Helicity‐dependent THz emission induced by ultrafast spin photocurrent in nodal‐line semimetal candidate Mg3Bi2

Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements,such as quantum communications,on-chip communication processing and storage.Here,we present the manipulation of helicity-dependent terahertz emission generated in a nodal line semimetal candidate Mg3Bi2 by using photon polarization states.The terahertz emission is mainly ascribed to the helicity-dependent photocurrent that is originated from circular photogalvanic effects,and the helicity-independent photocurrent that is attributed to linear photogalvanic effect.Our work will inspire more explorations into novel nodal line semimetals and open up new opportunities for developing ultrafast optoelectronics in the topological system.

Direct observation of the f–c hybridization in the ordered uranium films on W(110)

A key issue in metallic uranium and its related actinide compounds is the character of the f electrons, whether it is localized or itinerant.Here we grew well ordered uranium films on a W(110) substrate.The surface topography was investigated by scanning tunneling microscopy.The Fermi surface and band structure of the grown films were studied by angle-resolved photoemission spectroscopy.Large spectral weight can be observed around the Fermi level, which mainly comes from the f states.Additionally, we provided direct evidence that the f bands hybridize with the conduction bands in the uranium ordered films, which is different from previously reported mechanism of the direct f–f interaction.We propose that the above two mechanisms both exist in this system by manifesting themselves in different momentum spaces.Our results give a comprehensive study of the ordered uranium films and may throw new light on the study of the 5 f-electron character and physical properties of metallic uranium and other related actinide materials.

Electrical transport properties of cerium doped Bi_(2)Te_(3)thin films grown by molecular beam epitaxy

Introducing magnetism into topological insulators(TIs)can tune the topological surface states and produce exotic physical effects.Rare earth elements are considered as important dopant candidates,due to their large magnetic moments from heavily shielded 4f electrons.As the first element with just one 4f electron,cerium(Ce)offers an ideal platform for exploring the doping effect of f-electron in TIs.Here in this work,we have grown cerium-doped topological insulator Bi_(2)Te_(3)thin films on an Al_(2)O_(3)(0001)substrate by molecular beam epitaxy(MBE).Electronic transport measurements revealed the Kondo effect,weak anti-localization(WAL)effect and suppression of surface conducting channels by Ce doping.Our research shows the funda-mental doping effects of Ce in Bi_(2)Te_(3)thin films,and demonstrates that such a system could be a good platform for further re-search.

Atomically Smooth Ultrathin Films of Topological Insulator Sb_(2)Te_(3)

The growth and characterization of single-crystalline thin films of topological insulators(TIs)is an important step towards their possible applications.Using in situ scanning tunneling microscopy(STM)and angle-resolved photoemission spectroscopy(ARPES),we show that moderately thick Sb_(2)Te_(3)films grown layer-by-layer by molecular beam epitaxy(MBE)on Si(111)are atomically smooth,single-crystalline,and intrinsically insulating.Furthermore,these films were found to exhibit a robust TI electronic structure with their Fermi energy lying within the energy gap of the bulk that intersects only the Dirac cone of the surface states.Depositing Cs in situ moves the Fermi energy of the Sb_(2)Te_(3)films without changing the electronic band structure,as predicted by theory.We found that the TI behavior is preserved in Sb_(2)Te_(3)films down to five quintuple layers(QLs).

In situ Raman spectroscopy of topological insulator Bi2Te3 films with varying thickness

Topological insulators （TIs） are a new state of quantum matter with a band gap in bulk and conducting surface states. In this work, the Raman spectra of topological insulator Bi2Te3 films prepared by molecular beam epitaxy （MBE） have been measured by an in situ ultrahigh vacuum （UHV）-MBE-Raman spectroscopy system. When the thickness of Bi2Te3 films decreases from 40 quintuple-layers （QL） to 1 QL, the spectral characteristics of some Raman modes appearing in bulk Bi2Te3 vary and a new vibrational mode appears, which has not been reported in previous studies and might be related to quantum size effects and symmetry breaking. In addition, an obvious change was observed at 3 QL when a Dirac cone formed. These results offer some new information about the novel quantum states of TIs.

Electronic structure evolution accompanying heavy fermion formation in CeCu2Si2

The cooper pairs in the heavy-fermion superconductor CeCu2Si2 are formed of heavy fermions.Therefore,the heavy fermions are fundamental to the emergence of unconventional superconductivity and associated non-Fermi-liquid behavior in the normal state.The interplay between localization and itinerancy manifested on the electronic structure is key for understanding the heavyfermion behavior.Here,via the first-principle density functional theory(DFT)combined with single-site dynamical mean-field theory(DMFT),we investigate the temperature(T)evolution of the electronic structure of CeCu2Si2 in the normal state,focusing on the role of the 4f states in the low energy regime.Two characteristic temperature scales of this evolution,which accompanied the heavy-fermion formation,are established.The coherence onset temperature is around 130K,whereas the heavy-fermion band formation temperature is between 40 and 80K;both characteristic temperature scales are higher than the transport coherence temperature.Furthermore,the heavy-fermion formation is confirmed by calculating its effective mass variation with the temperature.Based on the calculated T-dependent evolution of the 4 f orbital occupancy and electronic structure,an explanation on the behavior of the temperature evolution of the correlation strength of CeCu2Si2 is provided.Our results offer a comprehensive microscopic picture of the heavy-fermion formation in CeCu2Si2,which is essential for further understanding the emergent superconducting pairing mechanism.