Employing inelastic x-ray scattering and neutron scattering techniques, we observed nematic and magnetic phase transitions with distinct characters in K5Fe4Ag6Te10. Upon cooling, the nematic order undergoes a strongly first-order phase transition followed by a magnetic transition at TN approximate to 34.7 K that is consistent with a second-order transition. The temperature difference between these two phase transitions is similar to 1.0(8) K. The observed phenomenon can be attributed to a distinctive first-order preemptive Ising-nematic transition, a characteristic unique to a quasi-two-dimensional scenario marked by strong out-of-plane spatial anisotropy due to weak coupling. Our studies establish K5Fe4Ag6Te10 as the first material in the family of iron pnictides and chalcogenides that possesses a nematic tricritical point preceding the magnetic one upon decreasing nematic coupling.

Based on a sample of (2712.4 ± 14.3) × 106 ψ(3686) events collected with the BESIII detector, a partial wave analysis of the decay ψ (3686)→ΛΣ^0π^0 + c.c. is performed to investigate Λ* and Σ* resonances in the π^0Σ^0 and π0Λ invariant mass distributions. Significant contributions are found from the Λ(1405), Λ(1520), Λ(1600), Λ(1670), Λ(1690), Λ(1800), Λ(1890), Λ(2325), Σ(1385), Σ(1660), Σ(1670), Σ(1750), and Σ(1910). The masses, widths, and production branching fractions for each component are determined. In addition, the branching fraction of ψ (3686)→ΛΣ^0π^0 + c.c. is measured to be (1.544 ± 0.013 ± 0.071) × 10−4 for the first time, where the first uncertainty is statistical and the second systematic.

The magnetic Weyl semimetal Co$_3$Sn$_2$S$_2$ is extensively investigated due to its giant anomalous Hall effect (AHE).Recent studies demonstrate that the AHE can be effectively tuned by multi-electron Ni doping.To reveal the underlying mechanism of this significant manipulation,it is crucial to explore the band structure modification caused by Ni doping. Here,we study the electrodynamics of both pristine and Ni-doped Co$_{3-x}$Ni$_x$Sn$_2$S$_2$ with $x=$0, 0.11 and 0.17 by infrared spectroscopy. We find that the inverted energy gap around the Fermi level($E_{F}$) gets smaller at $x=$0.11,which is supposed to enhance the Berry curvature and therefore increase the AHE.Then $E_{F}$ moves out of this gap at $x=$0.17. Additionally,the low temperature carrier density is demonstrated to increase monotonically upon doping,which is different from previous Hall measurement results. We also observe the evidences of band broadening and exotic changes of high-energy interband transitions caused by doping.Our results provide detailed information about the band structure of Co$_{3-x}$Ni$_x$Sn$_2$S$_2$ at different doping levels,which will help to guide further studies on the chemical tuning of AHE.

By analyzing (27.12 +/- 0.14) x 10(8) psi(3686) events accumulated with the BESIII detector, the decay eta(c)(2S) -> K+K-eta is observed for the first time with a significance of 6.2 sigma after considering systematic uncertainties. The product of the branching fractions of psi(3686) -> gamma eta(c)(2S) and eta(c)(2S) -> K+K-eta is measured to be B(psi(3686) -> gamma eta(c)(2S)) x B(eta(c)(2S) -> K+K-eta) = (2.39 +/- 0.32 +/- 0.34) x 10(-6), where the first uncertainty is statistical, and the second one is systematic. The branching fraction of eta(c)(2S) -> K+K-eta is determined to be B(eta(c)(2S) -> K+K-eta) = (3.42 +/- 0.46 +/- 0.48 +/- 2.44) x 10(-3), where the third uncertainty is due to the branching fraction of psi(3686) -> gamma eta(c)(2S). Using a recent BESIII measurement of B(eta(c)(2S) -> K+K-pi(0)+, we also determine the ratio between the branching fractions of eta(c)(2S) -> K+K-eta and eta(c)(2S) -> K+K- pi(0)to be 1.49 +/- 0.22 +/- 0.25, which is consistent with the previous result of BABAR at a comparable precision level.

Using (2.712±0.014)×10^9 ψ(3686) events collected with the BESIII detector operating at the BEPCII, we find an evidence of the η_c(2S)→ K^+ K^- η^' decay with a statistical significance of 3.1σ. Its decay branching fraction is measured to be (12.24±4.60(stat.)±2.37(syst.)±4.68(extr.))× 10^-4, where the first uncertainty is statistical, the second is systematic, and the third uncertainty is from the branching fraction of the ψ(3686)→γη_c(2S) decay. The upper limit on the product branching fraction B[ψ(3686)→γη_c(2S)] × B[η_c(2S)→ K^+ K^- η^'] is set to be 1.14 × 10^-6 at 90% confidence level. In addition, the branching fractions of χ_c1→ K^+ K^- η^' and χ_c2→ K^+ K^- η^' are updated to be (8.47±0.09(stat.)±0.47(syst.))× 10^-4 and (1.53±0.04(stat.)±0.08(syst.))× 10^-4, respectively. The precision is improved by twofold.

The three-dimensional Weyl semimetal Mn3Sn has attracted tremendous attention due to its great application potential. However, the complex magnetic structures at different temperature intervals make it extremely difficult to unravel the underlying electronic structures of Mn3Sn. Here, we perform temperature-dependent optical spectroscopy measurements on single-crystalline Mn3Sn to investigate its charge dynamics. We find that both the optical reflectivity R(omega) and conductivity sigma 1(omega) evolve very smoothly across the magnetic phase transition at TM = 285 K, where the giant anomalous Hall effect (AHE) at room temperature drops significantly. Furthermore, linearly increasing segments of sigma 1(omega) are observed in the whole temperature range from 300 K to 10 K, indicating that the existence of Weyl fermions is very robust against the magnetic phase transition. In addition, the Weyl points closest to the Fermi level EF are identified to be located about 101 meV away from EF at 10 K, and the associated Fermi velocity is about 2.50 x 107 cm/s. Our results reveal that the phase transition at TM only generates subtle modification to the band structure, which helps to further uncover the mechanism of the dramatic change of AHE in Mn3Sn.

HoSbTe was predicted to be a weak topological insulator whose spin-orbit coupling (SOC) gaps are reported to be as large as hundreds of meV. Utilizing infrared spectroscopy, we find that the compound has a metallic nature from 350 down to 10 K. Particularly, both its itinerant carrier density and scattering rate are demonstrated to decrease with temperature cooling, which is responsible for the appearance of a broad hump feature in the temperature-dependent resistivity around 200 K. More importantly, we reveal the appearance of a charge-density-wave (CDW) gap whose energy scale is identified to shift from 364 meV at 10 K to 252 meV at 350 K. An SOC-induced band splitting is observed as well, and its coexistence with the CDW gap in the same compound paves the way to explore more intriguing physics.

Utilizing femtosecond time-resolved pump probe spectroscopy, we investigated the ultrafast quasiparticle dynamics of the ferromagnetic Kondo lattice compound CeAgSb2. The photoinduced reflectivity demonstrates a positive relaxation process connected with simple metal physics at room temperature, while an additional negative decay channel associated with Kondo screening effect can be recognized immediately with temperature decreasing. The temperature-dependent amplitude of the latter exhibits a kink at T* approximate to 45 K, which is close to the energy scale of the first excited crystal field splitting Ce 4f level. We propose that an orbital crossover is responsible for this intriguing feature. The dominating 4f orbitals that participated in the Kodno c-f hybridization varies from the ground state below T* to multiple crystal field splitting levels. In the meantime, the hybridization gap changes from 12.6 to 220.4 K correspondingly, which is also in agreement with the fluence-dependent measurements.

The presence of magnetic ions was first believed to be detrimental to superconductivity. However, unconventional superconductivity has been widely induced by doping or applying external pressure in magnetic systems such as heavy fermion, cuprate and iron-based superconductors in which magnetic fluctuations are suggested to serve as the pairing glue for Cooper pairs. The discovery of superconductivity in the magnetic compounds CrAs and MnP under high pressures has further expanded this family of superconductors and provided new platforms for investigating the interplay between magnetism and superconductivity. CrAs and MnP represent the first superconductors among the transition metal Cr- and Mn-based compounds in which the electronic states near the Fermi level are dominated by Cr/Mn 3d electrons. Shortly after their discovery, new types of Cr-based quasi-one-dimensional superconductors A2Cr3As3 and ACr3As3 (A [Formula: see text] K, Rb, Cs or Na) were discovered at ambient pressure. The close proximity of superconductivity to magnetic instability in these systems suggests that spin fluctuations may play crucial roles in mediating the Cooper pairing. In this article we review the basic physical properties of these novel superconductors and the progress achieved in recent studies.

We report single crystal growth and physical properties characterization of YbFe_2Al_10 compounds. The measurements of resistivity, magnetic susceptibility, and specific heat show different behaviors from previous studies on polycrystal samples. A mixed valent characteristic with moderate mass enhancement is indicated. In particular, the optical spectroscopy measurement reveals formation of multiple hybridization energy gaps which become progressively pronounced at low temperature. The multiple hybridization energy gaps are likely caused by the hybridizations between the flat band from Yb 4f electrons and different bands of conduction electrons.

Three-dimensional topological insulators (3D TIs) represent states of quantum matters in which surface states are protected by time-reversal symmetry and an inversion occurs between bulk conduction and valence bands. However, the bulk-band inversion, which is intimately tied to the topologically nontrivial nature of 3D Tis, has rarely been investigated by experiments. Besides, 3D massive Dirac fermions with nearly linear band dispersions were seldom observed in TIs. Recently, a van der Waals crystal, ZrTe5, was theoretically predicted to be a TI. Here, we report an infrared transmission study of a high-mobility [∼33,000 cm2/(V ⋅ s)] multilayer ZrTe5 flake at magnetic fields (B) up to 35 T. Our observation of a linear relationship between the zero-magnetic-field optical absorption and the photon energy, a bandgap of ∼10 meV and a [Formula: see text] dependence of the Landau level (LL) transition energies at low magnetic fields demonstrates 3D massive Dirac fermions with nearly linear band dispersions in this system. More importantly, the reemergence of the intra-LL transitions at magnetic fields higher than 17 T reveals the energy cross between the two zeroth LLs, which reflects the inversion between the bulk conduction and valence bands. Our results not only provide spectroscopic evidence for the TI state in ZrTe5 but also open up a new avenue for fundamental studies of Dirac fermions in van der Waals materials.

Using infrared spectroscopy and ultrafast pump probe measurement, we have studied the two charge-density-wave (CDW) instabilities in the layered compound LaAgSb_{2}. The development of CDW energy gaps was clearly observed by optical spectroscopy, which removed most of the free carrier spectral weight. More interestingly, our time-resolved measurements revealed two coherent oscillations that softened by approaching the two phase transition temperatures, respectively. We addressed that these two oscillations come from the amplitude modes of CDW collective excitations, the surprisingly low energies (0.12 THz and 0.34 THz for the higher and lower temperature ones, respectively) of which are associated with the extremely small nesting wave vectors. Additionally, the amplitude and relaxation time of photoinduced reflectivity of LaAgSb_{2} single crystals stayed unchanged across the CDW phase transitions, which is quite rare and deserves further investigation.

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 $T_v$.

Received 19 June 2017DOI:https://doi.org/10.1103/PhysRevB.95.239907©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasKondo effectOptical conductivityCondensed Matter, Materials & Applied Physics

We report single crystal growth and physical properties characterization of YbFe$_2$Al$_{10}$ compounds. The measurements of resistivity, magnetic susceptibility, and specific heat show different behaviors from previous studies on polycrystal samples. A mixed valent characteristic with moderate mass enhancement is indicated. In particular, the optical spectroscopy measurement reveals formation of multiple hybridization energy gaps which become progressively pronounced at low temperature. The multiple hybridization energy gaps are likely caused by the hybridizations between the flat band from Yb 4$f$ electrons and different bands of conduction electrons.

Three dimensional (3D) topological Dirac materials are under intensive study recently. The layered compound ZrTe_5 has been suggested to be one of them by transport and ARPES experiments. Here, we perform infrared reflectivity measurement to investigate the underlying physics of this material. The derived optical conductivity exhibits linear increasing with frequency below normal interband transitions, which provides the first optical spectroscopic proof of a 3D Dirac semimetal. Apart from that, the plasma edge shifts dramatically to lower energy upon temperature cooling, which might be associated with the consequence of lattice parameter shrinking. In addition, an extremely sharp peak shows up in the frequency dependent optical conductivity, indicating the presence of a Van Hove singularity in the joint density of state.

RuP single crystals of MnP-type orthorhombic structure were synthesized by the Sn flux method. Temperature-dependent x-ray diffraction measurements reveal that the compound experiences two structural phase transitions, which are further confirmed by enormous anomalies shown in temperature-dependent resistivity and magnetic susceptibility. Particularly, the resistivity drops monotonically upon temperature cooling below the second transition, indicating that the material shows metallic behavior, in sharp contrast with the insulating ground state of polycrystalline samples. Optical conductivity measurements were also performed in order to unravel the mechanism of these two transitions. The measurement revealed a sudden reconstruction of band structure over a broad energy scale and a significant removal of conducting carriers below the first phase transition, while a charge-density-wave-like energy gap opens below the second phase transition.

We present a magnetoinfrared spectroscopy study on a newly identified three-dimensional (3D) Dirac semimetal ZrTe5. We observe clear transitions between Landau levels and their further splitting under a magnetic field. Both the sequence of transitions and their field dependence follow quantitatively the relation expected for 3D massless Dirac fermions. The measurement also reveals an exceptionally low magnetic field needed to drive the compound into its quantum limit, demonstrating that ZrTe5 is an extremely clean system and ideal platform for studying 3D Dirac fermions. The splitting of the Landau levels provides direct, bulk spectroscopic evidence that a relatively weak magnetic field can produce a sizable Zeeman effect on the 3D Dirac fermions, which lifts the spin degeneracy of Landau levels. Our analysis indicates that the compound evolves from a Dirac semimetal into a topological line-node semimetal under the current magnetic field configuration.