Abstract
Kondo lattice materials, where localized magnetic moments couple to itinerant electrons, provide a very rich backdrop for strong electron correlations. They are known to realize many exotic ...phenomena, with a dramatic example being recent observations of quantum oscillations and metallic thermal conduction in insulators, implying the emergence of enigmatic charge-neutral fermions. Here, we show that thermal conductivity and specific heat measurements in insulating YbIr
3
Si
7
reveal emergent neutral excitations, whose properties are sensitively changed by a field-driven transition between two antiferromagnetic phases. In the low-field phase, a significant violation of the Wiedemann-Franz law demonstrates that YbIr
3
Si
7
is a charge insulator but a thermal metal. In the high-field phase, thermal conductivity exhibits a sharp drop below 300 mK, indicating a transition from a thermal metal into an insulator/semimetal driven by the magnetic transition. These results suggest that spin degrees of freedom directly couple to the neutral fermions, whose emergent Fermi surface undergoes a field-driven instability at low temperatures.
Although not an intrinsic superconductor, graphite exhibits superconductivity when intercalated with certain dopants. Perhaps the most studied of these graphite-based superconductors are the alkali ...metal-graphite intercalation compounds (GICs) (ref. 2), of which the easiest to fabricate is C8K (ref. 3), with a transition temperature (ref. 2). By increasing the alkali metal concentration (through high-pressure fabrication techniques), the transition temperature can be increased up to 5 K in C2Na (ref. 4). Superconductivity in C6Yb and C6Ca with and 11.5 K, respectively, and at ambient conditions has been observed. Here we explore the architecture of the states near the Fermi level and identify characteristics of the electronic band structure generic to GICs. In addition to the expected charge transfer from the intercalant atoms to the graphene sheets, resulting from the occupation of the π bands, we find that in all of those-and only those-compounds that superconduct, an interlayer state, which is well-separated from the carbon sheets, also becomes occupied. We show that the energy of the interlayer band is controlled by a combination of its occupancy and the separation between the carbon layers.
We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less ...than 1 at. %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.
Understanding the microscopic origins of electronic phases in high-transition temperature (high-Tc) superconductors is important for elucidating the mechanism of superconductivity. In the ...paramagnetic tetragonal phase of BaFe2–xTxAs2 (where T is Co or Ni) iron pnictides, an in-plane resistivity anisotropy has been observed. Here, we use inelastic neutron scattering to show that low-energy spin excitations in these materials change from fourfold symmetric to twofold symmetric at temperatures corresponding to the onset of the in-plane resistivity anisotropy. Because resistivity and spin excitation anisotropies both vanish near optimal superconductivity, we conclude that they are likely intimately connected.
The observation of a separation between the antiferromagnetic phase boundary and the small-large Fermi surface transition in recent experiments has led to the proposal that frustration is an ...important additional tuning parameter in the Kondo lattice model of heavy fermion materials. The introduction of a Kondo (
K
) and a frustration (
Q
) axis into the phase diagram permits us to discuss the physics of heavy fermion materials in a broader perspective. The current experimental situation is analyzed in the context of this combined “
QK
” phase diagram. We discuss various theoretical models for the frustrated Kondo lattice, using general arguments to characterize the nature of the
f
-electron localization transition that occurs between the spin liquid and heavy Fermi liquid ground-states. We concentrate in particular on the Shastry–Sutherland Kondo lattice model, for which we establish the qualitative phase diagram using strong coupling arguments and the large-
N
expansion. The paper closes with some brief remarks on promising future theoretical directions.
Vanadium dioxide is a strongly correlated material that undergoes a metal-insulator transition from a high-temperature, rutile metal to a monoclinic insulating state at 67 °C. In recent years, ...experiments on single-crystal vanadium-dioxide nanowires grown by physical vapour deposition have shed light on the crucial role of strain in the structural and electronic phase diagram of this material, including evidence for a new M2 phase, but the detailed physics of this material is still not fully understood. The transition temperature can be reduced by doping with tungsten, but this process is not reversible. Here, we show that the metal-insulator transition in nanoscale beams of vanadium dioxide can be strongly modified by doping with atomic hydrogen using the catalytic spillover method. We also show that this process is completely reversible, and that the metal-insulator transition eventually vanishes when the doping exceeds a threshold value. Raman and conventional optical microscopy, electron diffraction and transmission electron microscopy provide evidence that the structure of the metallic post-hydrogenation state is similar to that of the rutile state. First-principles electronic structure calculations confirm that a distorted rutile structure is energetically favoured following hydrogenation, and also that such doping favours metallicity from both the Mott and Peierls perspectives. We anticipate that hydrogen doping will be a powerful tool for examining the metal-insulator transition and for engineering the properties of vanadium dioxide.
Strongly correlated materials are profoundly affected by the repulsive electron‐electron interaction. This stands in contrast to many commonly used materials such as silicon and aluminum, whose ...properties are comparatively unaffected by the Coulomb repulsion. Correlated materials often have remarkable properties and transitions between distinct, competing phases with dramatically different electronic and magnetic orders. These rich phenomena are fascinating from the basic science perspective and offer possibilities for technological applications. This article looks at these materials through the lens of research performed at Rice University. Topics examined include: Quantum phase transitions and quantum criticality in “heavy fermion” materials and the iron pnictide high temperature superconductors; computational ab initio methods to examine strongly correlated materials and their interface with analytical theory techniques; layered dichalcogenides as example correlated materials with rich phases (charge density waves, superconductivity, hard ferromagnetism) that may be tuned by composition, pressure, and magnetic field; and nanostructure methods applied to the correlated oxides VO2 and Fe3O4, where metal‐insulator transitions can be manipulated by doping at the nanoscale or driving the system out of equilibrium. We conclude with a discussion of the exciting prospects for this class of materials.
The relative importance of electron‐electron interactions, U, compared with the kinetic energy in the form of the bandwidth, D, delineates between weakly and strongly correlated materials. We discuss several types of strongly correlated materials, higlighting their rich physics and diverse properties. Our improved understanding of these systems opens the exciting possibility of controlling and applying their fascinating phases.
A quantum spin liquid is a state of matter where unpaired electrons’ spins, although entangled, do not show magnetic order even at the zero temperature. The realization of a quantum spin liquid is a ...long-sought goal in condensed-matter physics. Although neutron scattering experiments on the two-dimensional spin-1/2 kagome lattice ZnCu3(OD)6Cl2 and triangular lattice YbMgGaO4 have found evidence for the hallmark of a quantum spin liquid at very low temperature (a continuum of magnetic excitations), the presence of magnetic and non-magnetic site chemical disorder complicates the interpretation of the data. Recently, the three-dimensional Ce3+ pyrochlore lattice Ce2Sn2O7 has been suggested as a clean, effective spin-1/2 quantum spin liquid candidate, but evidence of a spin excitation continuum is still missing. Here, we use thermodynamic, muon spin relaxation and neutron scattering experiments on single crystals of Ce2Zr2O7, a compound isostructural to Ce2Sn2O7, to demonstrate the absence of magnetic ordering and the presence of a spin excitation continuum at 35 mK. With no evidence of oxygen deficiency and magnetic/non-magnetic ion disorder seen by neutron diffraction and diffuse scattering measurements, Ce2Zr2O7 may be a three-dimensional pyrochlore lattice quantum spin liquid material with minimum magnetic and non-magnetic chemical disorder.
We demonstrate that frustrated exchange interactions can produce exotic 3D crystals of vortex strings near the saturation field (H=H(sat)) of body- and face-centered cubic Mott insulators. The ...combination of cubic symmetry and frustration leads to a magnon spectrum of the fully polarized spin state (H>H(sat)) with degenerate minima at multiple noncoplanar Q vectors. This spectrum becomes gapless at the quantum critical point H=H(sat) and the magnetic ordering below H(sat) can be formally described as a condensate of a dilute gas of bosons. By expanding in the lattice gas parameter, we find that different vortex crystals span sizable regions of the phase diagrams for isotropic exchange and are further stabilized by symmetric exchange anisotropy.
We investigate topological Cooper pairing, including gapless Weyl and fully gapped class DIII superconductivity, in a three-dimensional doped Luttinger semimetal. The latter describes effective ...spin-3/2 carriers near a quadratic band touching and captures the normal-state properties of the 227 pyrochlore iridates and half-Heusler alloys. Electron-electron interactions may favor non- s -wave pairing in such systems, including even-parity d -wave pairing. We argue that the lowest energy d -wave pairings are always of complex (e.g., d + i d ) type, with nodal Weyl quasiparticles. This implies ϱ ( E ) ∼ | E | 2 scaling of the density of states (DoS) at low energies in the clean limit or ϱ ( E ) ∼ | E | over a wide critical region in the presence of disorder. The latter is consistent with the T dependence of the penetration depth in the half-Heusler compound YPtBi. We enumerate routes for experimental verification, including specific heat, thermal conductivity, NMR relaxation time, and topological Fermi arcs. Nucleation of any d -wave pairing also causes a small lattice distortion and induces an s -wave component; this gives a route to strain-engineer exotic s + d pairings. We also consider odd-parity, fully gapped p -wave superconductivity. For hole doping, a gapless Majorana fluid with cubic dispersion appears at the surface. We invent a generalized surface model with ν -fold dispersion to simulate a bulk with winding number ν . Using exact diagonalization, we show that disorder drives the surface into a critically delocalized phase, with universal DoS and multifractal scaling consistent with the conformal field theory (CFT) SO( n ) ν , where n → 0 counts replicas. This is contrary to the naive expectation of a surface thermal metal, and implies that the topology tunes the surface renormalization group to the CFT in the presence of disorder.
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