Chien-Wen Chuang

We study the high Curie temperature ferromagnet (${\mathrm{Cr}}_{0.35}{\mathrm{Sb}}_{0.65}{)}_2\mathrm{Te}{}_3$ ($T_C=192\mathrm{K}$), using $T$-dependent x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and angle-resolved photoemission spectroscopy (ARPES). The $T$-dependent (25–220 K) XAS-XMCD evolution of $\mathrm{Cr}3d$ and $\mathrm{Te}5p$ unoccupied site- and orbital-projected states shows a systematic modification, which we interpret as due to spin-splitting below $T{}_C$. The $T$-dependent XMCD intensity and leading-edge spin-sensitive shifts ${\gamma }_{\text{expt}}\left(T\right)$ follow bulk magnetization. ARPES measurements with $h\nu =78\mathrm{eV}$ show a metallic state with Sb $5p$ band dispersions at and near Fermi level ($E_F$), consistent with bulk band-structure calculations for ${\left({\mathrm{Cr}}_{0.33}{\mathrm{Sb}}_{0.67}\right)}_2\mathrm{Te}{}_3$. However, surface-sensitive ARPES with $h\nu =8.4\mathrm{eV}$ above and below $T{}_C$ show linear band dispersions just below $E_F$, suggesting a remnant of Dirac-type dispersions. Assuming the linear dispersion survives above $E_F$, it implies a topologically trivial ferromagnet as the estimated Dirac point energy lies above the largest ${\gamma }_{\text{expt}}(T=25\mathrm{K})$. The $\mathrm{Cr}3d$ XAS-XMCD spectra can be simulated by charge transfer multiplet cluster model calculations with an exchange field $H_{\text{ex}}$ which quantitatively reproduces the experimental XMCD. At $T=25\mathrm{K}$, the required exchange field $H_{\text{ex}}$ of $\sim 48\mathrm{T}$ corresponds to a Zeeman energy $\zeta =2.8\mathrm{meV}<T_C=192\mathrm{K}$ (= 16.5 meV) $\ll {\gamma }_{\text{expt}}\sim 140\mathrm{meV}$. The results indicate the role of $\mathrm{Cr}3d$ exchange interactions in causing spin-sensitive shifts in $\mathrm{Cr}3d$ states, and inducing comparable spin-sensitive shifts via hybridization in $\mathrm{Te}5p$ states of ${\left({\mathrm{Cr}}_{0.35}{\mathrm{Sb}}_{0.65}\right)}_2\mathrm{Te}{}_3$.

We performed angle-resolved photoemission spectroscopy with microfocused beam on a topological line-nodal compound ${\mathrm{CaSb}}_2$ which undergoes a superconducting transition at the onset $T_{\mathrm{c}}\sim$ 1.8 K, to clarify the Fermi-surface topology relevant to the occurrence of superconductivity. We found that a three-dimensional hole pocket at the $\mathrm{\Gamma }$ point is commonly seen for two types of single-crystalline samples fabricated by different growth conditions. On the other hand, the carrier-doping level estimated from the position of the chemical potential was found to be sensitive to the sample fabrication condition. The cylindrical electron pocket at the $Y\left(C\right)$ point predicted by the calculations is absent in one of the two samples, despite the fact that both samples commonly show superconductivity with similar $T_{\mathrm{c}}$'s. This suggests a key role of the three-dimensional hole pocket to the occurrence of superconductivity, and further points to an intriguing possibility to control the topological nature of superconductivity by carrier tuning in ${\mathrm{CaSb}}_2$.

We study the electronic structure of single crystal $1T-{\mathrm{TiSe}}_2$ using temperature ($T$)-dependent soft x-ray photoemission spectroscopy (PES) and natural circular dichroism (NCD) across the charge-density-wave (CDW) transition at $T_{\mathrm{cdw}}\sim 200$ K. We investigate detailed changes in energy positions and widths of the Ti $2p$ and Se $3d$ core-level spectra across $T_{\mathrm{cdw}}$. The Ti $2p-3d$-resonant PES shows a clear Ti $3d$ character two-hole correlation satellite in the valence band. A Cini-Sawatzky analysis indicates an effective attractive two-hole Coulomb interaction energy $U_{dd}$ =−1.3 eV. This attractive Coulomb energy is significantly larger than the known energy scale of excitonic correlations ($<$ 100 meV) in $1T-{\mathrm{TiSe}}_2$. On-resonant $T$-dependent measurements indicate that the strongly hybridized Ti $3d$-Se $4p$ bands show increased hybridization in the CDW phase. On-resonant valence band spectra show a finite, possibly experimental geometry-related NCD of the states at and near the Fermi level which do not show $T$ dependence across $T_{\mathrm{cdw}}$. Given the presence of excitonic effects and the periodic lattice distortion, our results suggest that theoretical models need to take into account the attractive Coulomb interaction and hybridization changes for a complete understanding of the CDW in $1T-{\mathrm{TiSe}}_2$.

We study the electronic structure of GdNi and HoNi, which are magnetic materials with a Curie temperature $T_c=69$ and $T_c=36\mathrm{K}$, respectively. These materials are useful for magnetic refrigerator applications at low temperature as they exhibit a large magnetocaloric effect near $T_c$. We have used hard x-ray photoemission spectroscopy (HAXPES) to investigate the core-level and valence-band electronic states of GdNi and HoNi. HAXPES measurements of the Gd and Ho $3d, 4d, 4p, 5p$, and $4s$ core-level spectra have been compared with atomic multiplet calculations of ${\mathrm{Gd}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ ionic configurations. The good match between the experimental and calculated spectra clarify the important role of spin-orbit coupling, as well as Coulomb and exchange interactions in the intermediate-coupling scheme. The core-level spectra also show plasmons in addition to the atomic multiplets. The Gd and Ho $4s$ spectra show clear evidence of exchange splitting. The Ni $2p$ and $3s$ spectra of GdNi and HoNi show a correlation satellite at a binding energy of 7 eV above their main peaks. The Ni $2p$ and Ni $3s$ spectra could be reproduced using charge transfer multiplet calculations. Valence-band HAXPES of GdNi and HoNi shows that the Gd $4f$ and Ho $4f$ features are also consistent with atomic multiplets and occur at high binding energies away from the Fermi level. The Ni $3d$ density of states are spread from the Fermi level to about 3 eV binding energy. The results indicate a partially filled Ni $3d$ band and show that the charge transfer model is not valid for describing the electronic structure of GdNi and HoNi.

GdNi is a ferrimagnetic material with a Curie temperature $T_C=69$ K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at $T=25$ K in the ferrimagnetic phase. We analyze the Gd $M_{4,5}$-edge ($3d-4f$) and Ni $L_{2,3}$-edge ($2p-3d$) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd $M_{4,5}$-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized $4f$ states. On the other hand, a model cluster calculation for Ni $L_{2,3}$-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest-neighbor Gd states, resulting in a $d$-electron count of 8.57. The Gd $M_{4,5}$-edge XMCD spectrum is consistent with a ground-state configuration of $S=7/2$ and $L=0$. The Ni $L_{2,3}$-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite total magnetic moment ($m_{tot}\sim 0.12{\mu }_B$) with the ratio $m_o/m_s\sim 0.11$. Valence band hard x-ray photoemission spectroscopy shows Ni $3d$ features at the Fermi level, confirming a partially filled $3d$ band, while the Gd $4f$ states are at high binding energies away from the Fermi level. The results indicate that the Ni $3d$ band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge-transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni $3d$ and Gd $4f$ states.