Multipolar interactions and related ordering phenomena have attracted great interest. Multipoles are related to exotic phases including possible multipole-fluctuation induced superconductivity and hidden-order phase. For a systematic investigation of the collective behaviors of multipole moments, f-electron systems are suitable choices since the interplay of spin and orbital degrees of freedom of f-electrons facilitates multipole formation.

In this work, we report Raman-scattering results of YbRu2Ge2 single crystals. This heavy-fermion metal has been suggested to enter a ferroquadrupolar (FQ) phase below T0=10K. We determined the crystal-field (CF) level scheme of Yb3+ ground multiplet in YbRu2Ge2. Importantly, the lowest-energy CF transition couples to the quadrupolar fluctuations above T0. From symmetry analysis, we further suggest that the FQ order has B1g symmetry.

Our study demonstrates the ability of Raman-scattering technique to probe the symmetry of dynamical fluctuations above unusual electronic phase transitions. Moreover, determination of the CF level scheme, and identification of the symmetry of the FQ order of YbRu2Ge2 will not only serve as basis for future study of this model system, but also inspire more interest in multipole-related phenomena.
19-02-11_2

The figure above shows a schematic energy diagram of the CF states (red horizontal lines) and the phonon modes (blue horizontal lines). The coupled CF transition and phonon modes are circled in purple. On the left are the angular electron-cloud distributions of the four CF states; on the right are the vibration patterns of the phonon modes.

We are exceptionally proud to announce that Mai Ye has won the Peter Lindenfeld Graduate Fellowship. The fellowship, established to honor Professor Emeritus Lindenfeld, supports a Physics and Astronomy graduate student who exemplifies the teacher-scholar model of learning pioneered by Prof. Lindenfeld. We want to celebrate Mai for his excellence in both scholarship and mentorship!

We are proud to announce that Alex's poster, "Resonant Raman Scattering in the Giant Rashba System BiTeI", won the Leszek Wielunski Award for Best Poster Presentation at the 33rd Annual Symposium of the Laboratory for Surface Modification (LSM). Congrats to Alex for winning this prestigious award!

We are proud to announce that Sean Kung's dissertation, "Collective excitations in the antisymmetric channel of Raman spectroscopy", is the recipient of the 2019 Northeast Association of Graduate Schools Outstanding Doctoral Dissertation Award. His story is highlighted in the Rutgers Physics News. Congrats to Sean for winning this prestigious award!

Calculated band structure projected onto the top quintuple layer for Jz=1/2. The blue arrow indicates an optical transition with circularly polarized light used in the experiment. (right) The photoluminescence spectra from right and left-handed chiral excitons with Jz=+1 and -1, respectively. The majority of the photoluminescence intensity preserves the helicity of the incident photon due to the chiral nature of the excitons, as shown in the schematic drawing.
Calculated band structure projected onto the top quintuple layer for Jz=1/2. The blue arrow indicates an optical transition with circularly polarized light used in the experiment. (right) The photoluminescence spectra from right and left-handed chiral excitons with Jz=+1 and -1, respectively. The majority of the photoluminescence intensity preserves the helicity of the incident photon due to the chiral nature of the excitons, as shown in the schematic drawing.

We observe composite particles -- chiral excitons -- residing on the surface of a topological insulator (TI), Bi2Se3. Unlike other known excitons composed of massive quasiparticles, chiral excitons are the bound states of surface massless electrons and surface massive holes, both subject to strong spin–orbit coupling which locks their spins and momenta into chiral textures. Due to this unusual feature, chiral excitons emit circularly polarized secondary light (photoluminescence) that conserves the polarization of incident light. This means that the out-of-plane angular momentum of a chiral exciton is preserved against scattering events during thermalization, thus enabling optical orientation of carriers even at room temperature. The discovery of chiral excitons adds to the potential of TIs as a platform for photonics and optoelectronics devices.

 

19-02-28_1
In the figure above, we show the eigenenergies and eigenstates derived from the model Hamiltonian calculation. (a) Evolution of the 4f-orbital energy with CF potential and SOC strength. [from left to center] Increasing CF potential in the absence of SOC (a1), and then increasing SOC strength in the existence of full CF potential (a2); [from left to center] increasing SOC in the absence of CF potential (a3), and then increasing CF potential in the existence of full SOC (a4). In this panel, the full SOC strength is ξ=610 cm−1 , and the full CF potential strengths are B4=-0.758 cm−1 and B6=-0.0165 cm−1 . (b) The wavefunctions and the angular electron-cloud distribution of the eigenstates. [left] The wavefunctions of the eigenstates when only CF potential is present. Red denotes positive value while blue denotes negative value; [middle] the angular electron-cloud distribution of the eigenstates when both SOC and CF potential are present; [right] the angular electron-cloud distribution of the eigenstates when only SOC is present.
Long-range order of multipoles, namely high-rank electric or magnetic moments, has been attracting great interest. For example, second-rank quadrupolar moments could lead to novel phenomena including quadrupolar Kondo effect and quadrupole-fluctuation-mediated superconductivity. f-electron systems are suitable choices to study multipolar interactions and ordering phenomena by virtue of the interplay of the spin and orbital degrees of freedom. CeB6, with its simple chemical composition, lattice structure, and electronic configuration, is considered a prototypical example of heavy-fermion metals with quadrupolar ordering. Additionally, CeB6 serves as stable electron-emitting cathodes due to its low work function. This material has found broad applications in surface analysis, metrology, X-ray sources and free electron lasers. Therefore, a study of CeB6 , which illustrates the rich physics of this compound in a detailed and transparent way, contributes to both academia and industry.

In this work, we report a comprehensive spectroscopic study of CeB6 to explore its electronic, phononic and magnetic excitations. We identify an intense photo-luminescence feature resulting from the resonant optical transition between Ce 4f and 5d bands. We preform a detailed temperature-dependent study of both intra- and inter-multiplet crystal-field (CF) excitations, and illustrate the interaction between light and CF states by a model Hamiltonian calculation. By studying lattice dynamics, we show the electron-phonon interaction dominates phonon-lineshape broadening, and moreover induces a summation mode of CF transition and lattice vibration. We observe quasi-elastic magnetic fluctuations developing below 20K. The temperature dependence of the corresponding Raman susceptibility is consistent with the magnetic susceptibility data. By comparing our data with those measured by other techniques, we identify this magnetic feature as a manifestation of ferromagnetic correlations induced by tendency towards quadrupolar ordering.

This systematic study of CeB6 will serve as basis for future study of this model system. Especially, we demonstrate two virtues of Raman scattering which have not been generally appreciated: 1, temperature dependence of the parameters of CF excitations reveals the interaction between f-electrons and itinerant electrons; 2, low-energy Raman response probes dynamical fluctuations related to exotic multipolar ordering. More importantly, this work demonstrates the spectroscopic approaches and theoretical analyses of studying rare-earth systems with localized f-electrons, especially systems with a single f-electron or hole localized on an rare-earth ion at high-symmetry crystallographic site, by secondary-emission spectroscopy. The same method has direct applicability to f-electron systems consisting of Ce or Yb ions, and can be further applied to a range of rare-earth materials.

 

We are exceptionally proud to announce that our Dr. Hesiang-Hsi (Sean) Kung has been selected to become the Rutgers nominee for the Northeastern Association of Graduate Schools 2019 Doctoral Dissertation Award! His story is highlighted in the School of Graduate Studies (SGS) Grad News.

Authors: W.-L. Zhang, Y. Song, W.-Y. Wang, C.-D. Cao, P.-C. Dai, C.-Q. Jin, and G. Blumberg

NaFeCuAs_Fig1
Abstract: We use polarization-resolved Raman scattering to study lattice dynamics in  NaFe0.53Cu0.47As single crystals. We identify four A1g phonon modes, at 126, 172, 183, and 197 cm-1, and four B3g phonon modes at 101, 139, 173, and 226
cm-1(D2h point group). The phonon spectra are consistent with the Ibam space group, which confirms that the Cu and Fe atoms form a stripe order. The temperature dependence of the phonon spectra suggests weak electron-phonon and magnetoelastic interactions.

 

Authors:  M.Ye, H.-S.Kim, J.-W.Kim, C.-J.Won, K.Haule, D.Vanderbilt, S.-W.Cheong and G. Blumberg

Figure 1
Abstract: 

We report ab-initio density functional theory calculation and Raman scattering results to explore the electronic structure of Ba5CuIr3O12 single crystals. This insulating iridate, consisting of face-sharing IrO6 octahedra forming quasi-one-dimensional chains, cannot be described by the local jeff = 1/2 moment picture commonly adopted for discussing electronic and magnetic properties of iridate compounds with IrO6 octahedra. The shorter Ir-Ir distance in the face-sharing geometry, compared to corner- or edge-sharing structures, leads to strong covalency between neighboring Ir. Then this strong covalency results in the formation of molecular orbitals (MO) at each Ir trimers as the low-energy electronic degree of freedom. The theoretically predicted three-peak structure in the joint density of states, a distinct indication of deviation from the jeff = 1/2 picture, is verified by observing the three-peak structure in the electronic excitation spectrum by Raman scattering.

 

We are proud to announce that Dr. Hsiang-Hsi (Sean) Kung has been awarded the 2018 Richard J. Plano Dissertation Prize. This Prize is awarded to the graduate student with the best Ph.D. dissertation this academic year. For more information please see here.

Also congratulations to undergraduate student Viktor Krapivin for receiving the Richard T. Weidner Physics Prize for outstanding academic performance. For more information please see here.
Viktor will be attending Stanford University in the graduate program in applied physics next year.