard-Holstein Hamiltonian), we reveal that sub-gap excitation pilots the formation of itinerant quasi-particles, which are suddenly dressed by an ultrafast reaction of the bosonic field.

Mott insulators in the strong spin-orbit coupling limit: from Heisenberg to a quantum compass and Kitaev models

We study the magnetic interactions in Mott-Hubbard systems with partially filled t_{2g} levels and with strong spin-orbit coupling. The latter entangles the spin and orbital spaces, and leads to a rich variety of the low energy Hamiltonians that extrapolate from the Heisenberg to a quantum compass model depending on the lattice geometry. This gives way to "engineer" in such Mott insulators an exactly solvable spin model by Kitaev relevant for quantum computation. We, finally, explain "weak" ferromagnetism, with an anomalously large ferromagnetic moment, in Sr2IrO4.

Structure of the spin-glass state of La2-xSrxCuO4: the spiral theory

Starting from the t-J model, we derive the effective field theory describing the spin dynamics in insulating La(2-x)Sr(x)CuO(4), x approximately < 0.055, at low temperature. The theory results in a disordered spiral ground state, in which the staggered component of the copper spins is confined in a plane determined by the spin anisotropies. The static spin structure factor obtained in our calculations is in perfect agreement with neutron scattering data over the whole range of doping in both, the Néel and the spin-glass phase. We show that topological defects (spin vortex-antivortex pairs) are an intrinsic property of the disordered spiral ground state.

Néel temperature of quasi-low-dimensional Heisenberg antiferromagnets

The Néel temperature T(N) of quasi-one- and quasi-two-dimensional antiferromagnetic Heisenberg models on a cubic lattice is calculated by Monte Carlo simulations as a function of interchain (interlayer) to intrachain (intralayer) coupling J(')/J down to J(')/J approximately = 10(-3). We find that T(N) obeys a modified random-phase approximationlike relation for small J(')/J with an effective universal renormalized coordination number, independent of the size of the spin. Empirical formulas describing T(N) for a wide range of J(') and useful for the analysis of experimental measurements are presented.

Theory of incommensurate magnetic correlations across the insulator-superconductor transition of underdoped La2-xSrxCuO4

The main feature in the elastic neutron scattering of La2-xSrxCuO4 is the existence of incommensurate peaks with positions that jump from 45 degrees to 0 degrees at 5% doping. We show that the spiral state of the t-t(')-t('')-J model with realistic parameters describes these data perfectly. We explain why in the insulator the peak is at 45 degrees while it switches to 0 degrees precisely at the insulator-metal transition. The calculated positions of the peaks are in agreement with the data in both phases.

Spin-flop transition and the anisotropic magnetoresistance of Pr(1.3-x)La(0.7)CexCuO4: unexpectedly strong spin-charge coupling in the electron-doped cuprates

We use transport and neutron-scattering measurements to show that a magnetic-field-induced transition from noncollinear to collinear spin arrangement in adjacent CuO2 planes of lightly electron-doped Pr(1.3-x)La(0.7)CexCuO4 (x=0.01) crystals affects significantly both the in-plane and out-of-plane resistivity. In the high-field collinear state, the magnetoresistance (MR) does not saturate but exhibits an intriguing fourfold-symmetric angular dependence, oscillating from being positive at B//[100] to being negative at B//[110]. The observed MR of more than 30% at low temperatures induced by a modest modification of the spin structure indicates an unexpectedly strong spin-charge coupling in electron-doped cuprates.

Magnon heat transport in doped La2CuO4

We present results of the thermal conductivity of La2CuO4 and La(1.8)Eu(0.2)CuO4 single crystals which represent model systems for the two-dimensional spin-1/2 Heisenberg antiferromagnet on a square lattice. We find large anisotropies of the thermal conductivity which are explained in terms of two-dimensional heat conduction by magnons within the CuO2 planes. Nonmagnetic Zn substituted for Cu gradually suppresses this magnon thermal conductivity kappa(mag). A semiclassical analysis of kappa(mag) is shown to yield a magnon mean free path which scales linearly with the reciprocal concentration of Zn ions.