Colossal anomalous Nernst effect in a correlated noncentrosymmetric kagome ferromagnet.

The transverse voltage generated by a temperature gradient in a perpendicularly applied magnetic field, termed the Nernst effect, has promise for thermoelectric applications and for probing electronic structure. In magnetic materials, an anomalous Nernst effect (ANE) is possible in a zero magnetic field. We report a colossal ANE in the ferromagnetic metal UCoRuAl, reaching 23 microvolts per kelvin.

Millimetre-long transport of photogenerated carriers in topological insulators.

Excitons are spin integer particles that are predicted to condense into a coherent quantum state at sufficiently low temperature. Here by using photocurrent imaging we report experimental evidence of formation and efficient transport of non-equilibrium excitons in BiSbSe nanoribbons. The photocurrent distributions are independent of electric field, indicating that photoexcited electrons and holes form excitons.

Evidence for singular-phonon-induced nematic superconductivity in a topological superconductor candidate SrBiSe.

Superconductivity mediated by phonons is typically conventional, exhibiting a momentum-independent s-wave pairing function, due to the isotropic interactions between electrons and phonons along different crystalline directions. Here, by performing inelastic neutron scattering measurements on a superconducting single crystal of SrBiSe, a prime candidate for realizing topological superconductivity by doping the topological insulator BiSe, we find that there exist highly anisotropic phonons, with the linewidths of the acoustic phonons increasing substantially at long wavelengths, but only for those along the [001] direction. This observation indicates a large and singular electron-phonon coupling at small momenta, which we propose to give rise to the exotic p-wave nematic superconducting pairing in the MBiSe (M = Cu, Sr, Nb) superconductor family.

Ultrahigh conductivity in Weyl semimetal NbAs nanobelts.

In two-dimensional (2D) systems, high mobility is typically achieved in low-carrier-density semiconductors and semimetals. Here, we discover that the nanobelts of Weyl semimetal NbAs maintain a high mobility even in the presence of a high sheet carrier density. We develop a growth scheme to synthesize single crystalline NbAs nanobelts with tunable Fermi levels.

Pressure Effects on the 4f Electronic Structure of Light Lanthanides.

Using the satellite structure of the Lγ_{1} line in nonresonant x-ray emission spectra, we probe the high-pressure evolution of the bare 4f signature of the early light lanthanides at ambient temperature. For Ce and Pr the satellite peak experiences a sudden reduction concurrent with their respective volume collapse (VC) transitions. These new experimental results are supported by calculations using state-of-the-art extended atomic structure codes for Ce and Pr, and also for Nd, which does not exhibit a VC.

Pressure-tuned Frustration of Magnetic Coupling in Elemental Europium.

Applying linear response and the magnetic force theorem in correlated density functional theory, the intersublattice exchange constants of antiferromagnetic Eu are calculated and found to vanish near the pressure of P_{c}=82  GPa, just where magnetic order is observed experimentally to be lost. The Eu 4f^{7} moment remains unchanged at high pressure, again in agreement with spectroscopic measurements, leaving the picture of perfect frustration of interatomic Ruderman-Kittel-Kasuya-Yoshida couplings in a broad metallic background, leaving a state of electrons strongly exchange coupled to arbitrarily oriented, possibly quasistatic local moments. This strongly frustrated state gives way to superconductivity at T_{c}=1.

Polarization induced Z2 and Chern topological phases in a periodically driving field.

Z2 and Chern topological phases such as newly discovered quantum spin Hall and original quantum Hall states hardly both co-exist in a single material due to their contradictory requirement on the time-reversal symmetry (TRS). We show that although the TRS is broken in systems with a periodically driving field, an effective TRS can still be defined provided the ac-field is linearly polarized or certain other conditions are satisfied. The controllable TRS provides us a route to manipulate contradictory phases by tuning the polarization.

Short range magnetic exchange interaction favors ferroelectricity.

Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science. To search multiferroics, currently most researches are focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism.

Spin generation via bulk spin current in three-dimensional topological insulators.

To date, spin generation in three-dimensional topological insulators is primarily modelled as a single-surface phenomenon, attributed to the momentum-spin locking on each individual surface. In this article, we propose a mechanism of spin generation where the role of the insulating yet topologically non-trivial bulk becomes explicit: an external electric field creates a transverse pure spin current through the bulk of a three-dimensional topological insulator, which transports spins between the top and bottom surfaces. Under sufficiently high surface disorder, the spin relaxation time can be extended via the Dyakonov-Perel mechanism.

Turning a band insulator into an exotic superconductor.

Understanding exotic, non-s-wave-like states of Cooper pairs is important and may lead to new superconductors with higher critical temperatures and novel properties. Their existence is known to be possible but has always been thought to be associated with non-traditional mechanisms of superconductivity where electronic correlations play an important role. Here we use a first principles linear response calculation to show that in doped Bi2Se3 an unconventional p-wave-like state can be favoured via a conventional phonon-mediated mechanism, as driven by an unusual, almost singular behaviour of the electron-phonon interaction at long wavelengths.

Calculation of multipolar exchange interactions in spin-orbital coupled systems.

A new method of computing multipolar exchange interaction in spin-orbit coupled systems is developed using multipolar tensor expansion of the density matrix in local density approximation+U electronic structure calculation. Within the mean field approximation, exchange constants can be mapped into a series of total energy calculations by the pair-flip approximation technique. The application to uranium dioxide shows an antiferromagnetic superexchange coupling in dipoles but a ferromagnetic one in quadrupoles which is very different from past studies.

Microscopic origin of stereochemically active lone pair formation from orbital selective external potential calculations.

The nature of the stereochemically active lone pair has long been a matter for debate. Here, by application of our recently developed orbital selective external potential (OSEP) method, we have studied the microscopic mechanism of stereochemically active lone pairs in various compounds. The OSEP method allows us to shift the energy level of a specific atomic orbital, therefore is helpful to identify unambiguously the role of this orbital in the chemical and physical properties of the system we are interested in.

Computational design of axion insulators based on 5d spinel compounds.

Based on density functional calculation using the local density approximation+U method, we predict that osmium compounds such as CaOs(2)O(4) and SrOs(2)O(4) can be stabilized in the geometrically frustrated spinel crystal structure. They show ferromagnetic order in a reasonable range of the on-site Coulomb correlation U and exotic electronic properties, in particular, a large magnetoelectric coupling characteristic of axion electrodynamics. Depending on U, other electronic phases including a 3D Weyl semimetal and Mott insulator are also shown to occur.

First-principles simulations of heavy fermion cerium compounds based on the Kondo lattice.

We propose a new framework for first-principles calculations of heavy-fermion materials. These are described in terms of the Kondo lattice Hamiltonian with the parameters extracted from a realistic density functional based calculation which is then solved using continuous-time quantum Monte Carlo method and dynamical mean field theory. As an example, we show our results for the Néel temperatures of cerium-122 compounds (CeX(2)Si(2) with X = Ru, Rh, Pd, Cu, Ag, and Au) where the general trend around the magnetic quantum critical point is successfully reproduced.

Doping driven (pi, 0) nesting and magnetic properties of Fe_{1+x}Te superconductors.

To understand newly discovered superconductivity in Fe-based systems, we investigate the electronic structure and magnetic properties of Fe_{1+x}Te using first-principles density functional calculations. While the undoped FeTe has the same Fermi surface nested at (pi,pi) as in Fe arsenides, doping by approximately 0.5 electrons reveals a novel square-type Fermi surface showing a strong (pi,0) nesting and leading to a different magnetic structure.

Anisotropy, itineracy, and magnetic frustration in high-Tc iron pnictides.

Using first-principles density functional theory calculations combined with insight from a tight-binding representation, dynamical mean field theory, and linear response theory, we have extensively investigated the electronic structures and magnetic interactions of nine ferropnictides representing three different structural classes. The calculated magnetic interactions are found to be short range, and the nearest (J_{1a}) and next-nearest (J2) exchange constants follow the universal trend of J_{1a}/2J_{2} approximately 1, despite their itinerant origin and extreme sensitivity to the z position of As. These results bear on the discussion of itineracy versus magnetic frustration as the key factor in stabilizing the superconducting ground state.

Screening of magnetic moments in PuAm alloy: local density approximation and dynamical mean field theory study.

The puzzling absence of Pu magnetic moments in a PuAm environment is explored using the self-consistent dynamical mean field theory calculations in combination with the local density approximation. We argue that delta-Pu-Am alloys provide an ideal testbed for investigating the screening of moments from the single impurity limit to the dense limit. Several important effects can be studied: volume expansion, shift of the bare Pu on-site f energy level, and the reduction of the hybridization cloud resulting from the collective character of the Kondo effect in the Anderson lattice.

Electron-hole symmetry and magnetic coupling in antiferromagnetic LaFeAsO.

When either electron or hole doped at concentrations x approximately 0.1, the LaFeAsO family displays remarkably high temperature superconductivity with Tc up to 55 K. In the most energetically stable Q-->M=(pi,pi,0) antiferromagnetic (AFM) phase comprised of tetragonal-symmetry breaking alternating chains of aligned spins, there is a deep pseudogap in the Fe 3d states centered at the Fermi energy arising from light carriers (m* approximately 0.

Origin of low thermal conductivity in nuclear fuels.

Using a novel many-body approach, we report lattice dynamical properties of UO2 and PuO2 and uncover various contributions to their thermal conductivities. Via calculated Grüneisen constants, we show that only longitudinal acoustic modes having large phonon group velocities are efficient heat carriers. Despite the fact that some optical modes also show their velocities which are extremely large, they do not participate in the heat transfer due to their unusual anharmonicity.

Calculated momentum dependence of Zhang-Rice states in transition metal oxides.

Using a combination of local density functional theory and cluster exact diagonalization based dynamical mean field theory, we calculate many-body electronic structures of several Mott insulating oxides including undoped high T(c) materials. The dispersions of the lowest occupied electronic states are associated with the Zhang-Rice singlets in cuprates and with doublets, triplets, quadruplets, and quintets in more general cases. Our results agree with angle resolved photoemission experiments including the decrease of the spectral weight of the Zhang-Rice band as it approaches k=0.

Calculation of magnetic exchange interactions in Mott-Hubbard systems.

An efficient method of computing magnetic exchange interactions in systems with strong correlations is introduced. It is based on a magnetic force theorem that evaluates linear response due to rotations of magnetic moments and uses a generalized spectral density functional framework allowing us to explore several approximations ranging from local density functional to exact diagonalization based dynamical mean field theory. Applications to spin waves and magnetic transition temperatures of 3d metal mono-oxides as well as high-T(c) superconductors are in good agreement with experiment.

Local self-energy approach for electronic structure calculations.

Using a novel self-consistent implementation of Hedin's perturbation theory, we calculate space- and energy-dependent self-energy for a number of materials. We find it to be local in real space and rapidly convergent on second- to third-nearest neighbors. Corrections beyond are evaluated and shown to be completely localized within a single unit cell.

Many-body electronic structure of americium metal.

We report computer based simulations of energetics, spectroscopy, and electron-phonon interaction of americium using a novel spectral density functional method. This approach gives rise to a new concept of a many-body electronic structure and reveals the unexpected mixed valence regime of Am 5f6 electrons which under pressure acquire the 5f7 valence state. This explains the unique properties of Am and addresses the fundamental issue of how the localization delocalization edge is approached from the localized side in a closed shell system.

Critical temperature and enhanced isotope effect in the presence of paramagnons in phonon-mediated superconductors.

We reconsider the long-standing problem of the effect of spin fluctuations on the critical temperature and isotope effect in a phonon-mediated superconductor. Although the general physics of the interplay between phonons and paramagnons has been rather well understood, the existing approximate formulas fail to describe the correct behavior of Tc for general phonon and paramagnon spectra. Using a controllable approximation, we derive an analytical formula for Tc which agrees well with exact numerical solutions of the Eliashberg equations for a broad range of parameters.

The alpha--> gamma transition in Ce: a theoretical view from optical spectroscopy.

Using a novel approach to calculate optical properties of strongly correlated systems, we address the old question of the physical origin of the alpha--> gamma transition in Ce. We find that the Kondo collapse model, involving both the f and the spd electrons, describes the optical data better than a Mott transition picture involving the f electrons only. Our results compare well with existing experiments on thin films.