arXiv subjects

G. Blumberg

Publications and source records attributed to G. Blumberg.

At least 19 recordsLinked to original sources

arXiv2026

Chirality-resolved spectroscopy of Caroli-de Gennes-Matricon states in multiband FeTe$_{1-x}$Se$_{x}$ superconductors

We employ terahertz Faraday magneto-optical spectroscopy to probe the relaxation dynamics of quantized helical Caroli-de Gennes-Matricon (CdGM) states in epitaxial FeTe$_{1-x}$Se$_x$ thin films, nodeless multiband superconductors with short coherence lengths in the moderately clean limit. By exploiting polarization-selective optical transitions, we directly resolve the helicity and band origin of vortex-core quasiparticles. We observe long-lived CdGM resonances with opposite circular polarizations for electron- and hole-like bands. This enables independent, band-resolved determination of quasiparticle lifetimes, vortex masses, coherence lengths, and upper critical fields, and reveals their systematic evolution with isovalent substitution. The results establish terahertz magneto-optics as a direct probe of helical vortex-core excitations and provide dynamical evidence for multiband CdGM states in iron-based superconductors.

cond-mat.supr-con↗

arXiv2024

Modified Martin-Puplett interferometer for magneto-optical Kerr effect measurements at sub-THz frequencies

We present magneto-optical Kerr effect (MOKE) spectrometer based on a modified Martin-Puplett interferometer, utilizing continuous wave sub-THz low-power radiation in broad frequency range. This spectrometer is capable of measuring the frequency dependence of the MOKE response function, both the Kerr rotation and ellipticity, simultaneously with sub-milliradian accuracy without the need for reference measurement. The instrument's versatility allow it to be coupled to a cryostat with optical windows enabling studies of a variety of quantum materials such as unconventional superconductors, two-dimensional electron gas systems, quantum magnets, and other system showing optical Hall response, at sub-Kelvin temperatures and in high magnetic fields. We demonstrate the functionality of the MOKE spectrometer using an undoped InSb wafer as a test sample.

cond-mat.mtrl-sci↗

arXiv2024

Spin-Mediated Direct Photon Scattering by Plasmons in BiTeI

We use polarization resolved Raman spectroscopy to demonstrate that for a 3D giant Rashba system the bulk plasmon collective mode can directly couple to the Raman response even in the long wavelength $\mathbf q \rightarrow 0$ limit. Although conventional theory predicts the plasmon spectral weight to be suppressed as the square of its quasi-momentum and thus negligibly weak in the Raman spectra, we observe a sharp in-gap plasmon mode in the Raman spectrum of BiTeI below the Rashba continuum. This coupling, in a polar system with spin-orbit coupling, occurs without assistance from phonons when the incoming photon excitation is resonant with Rashba-split intermediate states. We discuss the distinctive features of BiTeI's giant Rashba system band structure that enable the direct observation of plasmon in Raman scattering.

cond-mat.str-el↗

arXiv2024

Electronic and Vibrational Excitations on the Surface of the Three-Dimensional Topological Insulator Bi$_2$Te$_{3-x}$Se$_{x}$ (x = 0, 2, 3)

We study surface states in the three-dimensional topological insulators Bi$_2$Te$_{3-x}$Se$_{x}$ (x = 0, 2, 3) by polarization resolved resonant Raman spectroscopy. By tracking the spectral intensity of the surface phonon modes with respect to the incident photon energy, we show that the surface phonons are qualitatively similar to their bulk counterparts. Using the resonant Raman excitation profile, we estimated the binding energy of the surface conduction bands relative to bulk conduction bands. In addition, we selectively excite the surface-to-bulk electronic continuum near the Fermi energy in Bi$_2$Se$_3$ to determine the strength of Fano interaction between the most prominent surface phonon and the surface-to-bulk continuum.

cond-mat.mtrl-sci↗

arXiv2022

Chiral Electronic Excitations in a Quasi-2D Rashba System BiTeI

The optical transitions between spin-polarized bands of the quasi-two dimensional Rashba system BiTeI are investigated using polarization resolved resonant Raman spectroscopy. We detect chiral excitations between states with opposite helicity and compare spectra to calculations within a three-band model. Using the resonant Raman excitation profile, we deduce the Rashba parameters and band gaps of the higher conduction bands near the Fermi level, and compare the parameters to values obtained by ab initio density function theory (DFT).

cond-mat.str-el↗

arXiv2021

Quadrupolar charge dynamics in the nonmagnetic FeSe$_{1-x}$S$_x$ superconductors

We use polarization-resolved electronic Raman spectroscopy to study charge dynamics in non-magnetic FeSe$_{1-x}$S$_x$ superconductor. We observe two features of the $XY$ quadrupole symmetry: a low-energy quasi-elastic peak (QEP) and an electronic continuum. The QEP exhibits critical enhancement upon cooling towards the structural transition at $T_S(x)$. Below $T_S(x)$, the QEP diminishes gradually, and a gap with temperature evolution reminiscent of a mean-field order parameter opens in the continuum. The intensity of the QEP develops with increasing sulfur doping $x$ and maximizes at $x\approx$ 0.15, while the gap magnitude decreases with the suppression of $T_S(x)$. We interpret the development of the gap in the quadrupole scattering channel as the formation of a stripe quadrupole order: a wave of quadrupole moment without charge or spin modulation.

cond-mat.supr-con↗

arXiv2021

Comment on "Direct observation of excitonic instability in Ta$_2$NiSe$_5$" [arXiv:2007.08212 (2020)]

The origin of the second order phase transition at 328K in Ta$_2$NiSe$_5$, a prominent candidate for direct gap excitonic insulator, remains under fervent debate. The driving force for the transition can be revealed by identification of the soft mode's origin that may be deducted from polarization resolved Raman scattering experiments. Such studies were recently reported in [arXiv:2007.07344 (2020)], [arXiv:2102.07912 (2021)], [arXiv:2007.01723 (2020)] and [arXiv:2007.08212 (2020)]. In this Comment, it is shown that the parameters derived in a recent arXiv by Kwangrae Kim et. al. [arXiv:2007.08212 (2020)], including the Weiss temperature for excitonic transition, are based on inconsistent data.

cond-mat.str-el↗

arXiv2020

Random singlet state in Ba$_5$CuIr$_3$O$_{12}$ single crystals

We study the thermodynamic and high-magnetic-field properties of the magnetic insulator Ba$_5$CuIr$_3$O$_{12}$, which shows no magnetic order down to 2 K consistent with a spin liquid ground state. While the temperature dependence of the magnetic susceptibility and the specific heat shows only weak antiferromagnetic correlations, we find that the magnetization does not saturate up to a field of 59 Tesla, leading to an apparent contradiction. We demonstrate that the paradox can be resolved, and all of the experimental data can be consistently described within the framework of random singlet states. We demonstrate a generic procedure to derive the exchange coupling distribution $P(J)$ from the magnetization measurements and use it to show that the experimental data is consistent with the power-law form $P(J)\sim J^{-α}$ with $α\approx 0.6 $. Thus, we reveal that high-magnetic-field measurements can be essential to discern quantum spin liquid candidates from disorder dominated states that do not exhibit long-range order.

cond-mat.str-el↗

arXiv2019

Superconductivity and phonon self-energy effects in Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$

We study Fe$_{1+y}$Te$_{0.6}$Se$_{0.4}$ multi-band superconductor with $T_c=14$K by polarization-resolved Raman spectroscopy. Deep in the superconducting state, we detect pair-breaking excitation at 45cm$^{-1}$ ($2Δ=5.6$meV) in the $XY$($B_{2g}$) scattering geometry, consistent with twice of the superconducting gap energy (3 meV) revealed by ARPES on the hole-like Fermi pocket with $d_{xz}/d_{yz}$ character. We analyze the superconductivity induced phonon self-energy effects for the $B_{1g}$(Fe) phonon and estimate the electron-phonon coupling constant $λ^Γ\approx 0.026$, which is insufficient to explain superconductivity with $T_c=14$K.

cond-mat.supr-con↗

arXiv2019

Raman spectroscopy of $f$-electron metals: an example of CeB$_{6}$

We performed an optical spectroscopy study of electronic and magnetic excitations for a rare-earth system with a single electron quasi-localized in the f-shell on an ion at high-symmetry crystallographic site in application to CeB$_{6}$ heavy-fermion metal. We carried out group-theoretical classification of the electronic crystal field (CF) transitions and assessed their coupling to light cross-sections for polarization resolved Raman scattering processes. We discuss applicability of symmetrized Raman susceptibility to studies of exotic charge and spin high multiplet ordering phases in f-electron systems. We study temperature effects on intra- and inter-multiplet CF transitions and also on the coupling between the CF excitations with the lattice vibrations. We acquired temperature dependence of the low-frequency polarization resolved Raman response and obtained the static Raman susceptibility for all Raman-allowed symmetry channels: A$_{1g}$, E$_{g}$, T$_{1g}$, and T$_{2g}$ of the cubic O$_{h}$ point group. We demonstrate that for CeB$_{6}$ system only T$_{1g}$-symmetry static Raman susceptibility shows an anomalous temperature dependence which is consistent with the magnetic susceptibility data measured by other techniques. This anomalous behavior in the T$_{1g}$-channel signifies the presence of long wavelength magnetic fluctuations, while the lack of susceptibility enhancement for all the remaining symmetry channels indicates that long wavelength charge quadrupole fluctuations at low-temperature are weak.

cond-mat.str-el↗

arXiv2019

Lattice dynamics, crystal-field excitations and quadrupolar fluctuations of YbRu$_2$Ge$_2$

We report Raman-scattering results of YbRu$_2$Ge$_2$ single crystals to explore the phononic and crystal-field (CF) excitations. This heavy-fermion metal is suggested to enter a ferroquadrupolar (FQ) phase below T$_0$=10 K. The tetragonal CF potential splits the Yb$^{3+}$ $^2F_{7/2}$ ground multiplet into two $Γ_6$ and two $Γ_7$ Kramers doublets. We establish the following CF level scheme of the ground multiplet: a $Γ_6$ ground state, with the two $Γ_7$ states at 2 cm$^{-1}$, 95 cm$^{-1}$ and the other $Γ_6$ state at 239 cm$^{-1}$. The $\sim$2 cm$^{-1}$ separation between the CF ground and first excited states is in agreement with the previously proposed quasi-quartet CF ground state. The intensity of the lowest-energy CF transition remarkably increases on cooling, indicating a coupling of this CF excitation to the quadrupolar fluctuations above T$_0$. From symmetry analysis, we suggest that the FQ order has B$_{1g}$ symmetry. Moreover, temperature-dependent study of four Raman-active phonon modes shows that the intensities of the A$_{1g}$ and one E$_{g}$ modes increase significantly on cooling, which is explained by a near-resonant coupling between these two phonon modes and CF transitions.

cond-mat.str-el↗

arXiv2019

Observation of Chiral Surface Excitons in a Topological Insulator Bi$_2$Se$_3$

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin-orbit interaction in solids composed of heavy elements. Here, we study the composite particles -- chiral excitons -- formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical three-dimensional topological insulator (TI), Bi$_2$Se$_3$. Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of meVs, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin-orbit coupling. A theoretical model based on such proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin-orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.

cond-mat.mes-hall↗

arXiv2018

Raman scattering study of NaFe$_{0.53}$Cu$_{0.47}$As

We use polarization-resolved Raman scattering to study lattice dynamics in NaFe$_{0.53}$Cu$_{0.47}$As single crystals. We identify 4 $A_{1g}$ phonon modes at 125, 172, 183 and 197 cm$^{-1}$, and 4 $B_{3g}$ phonon modes at 101, 138, 173, 226 cm$^{-1}$. The phonon spectra are consistent with the $Ibam$ 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 magneto-elastic interactions.

cond-mat.supr-con↗

arXiv2018

High Tc superconductivity in CaKFe$_4$As$_4$ in absence of nematic fluctuations

We employ polarization-resolved Raman spectroscopy to study multi-band stoichiometric superconductor CaKFe$_4$As$_4$. The B$_{2g}$ symmetry Raman response shows no signatures of Pomeranchuk-like electronic nematic fluctuations which is observed for many other Fe-based superconductors. In the superconducting state, we identify three pair-breaking peaks at 13.8, 16.9 and 21 meV and full spectral weight suppression at low energies. The pair-breaking peak energies in Raman response are about 20% lower than twice the gap energies as measured by single-particle spectroscopy, implying a sub-dominant $d$-wave symmetry interaction. We analyze the superconductivity induced phonon self-energy effects and give an estimation of weak electron-phonon coupling constant $λ^Γ$=0.0015.

cond-mat.supr-con↗

arXiv2017

On the origin of critical nematic fluctuations in pnictide superconductors

We employ polarization-resolved Raman spectroscopy to study critical nematic fluctuations in Ba(Fe$_{1-x}$Au$_x$)$_2$As$_2$ superconductors above and across well separated tetragonal to orthorhombic phase transition at temperature $T_S(x)$ and the Néel transition at $T_N(x)$. The static Raman susceptibility in $XY$ symmetry channel increases upon cooling from room temperature following the Curie-Weiss law, with Weiss temperature $T_θ(x)$ several tens of degrees lower than $T_S(x)$. Data reveals a hidden nematic quantum critical point at $x_{c} = 0.031$ when the system becomes superconducting, indicating a direct connection between quantum critical nematic fluctuations and unconventional superconductivity. We attribute the origin of the nematicity to charge quadrupole fluctuations due to electron transfer between the nearly degenerate $d_{xz}/d_{yz}$ orbitals.

cond-mat.supr-con↗

arXiv2017

Anomalous magneto-elastic coupling in Au-doped BaFe2As2

We used polarization-resolved Raman scattering to study magneto-elastic coupling in Ba(Fe$_{1-x}$Au$_{x}$)$_2$As$_2$ crystals as a function of light Au-doping, materials for which temperatures of the structural transition ($T_S$) and of the magnetic ordering transition ($T_N$) split. We study the appearance of the $A_g$(As)phonon intensity in the $XY$ scattering geometry that is very weak just below $T_S$, but for which the intensity is significantly enhanced below $T_N$. In addition, the $A_g$(As) phonon shows an asymmetric line shape below $T_N$ and an anomalous linewidth broadening upon Au-doping in the magnetic phase. We demonstrate that the anomalous behavior of the $A_g$(As) phonon mode in the $XY$ scattering geometry can be consistently described by a Fano model involving the $A_g$(As) phonon mode interacting with the $B_{2g}$ symmetry-like magnetic continuum in which the magneto-elastic coupling constant is proportional to the magnetic order parameter.

cond-mat.supr-con↗

arXiv2017

Magneto-elastic coupling in Fe-based superconductors

We used polarization-resolved Raman scattering to study the magneto-elastic coupling in the parent compounds of several families of Fe-based superconductors (BaFe2As2, EuFe2As2, NaFeAs, LiFeAs, FeSe and LaFeAsO). We observe an emergent Ag-symmetry As phonon mode in the XY scattering geometry whose intensity is significantly enhanced below the magneto-structural transition only for compounds showing magnetic ordering. We conclude that the small lattice anisotropy is insufficient to induce the in-plane electronic polarizability anisotropy necessary for the observed phonon intensity enhancement, and interpret this enhancement below the Neel temperature in terms of the anisotropy of the magnetic moment and magneto-elastic coupling. We evidence a Fano line- shape in the XY scattering geometry resulting from a strong coupling between the Ag (As) phonon mode and the B2g symmetry-like electronic continuum. Strong electron-phonon coupling may be relevant to superconductivity.

cond-mat.supr-con↗

arXiv2017

Chiral Spin Mode on the Surface of a Topological Insulator

Using polarization-resolved resonant Raman spectroscopy, we explore collective spin excitations of the chiral surface states in a three dimensional topological insulator, Bi$_2$Se$_3$. We observe a sharp peak at 150 meV in the pseudovector $A_2$ symmetry channel of the Raman spectra. By comparing the data with calculations, we identify this peak as the transverse collective spin mode of surface Dirac fermions. This mode, unlike a Dirac plasmon or a surface plasmon in the charge sector of excitations, is analogous to a spin wave in a partially polarized Fermi liquid, with spin-orbit coupling playing the role of an effective magnetic field.

cond-mat.mtrl-sci↗