Search Results (37)

Complex phase diagrams are a generic feature of quantum materials that display high-temperature superconductivity. In addition to d-wave superconductivity (or other unconventional states), these phase diagrams typically include various forms of charge-ordered phases, including charge-density waves and/or spin-density waves, as well as electronic nematic states. In most cases, these phases have critical temperatures comparable in magnitude to that of the superconducting state and appear in a “pseudo-gap” regime. In these systems, the high temperature state does not produce a good metal with well-defined quasiparticles but a ”strange metal”. These states typically arise from doping a strongly correlated Mott insulator. With my collaborators, I have identified these behaviors as a problem with “Intertwined Orders”. A pair-density wave is a type of superconducting state that embodies the physics of intertwined orders. Here, I discuss the phenomenology of intertwined orders and the quantum materials that are known to display these behaviors. Full article

The electronic and phonon bands of Skaergaardite are investigated using density functional theory (DFT) as implemented in Quantum ESPRESSO. Skaergaardite is a copper palladium mineral (CuPd) found in the Skaergaard intrusion with a CsCl-type (B2) structure. Due to its porous structure, it presents a large surface area available for interactions, which makes it a promising catalyst. The PBE-GGA functional with a Hubbard-like localized term (DFT+U) is combined with ultrasoft and norm-conserving pseudopotentials, and a conventional approach with a dense Monkhorst–Pack grid of k-points 12 × 12 × 12 is applied. The electronic valence bands are mainly constituted by 3d orbitals of Cu and 4d orbitals of Pd and a pseudo-gap can be recognized. With respect to DFT, DFT+U causes a general downward shift in the valence band. The acoustic and optical phonon branches are separated by a few cm⁻¹ gap at about 150 cm⁻¹ and show a density of state curve typical of ordered materials. These results highlight the reliability of DFT+U in studying bimetallic systems with scarce experimental benchmarks, offering insights into the behavior of Skaergaardite and its potential applications in material science such as reduction reactions and hydrogen storage. Full article

Na−O₂ batteries are plagued by high cathodic oxygen reduction (ORR)/oxygen evolution (OER) overpotentials during discharging/charging. Herein, we constructed six carbide/nitride MXenes (M₂XO₂, M = Ti, Zr, and Hf, X = C, and N) and investigated their performance as cathodes for Na−O₂ batteries by first-principles calculations. M₂CO₂ MXenes have a pseudogap, showing semiconducting properties, while M₂NO₂ MXenes are conductive. The nucleophilic O on the M₂XO₂ surfaces prefers to bind with the Na atoms of Na_xO₂ intermediates to activate the Na−O bonds, improving the sodium deintercalation. For all M₂XO₂ MXenes, the OER overpotential is higher than the ORR overpotential, forming a performance bottleneck of Na−O₂ batteries. The overpotentials originate from the too-strong adsorption of Na_xO₂ on M₂XO₂ MXenes. Lowering the O p-band center of the M₂XO₂ MXenes can weaken the Na_xO₂ adsorption, thereby reducing the overpotential. Consequently, the overpotentials of the M₂CO₂ carbides are lower than those of the M₂NO₂ nitrides and further decrease with a decreasing M atomic number. The Ti₂CO₂ MXene shows extremely low ORR, OER, and total overpotentials (0.23, 0.32, and 0.55 V), suggesting a huge potential as cathodes in Na−O₂ batteries. Full article

Inspired by experiments manifesting unconventional metallic behavior in ${\mathrm{MoO}}_{2\pm \delta }$ and superconductivity in ${\mathrm{K}}_x$${\mathrm{MoO}}_{2-\delta }$, we present $t_{2g}$-DFT+DMFT results for monoclinic ${\mathrm{MoO}}_2$. We unearth the role played by multi-orbital, many-particle physics in understanding the emergence of $4d$-orbital selectivity with coexisting pseudogapped, resilient, and Fermi-liquid quasiparticles, which might host unconventional superconductivity in K-doped ${\mathrm{MoO}}_2$ bulk crystals at low temperatures. Our findings highlight the capability of DFT+DMFT to bridge the gap between electronic structure and electric transport in multi-orbital Hubbard models, providing insights into spin and charge fluctuations, as well as their role in orbital-selective non-Fermi liquid formation. Full article

Periodic lattice distortion, known as the charge density wave, is generally attributed to electron–phonon coupling. This correlation is expected to induce a pseudogap at the Fermi level in order to gain the required energy for stable lattice distortion. The transition metal dichalcogenide 1T-VSe₂ also undergoes such a transition at 110 K. Here, we present detailed angle-resolved photoemission spectroscopy experiments to investigate the electronic structure in 1T-VSe₂ across the structural transition. Previously reported warping of the electronic structure and the energy shift of a secondary peak near the Fermi level as the origin of the charge density wave phase are shown to be temperature independent and hence cannot be attributed to the structural transition. Our work reveals new states that were not resolved in previous studies. Earlier results can be explained by the different dispersion natures of these states and temperature-induced broadening. Only the overall size of the Fermi surface is found to change across the structural transition. These observations, quite different from the charge density wave scenario commonly considered for 1T-VSe₂ and other transition metal dichalcogenides, bring fresh perspectives toward correctly describing structural transitions. Therefore, these new results can be applied to material families in which the origin of the structural transition has not been resolved. Full article

We conduct ab-initio electronic structure calculations to explore a novel category of magnetic Heusler compounds, comprising solely $3d$ transition metal atoms and characterized by high spin magnetic moments. Specifically, we focus on Co${}_{2}YZ$ Heusler compounds, where Y and Z represent transition metal atoms such that the order of the valence is Co > Y > Z. We show that these compounds exhibit a distinctive region of very low density of minority-spin states at the Fermi level when crystallizing in the $L{2}_1$ lattice structure. The existence of this pseudogap leads most of the studied compounds to a Slater–Pauling-type behavior of their total spin magnetic moment. Co${}_2$FeMn is the compound that presents the largest total spin magnetic moment in the unit cell reaching a very large value of 9 ${\mathsf{\mu }}_{\mathrm{B}}$. Our findings suggest that these compounds are exceptionally promising materials for applications in the realms of spintronics and magnetoelectronics. Full article

The connectivity and inherent frustration of the kagome lattice can produce interesting electronic structures and behaviors in compounds containing this structural motif. Here we report the properties of Pt${}_3$X${}_2$ (X = In and Tl) that adopt a double-layer kagome net structure related to that of the topologically nontrivial high-temperature ferromagnet Fe₃Sn₂ and the density wave hosting compound V₃Sb₂. We examined the structural and physical properties of single crystal Pt₃Tl₂ and polycrystalline Pt₃In₂ using X-ray and neutron diffraction, magnetic susceptibility, heat capacity, and electrical transport measurements, along with density functional theory calculations of the electronic structure. Our calculations show that Fermi levels lie in pseudogaps in the densities of states with several bands contributing to transport, and this is consistent with our Hall effect, magnetic susceptibility, and heat capacity measurements. Although electronic dispersions, characteristic of simple kagome nets with nearest-neighbor hopping, are not clearly seen, likely due to the extended nature of the Pt $5d$ states, we do observe moderately large and non-saturating magnetoresistance values and quantum oscillations in the magnetoresistance and magnetization associated with the kagome nets of Pt. Full article

First-principles calculations for underdoped La_2−xSr_xCuO₄ (LSCO) have revealed a Fermi surface consisting of spin-triplet (KS) particles at the antinodal Fermi-pockets and spin-singlet (SS) particles at the nodal Fermi-arcs in the presence of AF local order. By performing a unique method of calculating the electronic-spin state of overdoped LSCO and by measurement of the spin-correlation length by neutron inelastic scattering, the origin of the phase-diagram, including the pseudogap phase in the high temperature superconductor, Sr-doped copper-oxide LSCO, has been elucidated. We have theoretically solved the long-term problem as to why the angle-resolved photoemission spectroscopy (ARPES) has not been able to observe Fermi pockets in the Fermi surface of LSCO. As a result, we show that the pseudogap region is bounded below the characteristic temperature T*(x) and above the superconducting transition temperature T_c(x) in the T vs. x phase diagram, where both the AF order and the KS particles in the Fermi pockets vanish at T*(x), whilst KS particles contribute to d-wave superconductivity below T_c. We also show that the relationship T*(x_c) = T_c(x_c) holds at x_c = 0.30, which is consistent with ARPES experiments. At T*(x), a phase transition occurs from the pseudogap phase to an unusual metallic phase in which only the SS particles exist. Full article

We review the magneto-structural properties of electron–electron correlated quasi-one- dimensional (1D) molecular organics. These weakly localized quarter-filled metallic-like systems with pronounced spin 1/2 antiferromagnetic (AF) interactions in stack direction exhibit a spin charge decoupling where magnetoelastic coupling picks up spin 1/2 to pair into S = 0 singlet dimers. This is well illustrated by the observation of a spin-Peierls (SP) instability in the (TMTTF)2X Fabre salts and related salts with the o-DMTTF donor. These instabilities are revealed by the formation of a pseudo-gap in the spin degrees of freedom triggered by the development of SP structural correlations. The divergence of these 1D fluctuations, together with the interchain coupling, drive a 3D-SP ground state. More surprisingly, we show that the Per2-M(mnt)2 system, undergoing a Kondo coupling between the metallic Per stack and the dithiolate stack of localized AF coupled spin ½ (for M = Pd, Ni, Pt), enhances the SP instability. Then, we consider the zig-zag spin ladder DTTTF2-M(mnt)2 system, where unusual singlet ground state properties are due to a combination of a 4k_F charge localization effect in stack direction and a 2k_F SP instability along the zig-zag ladder. Finally, we consider some specific features of correlated 1D systems concerning the coexistence of symmetrically different 4k_F BOW and 4k_F CDW orders in quarter-filled organics, and the nucleation of solitons in perturbed SP systems. Full article

Fast and local probes, such as X-ray spectroscopy, X-ray diffraction (XRD), and X-ray microscopy, have provided direct evidence for nanoscale phase separation in high temperature perovskite superconductors composed of (i) free particles coexisting with (ii) Jahn Teller polarons (i.e., charges associated with local lattice distortions) not detected by slow experimental methods probing only delocalized states. Moreover, these experimental probes have shown the formation of a superstripes phase in the pseudogap regime below T* in cuprates. Here, we focus on the anomalous temperature dependence of short range X-ray diffraction CDW reflection satellites with high momentum transfer, probing both charge and lattice fluctuations in superconducting HgBa₂CuO_4+y (Hg1201) in the pseudogap regime below T* and above T_c. We report compelling evidence of the anomalous anticorrelation of the coherence volume with the peak maximum amplitude of the CDW XRD satellite by cooling below T*. This anomalous temperature trend of the short-range striped Jahn Teller polaronic CDW puddles is in agreement with predictions of the Q-ball theory of the quark gluon plasma extended to cuprates, providing compelling evidence for non topological soliton puddles of striped condensate of pairs in the pseudogap phase. Full article

It is demonstrated, that recently proposed by the author Q-ball mechanism of the pseudogap state and high-Tc superconductivity in cuprates may be detected in micro X-ray diffraction, since it imposes inverse correlations between the size and scattering intensities of the Q-ball charge-density-wave (CDW) fluctuations in these compounds. The Q-ball charge Q gives the number of condensed elementary bosonic excitations in a CDW fluctuation of finite amplitude. The attraction between these excitations inside Euclidean Q-balls is self-consistently triggered by the simultaneous condensation of Cooper/local pairs. Euclidean Q-ball solutions, analogous to the famous Q-balls of squarks in the supersymmetric standard model, arise due to the global invariance of the effective theory under the $U\left(1\right)$ phase rotation of the Fourier amplitudes of the short-range CDW fluctuations. A conserved ‘Noether charge’ Q along the Matsubara time axis equals $Q\propto T{M}^{2}V$, where the temperature T, Q-ball’s volume V, and fluctuation amplitude M enter. Several predictions are derived in an analytic form that follow from this picture. The conservation of the charge Q leads to an inverse proportionality between the volume V and X-ray scattering intensity ∼$M^2$ of the CDW puddles found in micro X-ray scattering experiments. The theoretical temperature dependences of the most probable Q value of superconducting Q-balls and their size and scattering amplitudes fit well the recent X-ray diffraction data in the pseudogap phase of high-Tc cuprates. Full article

We present a numerical study on the non-Fermi liquid behavior of a three-dimensional strongly correlated system. The Hubbard model in a simple cubic lattice is simulated by the dynamical cluster approximation; in particular, the quasi-particle weight is calculated at finite dopings for a range of temperatures. By fitting the quasi-particle weight to the marginal Fermi liquid form at finite doping near the putative quantum critical point, we find evidence of a separatrix between Fermi liquid and non-Fermi liquid regions. Our results suggest that a marginal Fermi liquid and possibly a quantum critical point exist in the non-symmetry broken solution of the three-dimensional interacting electron systems. We also calculate the spectral function, close to the half-filling, and we obtain evidence of pseudogap. Full article

We show that magnetic monopoles appear naturally in granular quantum matter. Their condensation leads to a new state of matter, superinsulation, in which Cooper pairs are bound into purely electric pions by strings of electric flux. These electric flux tubes, the dual of Abrikosov vortices, prevent the separation of charge–hole pairs, thereby causing an infinite resistance, even at finite temperatures, the dual behaviour of superconductors. We will discuss the electric Meissner effect, asymptotic freedom and their measurements and describe the recent direct detection of a linear, confining potential by dynamic relaxation experiments. Finally, we consider dyons, excitations carrying both a magnetic and an electric charge, and show that a condensate of such dyons leads to a possible solution of the mysteries of the pseudogap state of high-$T_c$ cuprates. Full article

We report the ground state stabilization and corresponding electrical transport and magnetotransport properties of a 2D metallic system with an isotropic Fermi surface reconstructed by a charge density wave. The onset of the charge density wave is a spontaneous process, stabilized by the condensation energy gain due to the self-consistent mechanism of topological reconstruction of the Fermi surface and opening of the pseudo-gap around it. We address the signature of the uni-axial reconstruction in terms of the measurable quantities, such as the intra-band transport properties, including the one-particle density of states, the total and effective concentration of electrons, and the Hall coefficient. Additionally, we analyze the magnetotransport properties of the system reconstructed by the bi-axial, checkerboard-like charge density wave, under conditions of magnetic breakdown. It manifests huge quantum oscillations in diagonal components of magnetoconductivity, while the Hall conductivity changes sign, varying the external magnetic field with a finite region of vanishing Hall coefficient in between. Full article

We investigated Heusler-type Ru₂VAl, a candidate material for next-generation thermoelectric conversion, by first-principle calculations of its thermoelectric conversion properties and direct experimental observations of its electronic structures, employing photoemission and infrared spectroscopy. Our results show that Ru₂VAl has a wider pseudogap near the Fermi level compared to Fe₂VAl. Accordingly, a higher thermoelectric conversion performance can be expected in Ru₂VAl at higher temperatures. Full article