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Register a new userNovel $J_{rm{eff}}$=3/2 Metallic Phase and Unconventional Superconductivity in GaTa$_4$Se$_8$
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By means of density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations and resonant inelastic x-ray scattering (RIXS) experiments, we investigate the high-pressure phases of the spin-orbit-coupled $J_{rm{eff}}=3/2$ insulator GaTa$_4$Se$_8$. Its metallic phase, derived from the Mott state by applying pressure, is found to carry $J_{rm{eff}}=3/2$ moments. The characteristic excitation peak in the RIXS spectrum maintains its destructive quantum interference of $J_{rm{eff}}$ at the Ta $L_2$-edge up to 10.4 GPa. Our exact diagonalization based DFT+DMFT calculations including spin-orbit coupling also reveal that the $J_{rm{eff}}=3/2$ character can be clearly identified under high pressure. These results establish the intriguing nature of the correlated metallic magnetic phase, which represents the first confirmed example of $J_{rm{eff}}$=3/2 moments residing in a metal. They also indicate that the pressure-induced superconductivity is likely unconventional and influenced by these $J_{rm{eff}}=3/2$ moments. Based on a self-energy analysis, we furthermore propose the possibility of doping-induced superconductivity related to a spin-freezing crossover.
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GaTa$_4$Se$_8$ belongs to the lacunar spinel family. Its crystal structures is still a puzzle though there have been intensive studies on its novel properties, such as the Mott insulator phase and superconductivity under pressure. In this work, we investigate its phonon spectra through first-principle calculations and proposed it most probably has crystal structure phase transition, which is consistent with several experimental observations. For the prototype lacunar spinel with cubic symmetry of space group $Fbar{4}3m$, its phonon spectra have three soft modes in the whole Brillouin zone, indicating the strong dynamical instability of such crystal structure. In order to find the dynamically stable crystal structure, further calculations indicate two new structures of GaTa$_4$Se$_8$, corresponding to $R3m$ and $Pbar{4}2_{1}m$, verifying that at the ambient pressure, there does exist structure phase transition of GaTa$_4$Se$_8$ from $Fbar{4}3m$ to other structures when the temperature is lowered. We also performed electronic structure calculation for $R3m$ and $Pbar{4}2_{1}m$ structure, showing that $Pbar{4}2_{1}m$ structure GaTa$_4$Se$_8$ is band insulator, and obtained Mott insulator state for $R3m$ structure by DMFT calculation under single-band Hubbard model picture when interaction parameter U is larger than 0.40 eV vs. band width of 0.25 eV. It is reasonable to assume that while lowering the temperature, $Fbar{4}3m$ structure GaTa$_4$Se$_8$ becomes $R3m$ structure GaTa$_4$Se$_8$ first, then $Pbar{4}2_{1}m$ structure GaTa$_4$Se$_8$, because of the symmetry of $Pbar{4}2_{1}m$ is lower than $R3m$ after Jahn-Teller distortion. The structure transition may explain the magnetic susceptibility anomalous at low temperature.
Heavy transition metal magnets with $J_{rm eff}$ $=$ 1/2 electronic ground states have attracted recent interest due to their penchant for hosting new classes of quantum spin liquids and superconductors. Unfortunately, model systems with ideal $J_{rm eff}$ $=$ 1/2 states are scarce due to the importance of non-cubic local distortions in most candidate materials. In this work, we identify a family of iridium halide systems [i.e. K$_2$IrCl$_6$, K$_2$IrBr$_6$, (NH$_4$)$_2$IrCl$_6$, and Na$_2$IrCl$_6 cdotp $ 6(H$_2$O)] with Ir$^{4+}$ electronic ground states in extremely close proximity to the ideal $J_{rm eff}$ $=$ 1/2 limit, despite a variation in the low-temperature global crystal structures. We also find ordered magnetic ground states for the three anhydrous systems, with single crystal neutron diffraction on K$_2$IrBr$_6$ revealing Type-I antiferromagnetism. This spin configuration is consistent with expectations for significant Kitaev exchange in a face-centered-cubic magnet.
Superconductivity without phonons has been proposed for strongly correlated electron materials that are tuned close to a zero-temperature magnetic instability of itinerant charge carriers. Near this boundary, quantum fluctuations of magnetic degrees of freedom assume the role of phonons in conventional superconductors, creating an attractive interaction that glues electrons into superconducting pairs. Here we show that superconductivity can arise from a very different spectrum of fluctuations associated with a local or Kondo-breakdown quantum-critical point that is revealed in isotropic scattering of charge carriers and a sub-linear temperature-dependent electrical resistivity. At this critical point, accessed by applying pressure to the strongly correlated, local-moment antiferromagnet CeRhIn5, magnetic and charge fluctuations coexist and produce electronic scattering that is maximal at the optimal pressure for superconductivity. This previously unanticipated source of pairing glue opens possibilities for understanding and discovering new unconventional forms of superconductivity.
We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected at Tsc=1.65K as sharp drop of the resistivity in a high quality sample of RRR=35. The specific heat shows a large jump at Tsc indicating strong coupling. The large Sommerfeld coefficient, 117mJ K-2mol-1 extrapolated in the normal state and the temperature dependence of C/T below Tsc are the signature of unconventional SC. The discrepancy in the entropy balance at Tsc between SC and normal states points out that hidden features must occur. Surprisingly, a large residual value of the Sommerfeld coefficient seems quite robust (gamma_0/gamma ~ 0.5). The large upper critical field Hc2 along the three principal axes favors spin-triplet SC. For H // b-axis, our experiments do not reproduce the huge upturn of Hc2 reported previously. This discrepancy may reflect that Hc2 is very sensitive to the sample quality. A new perspective in UTe2 is the proximity of a Kondo semiconducting phase predicted by the LDA band structure calculations.
Transition metal oxides exhibit various competing phases and exotic phenomena depending on how their reaction to the rich degeneracy of the $d$-orbital. Large spin-orbit coupling (SOC) reduces this degeneracy in a unique way by providing a spin-orbital-entangled ground state for 4$d$ and 5$d$ transition metal compounds. In particular, the spin-orbital-entangled Kramers doublet, known as the $J_{mathbf{eff}}$=1/2 pseudospin, appears in layered iridates and $alpha$-RuCl$_3$, manifesting a relativistic Mott insulating phase. Such entanglement, however, seems barely attainable in 3$d$ transition metal oxides, where the SOC is small and the orbital angular momentum is easily quenched. From experimental and theoretical evidence, here we report on the CuAl$_2$O$_4$ spinel as the first example of a $J_{mathbf{eff}}$=1/2 Mott insulator in 3$d$ transition metal compounds. Based on the experimental study, including synthesis of the cubic CuAl$_2$O$_4$ single crystal, density functional theory and dynamical mean field theory calculations reveal that the $J_{mathbf{eff}}$=1/2 state survives the competition with an orbital-momentum-quenched $S$=1/2 state. The electron-addition spectra probing unoccupied states are well described by the $j_{mathbf{eff}}$=1/2 hole state, whereas electron-removal spectra have a rich multiplet structure. The fully relativistic entity found in CuAl$_2$O$_4$ provides new insight into the untapped regime where the spin-orbital-entangled Kramers pair coexists with strong electron correlation.
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