Hybridization Energy Gap Research Articles

Pressure-driven change of ground state of Ce3Pd3Bi4 : A DFT+DMFT study

A candidate material for strongly correlated topological materials, ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{3}{\mathrm{Bi}}_{4}$ has attracted much attention, but its ground state remains controversial. Compared with the typical Kondo insulator ${\mathrm{Ce}}_{3}{\mathrm{Pt}}_{3}{\mathrm{Bi}}_{4}$, two possibilities of ground states are proposed: ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{3}{\mathrm{Bi}}_{4}$ is either a spin--orbit-driven topological semimetal or a Kondo insulator with less Kondo coupling strength than platinum. Here, we performed density functional theory $(\mathrm{DFT})+\mathrm{dynamical}$ mean field theory (DMFT) calculations on ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{3}{\mathrm{Bi}}_{4}$ under different pressures to clarify its ground state, as pressure can tune the strength of Kondo coupling without affecting the strength of spin-orbit coupling. We found ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{3}{\mathrm{Bi}}_{4}$ has a metallic ground state and becomes insulating with increasing pressure at a low temperature. And as the pressure increased to 2 GPa, a hybridization energy gap can be observed at 10 K. As the pressure increased to 5 GPa, the electronic structure of ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{3}{\mathrm{Bi}}_{4}$ is even similar to that of the Kondo insulator ${\mathrm{Ce}}_{3}{\mathrm{Pt}}_{3}{\mathrm{Bi}}_{4}$ under ambient pressure, and a clear hybridization energy gap $(\ensuremath{\sim}3\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$ appears at 20 K. Our results not only demonstrate that the key factor controlling the different ground states between the two compounds is Kondo physics, rather than spin-orbit coupling, but also confirm that ${\mathrm{Ce}}_{3}{\mathrm{Pd}}_{3}{\mathrm{Bi}}_{4}$ is an ideal material to tuning the ground states by changing the strength of hybridization by pressure.

Dramatic change of photoexcited quasiparticle relaxation dynamics across Yb valence state transition in YbInCu4

YbInCu$_4$ undergoes a first order structural phase transition near $T_v$=40 K associated with an abrupt change of Yb valence state. We perform ultrafast pump-probe measurement on YbInCu$_4$ and find that the expected heavy fermion properties arising from the \emph{c-f} hybridization exist only in a limited temperature range above $T_v$. Below $T_v$, the compound behaves like a normal metal though a prominent hybridization energy gap is still present in infrared measurement. We elaborate that those seemingly controversial phenomena could be well explained by assuming that the Fermi level suddenly shifts up and becomes far away from the flat \emph{f}-electron band as well as the indirect hybridization energy gap in the mixed valence state below $T_v$.

Quantum interference by localized scattering waves of gapless helical modes in narrow strips of topological insulators

Quantum interference in scattering from a potential offset is investigated in narrow strips of two-dimensional systems described by the Bernevig-Hughes-Zhang Hamiltonian. Attention is focused on the situations where the transmission in the scattering region takes place around the Dirac point of topological insulators when the hybridization energy gap is eliminated by utilizing transverse interference. Apart from conventional periodic transmission modulation that takes place when the length of the potential offset region is varied, resonant disappearances of reflection occur for short potential offsets. The anomalous resonance appears not only for the four-band Hamiltonian but also for the two-band Hamiltonian, manifesting the generality of the phenomenon. Evanescent-like waves excited around the potential steps are indicated to be responsible for the anomalous behavior. The interference states can couple with each other and generic reduction in the amplitude of transmission modulation occurs upon coupling with the periodic modulation.

Klein tunneling of helical edge states in narrow strips of a two-dimensional topological insulator

The quantum transmission of helical edge states across a square potential barrier is numerically investigated in narrow channels of a two-dimensional topological insulator. Although the transmission probability in general decreases when a potential offset is introduced in the middle of the channels, the transmission remains almost perfect regardless of the amplitude and length of the potential offset when the hybridization energy gap is closed by tuning the off-diagonal spin–orbit terms in the effective four-band Hamiltonian. The approximate absence of scattering resembling the Klein tunneling, where the transmission is unimpeded as an electron propagates relativistically as a hole in the barrier without decay, improves further when an interference condition is satisfied within the barrier. The dependence of the residual reflection on the Fermi level reveals anomalous characteristics in the Klein tunneling regime.

Hybridization gap versus hidden-order gap in URu2Si2as revealed by optical spectroscopy

We present the in-plane optical reflectance measurement on single crystals of URu$_2$As$_2$. The study revealed a strong temperature-dependent spectral evolution. Above 50 K, the low frequency optical conductivity is rather flat without a clear Drude-like response, indicating a very short transport life time of the free carriers. Well below the coherence temperature, there appears an abrupt spectral weight suppression below 400 cm$^{-1}$, yielding evidence for the formation of a hybridization energy gap arising from the mixing of the conduction electron and narrow f-electron bands. A small part of the suppressed spectral weight was transferred to the low frequency side, leading to a narrow Drude component, while the majority of the suppressed spectral weight was transferred to the high frequency side centered near 4000 cm$^{-1}$. Below the hidden order temperature, another very prominent energy gap structure was observed, which leads to the removal of a large part of the Drude component and a sharp reduction of the carrier scattering rate. The study revealed that the hybridization gap and the hidden orger gap are distinctly different: they occur at different energy scales and exhibit completely different spectral characteristics.

Electronic structure of mixed-valence semiconductors in theLSDA+Uapproximation. II. SmB6andYbB12

The electronic structure and optical spectra of ${\mathrm{SmB}}_{6}$ and ${\mathrm{YbB}}_{12}$ are investigated theoretically from first principles, using the fully relativistic Dirac LMTO band structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as with the so-called $\mathrm{LSDA}+U$ approach. The energy band structures of ${\mathrm{SmB}}_{6}$ and ${\mathrm{YbB}}_{12}$ are related to their measured XPS and UPS spectra as well as optical spectra. The $\mathrm{LSDA}+U$ band structure calculations produce a small hybridization energy gap in ${\mathrm{SmB}}_{6}$ and ${\mathrm{YbB}}_{12}$ for samarium and ytterbium ions in the divalent state.

Impurity effect on the metal-insulator transition in Kondo insulators

In this paper we study the effect of transition impurities on the metal-insulator transition in Kondo insulators. We use a two-subband model Hamiltonian in which a strongly correlated electron narrow subband is coupled to a wide conduction-electron subband. In the model, the disorder is treated by using the off-diagonal coherent potential approximation with a simple parametrization of the electron energy hopping. We obtain a criterion for the opening of the hybridization energy gap as a function of impurity concentration. In the particular case of ${\mathrm{CeRh}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{x}\mathrm{Sb},$ the insulator-metal transition occurs at critical Pd concentration of 10%, in agreement with experimental data.

Observation of a hybridization energy gap in mixed-valent TmSe

Inelastic neutron scattering studies have been performed on single crystals of TmSe. An excitation at h..omega..approx.10 meV is observed whose momentum, temperature, and sample dependences provide evidence that this spectral feature is due to an excitation across a hybridization gap in TmSe.