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Register a new userContrasting pressure evolutions of $f$ electron hybridized states in CeRhIn$_5$ and YbNi$_3$Ga$_9$: an optical conductivity study
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Optical conductivity [$sigma(omega)$] of CeRhIn$_5$ and YbNi$_3$Ga$_9$ have been measured at external pressures to 10 GPa and at low temperatures to 6 K. Regarding CeRhIn$_5$, at ambient pressure the main feature in $sigma(omega)$ is a Drude peak due to free carriers. With increasing pressure, however, a characteristic mid-infrared (mIR) peak rapidly develops in $sigma(omega)$, and its peak energy and width increase with pressure. These features are consistent with an increased conduction ($c$)-$f$ electron hybridization at high pressure, and show that the pressure has tuned the electronic state of CeRhIn$_5$ from very weakly to strongly hybridized ones. As for YbNi$_3$Ga$_9$, in contrast, a marked mIR peak is observed already at ambient pressure, indicating a strong $c$-$f$ hybridization. At high pressures, however, the mIR peak shifts to lower energy and becomes diminished, and seems merged with the Drude component at 10 GPa. Namely, CeRhIn$_5$ and YbNi$_3$Ga$_9$ exhibit some opposite tendencies in the pressure evolutions of $sigma(omega)$ and electronic structures. These results are discussed in terms of the pressure evolutions of $c$-$f$ hybridized electronic states in Ce and Yb compounds, in particular in terms of the electron-hole symmetry often considered between Ce and Yb compounds.
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A key issue in heavy fermion research is how subtle changes in the hybridization between the 4$f$ (5$f$) and conduction electrons can result in fundamentally different ground states. CeRhIn$_5$ stands out as a particularly notable example: replacing Rh by either Co or Ir, located above or below Rh in the periodic table, antiferromagnetism gives way to superconductivity. In this photoemission study of CeRhIn$_5$, we demonstrate that the use of resonant ARPES with polarized light allows to extract detailed information on the 4$f$ crystal field states and details on the 4$f$ and conduction electron hybridization which together determine the ground state. We directly observe weakly dispersive Kondo resonances of $f$-electrons and identify two of the three Ce $4f_{5/2}^{1}$ crystal-electric-field levels and band-dependent hybridization, which signals that the hybridization occurs primarily between the Ce $4f$ states in the CeIn$_3$ layer and two more three-dimensional bands composed of the Rh $4d$ and In $5p$ orbitals in the RhIn$_2$ layer. Our results allow to connect the properties observed at elevated temperatures with the unusual low-temperature properties of this enigmatic heavy fermion compound.
Optical conductivity [$sigma(omega)$] of YbCu$_2$Ge$_2$ has been measured at external pressures ($P$) to 20 GPa, to study the $P$ evolution of $f$ electron hybridized states. At $P$=0, $sigma(omega)$ shows a marked mid-infrared (mIR) peak at 0.37 eV, which is due to optical excitations from $f^{14}$ (Yb$^{2+}$) state located below the Fermi level. With increasing $P$, the mIR peak shows significant shifts to lower energy, reaching 0.18 eV at $P$=20 GPa. This result indicates that the $f^{14}$ energy level increases toward the Fermi level with $P$. Such a shift of the $f$ electron level with $P$ has been expected from theoretical considerations, but had never been demonstrated by spectroscopic experiment under high $P$. The obtained results are also analyzed in terms of the $P$ evolution of the conduction-$f$ electron hybridization.
We discuss recent results on the heavy fermion superconductor CeRhIn$_5$ which presents ideal conditions to study the strong coupling between the suppression of antiferromagnetic order and the appearance of unconventional superconductivity. The appearance of superconductivity as function of pressure is strongly connected to the suppression of the magnetic order. Under magnetic field, the re-entrance of magnetic order inside the superconducting state shows that antiferromagnetism nucleates in the vortex cores. The suppression of antiferromagnetism in CeRhIn$_5$ by Sn doping is compared to that under hydrostatic pressure.
CeRhIn$_5$ provides a textbook example of quantum criticality in a heavy fermion system: Pressure suppresses local-moment antiferromagnetic (AFM) order and induces superconductivity in a dome around the associated quantum critical point (QCP) near $p_{c} approx 23,$kbar. Strong magnetic fields also suppress the AFM order at a field-induced QCP at $B_{rm c}approx 50,$T. In its vicinity, a nematic phase at $B^*approx 28,$T characterized by a large in-plane resistivity anisotropy emerges. Here, we directly investigate the interrelation between these phenomena via magnetoresistivity measurements under high pressure. As pressure increases, the nematic transition shifts to higher fields, until it vanishes just below $p_{rm c}$. While pressure suppresses magnetic order in zero field as $p_{rm c}$ is approached, we find magnetism to strengthen under strong magnetic fields due to suppression of the Kondo effect. We reveal a strongly non-mean-field-like phase diagram, much richer than the common local-moment description of CeRhIn$_5$ would suggest.
We present core level non-resonant inelastic x-ray scattering (NIXS) data of the heavy fermion compounds CeCoIn$_5$ and CeRhIn$_5$ measured at the Ce $N_{4,5}$-edges. The higher than dipole transitions in NIXS allow determining the orientation of the $Gamma_7$ crystal-field ground-state orbital within the unit cell. The crystal-field parameters of the Ce$M$In$_5$ compounds and related substitution phase diagrams have been investigated in great detail in the past; however, whether the ground-state wavefunction is the $Gamma_7^+$ ($x^2,-,y^2$) or $Gamma_7^-$ ($xy$ orientation) remained undetermined. We show that the $Gamma_7^-$ doublet with lobes along the (110) direction forms the ground state in CeCoIn$_5$ and CeRhIn$_5$. For CeCoIn$_5$, however, we find also some contribution of the first excited state crystal-field state in the ground state due to the stronger hybridization of 4$f$ and conduction electrons, suggesting a smaller $alpha^2$ value than originally anticipated from x-ray absorption. A comparison is made to the results of existing density functional theory plus dynamical mean-field theory calculations.
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