Session T63: Normal State Properties of Unconventional Superconductors

Plasmon like behavior of optimally doped Bi2212 at small in-plane momentum measured with M-EELS

Optimally doped Bi_2.1Sr_1.9CaCu₂O_8+x (Bi-2212,T_c=91K) is a so-called “strange metal” at 300K. Using momentum-resolved electron energy-loss spectroscopy (M-EELS) measurements, we recently showed that, while Bi-2212 exhibits a well-defined plasmon spectrum at moderate momentum (q~0.1 r.l.u.), at large q (>0.2 r.l.u.) it exhibits a featureless, frequency-independent continuum^[1]. Here we examine the M-EELS data from Bi-2212 at much smaller momentum, 0.01 < q < 0.1 r.l.u. ^[2]. In this regime the scattering matrix elements are extremely energy-dependent, making interpretation of the spectra difficult. Using Jain and Allen’s model of a layered conductor^[3], accounting for the finite q resolution of the measurement, we show that the M-EELS data are consistent with a 1 eV plasmon over this entire q range. These results are qualitatively consistent with previous ellipsometry and transmission EELS measurements on this material.

[1] Husain, Ali A., et al. "Crossover of charge fluctuations across the strange metal phase diagram." Physical Review X 9.4 (2019): 041062.

[2]Husain, Ali Abdullah. Charge fluctuations of the strange metal in space and time. Diss. University of Illinois at Urbana-Champaign, 2020.

[3] Jain, Jainendra K., and Philip B. Allen. "Dielectric response of a semi-infinite layered electron gas and Raman scattering from its bulk and surface plasmons." Physical Review B 32.2 (1985): 997.   

17O NMR studies of Sr2RuO4 with applied uniaxial stresses beyond the Lifshitz transition

Experiments on uniaxially stressed Sr₂RuO₄ have resulted in substantial physical properties changes, including a factor 2.5 increase in the superconducting T_c, and an apparent breakdown of Fermi Liquid behavior in the normal state. Both are attributed to to a strain-induced passing of the Fermi energy through a van Hove singularity (vHs) [1]. Recent μSR measurements gave evidence for a magnetic phase transition at even greater stress [2]. Presented here are the results of ¹⁷O NMR spectroscopy and relaxation measurements. Enhanced relaxation rates characterize a phase boundary consistent with the proposed line of transitions. Nevertheless, within the low-symmetry phase, the NMR spectra are not broadened and (1/T₁T) follows standard Fermi liquid behavior. We discuss the implications of these results, how they relate to the possible type of magnetic order, and the relationship of the reported magnetic order to the superconducting state.

[1] Steppke, A. et al. Science 355, eaaf9398 (2017).

[2] Grinenko, V., Ghosh, S., Sarkar, R. et al. Nat. Phys. 17, 748–754 (2021).   

Tunable charge order by pressure and isovalent substitution in electronic nematic superconducting system Ba(1-x)Sr(x)Ni2As2

The discoveries of charge order in the cuprates and nematic order in the iron-based superconductors have pointed towards the possibilities of both orderings being tied to mechanisms of high Tc superconductivity. The Ba(1-x)Sr(x)Ni2As2 system, closely related in structure to the BaFe2As2 system, has been shown to exhibit both types of ordering without the presence of any magnetic order. We report single crystal X-ray diffraction that shows the evolution of two commensurate charge orderings and one incommensurate charge ordering in the Ba(1-x)Sr(x)Ni2As2 system. Additionally, we present pressure experiments that study the relationship between Sr substitution and applied pressure in order to understand the evolution of the phase diagram. The evolution of the charge order in this system correlates well with the already reported evolutions of nematicity and superconducivity, suggesting a strong link between the three phases.   

Phonon Thermal Hall Transport from Coupling to Spin Defects

Recent thermal-Hall transport experiments in certain metals as well as Mott insulators have identified phonons as the major heat carrier, but the mechanism of phonon chirality remains unclear. In this work, we investigate the scenario of phonon coupling to spin-1/2 defects in the material. In our theory, we assume the spin defects break lattice inversion symmetry and therefore gives rise to a direct coupling between the defect spin and lattice displacement. When the spin is polarized by an external magnetic field, phonons acquire a chiral self energy, and it is strongest when the phonon energy resonates with the spin-flip transition. We discuss two mechanisms via which the phonons can demonstrate thermal-Hall effect: First, the phonons move in an effective Berry-curvature induced by the spin defect. Second, the phonons skew scatter on the spin defect.   

Strain-induced nematic response of the CDW in Sr-doped BaNi2As2

Ba1-xSrxNi2As2 (BSNA) is a pnictide superconductor that exhibits several charge density wave (CDW) phases and a diverging nematic susceptibility observed in transport experiments under applied uniaxial strain. The nematic susceptibility was found to become strain hysteretic in the incommensurate CDW (I-CDW) phase, suggesting that nematicity and the I-CDW phase are coupled, which is surprising because the I-CDW phase does not break C4 symmetry. To explore this coupling, we performed an x-ray scattering study of the CDW phases in BSNA x=0.27 under applied uniaxial strain. We find a large nematic response in the CDW as well, detected as a breaking of the C4 symmetry induced by the applied strain field. Our results indicate that the CDW is strongly coupled to the nematic order parameter and may even be the driver of nematic phenomena in this material.   

Hybrid plasmon-phonon excitation in Sr­Ti1-xNbxO3

Sr­Ti_1-xNb_xO₃ is known to be one of the most dilute superconductors with properties paralleling copper-oxides. It has perplexing optical properties, including an anomalously broad plasma edge that decreases in frequency with increasing temperature. Here we present a momentum-resolved electron energy loss spectroscopy (M-EELS) study of plasmon excitations in the normal state of Sr­Ti_1-xNb_xO₃ (x=0, 0.002, 0.01, 0.014). Measurements at large momentum transfer of the undoped insulating sample, SrTiO₃, reveal a multiphonon continuum typical of anharmonic insulators. As electrons are added via doping, this spectral weight does not form a simple plasmon, but rather becomes enhanced and shifts to slightly higher energy. Doping makes the continuum stronger and visible over a wider range of momentum, all the way down to zero momentum. We identify this mode, which has been identified in optics as a plasmon, as a hybrid excitation of doped carriers mixed with an anharmonic phonon continuum.   

Investigation of electronic nematicity in Sr2RuO4 via time-resolved optical pump-probe spectroscopy

Despite intensive study, the unconventional superconductivity in the layered oxide perovskite

Sr₂RuO₄ (SRO) still eludes explanation. An understanding of the superconducting state is likely

to require a complete and accurate description of the normal state out of which electron pairing

emerges. Previous angle-resolved transport studies have uncovered an apparent electronic

anisotropy in thin films of SRO which emerges well above the superconducting critical

temperature [Wu et al., PNAS 117, 10654 (2020)]. Here, we use time-resolved optical pump-

probe spectroscopy experiments to study the ultrafast electronic dynamics in SRO thin films.

Remarkably, we find that the in-plane four-fold rotational symmetry of the crystal lattice is

reduced to a two-fold symmetry in the transient response. This finding suggests spontaneously

broken rotational symmetry or a large nematic susceptibility exists in the normal state of SRO.

We discuss the implications of this normal-state nematic symmetry breaking on the much-

debated superconducting order parameter.   

Nonlinear optical spectroscopy studies of a Kagome lattice material RbV3Sb5

The transition-metal Kagome lattice materials provide an exciting platform to investigate the quantum interactions among electron correlation, topology, and geometric frustration. The recently discovered Kagome superconductors AV₃Sb₅ (A=K, Rb, Cs) with nontrivial topological band structures and multiply intertwined broken symmetry states have attracted tremendous research interest. A charge density wave (CDW) order with broken time-reversal symmetry and other point group symmetries has been observed in AV₃Sb₅, which possible suggests the presence of a hidden ordered state and yet awaits comprehensive understanding. Optical second-harmonic generation (SHG), a doubling frequency generation process by nonlinear light-matter interactions inside the material of interest, can directly probe point group symmetry changes across phase transitions and therefore has been used to search for otherwise hidden orders. In this talk, I will present our results on the Kagome superconductor RbV₃Sb₅ measured by rotation anisotropy (RA) SHG. I will particularly focus on the temperature dependence of RA SHG that reveals the full point group symmetry evolutions in RbV₃Sb₅ across various transition temperatures.   

NMR study of metallic strontium titanate

Electron-doped strontium titanate (SrTiO₃) is a peculiar metal and superconductor due to extremely low densities of high-mobility carriers and the proximity to a ferroelectric quantum-critical point. In particular, the normal-state transport properties are not well understood. Transport measurements find a robust T²-dependent resistivity, as might be expected for a Fermi liquid. However, the Fermi energy is so low that Fermi-liquid theory cannot explain the observed behavior. Nuclear magnetic resonance (NMR) is a powerful local probe that has not yet been used to investigate this issue. Here we present a ^47,49Ti NMR study across the temperature-doping phase diagram of Nb- and oxygen-vacancy-doped SrTiO₃. From Knight shift measurements, we gain insight into the local electronic spin susceptibility. We discuss the results within Fermi-liquid and polaronic models of metallic SrTiO₃.   

Non-Fermi liquid phase and linear-in- temperature scattering rate in overdoped two dimensional Hubbard model

Understanding electronic properties that violate the Landau Fermi liquid paradigm in cuprate superconductors remains a major challenge in condensed matter  physics. The strange metal state in overdoped cuprates that exhibits linear-in-temperature scattering rate and dc resistivity is a particularly puzzling example. Here, we compute the electronic scattering rate in the two-dimensional Hubbard model using cluster generalization of dynamical mean-field theory. We present a global phase diagram documenting an apparent non-Fermi liquid phase, in between the pseudogap and Fermi liquid phase in the doped Mott insulator regime. We discover that in this non-Fermi liquid phase, the electronic scattering rate  $\gamma_k(T)$  can display linear temperature dependence as temperature $T$ goes to zero.  In the temperature range that we can access, the $T-$ dependent  scattering rate is isotropic on the Fermi surface, in agreement with recent experiments.  Using fluctuation diagnostic techniques, we identify antiferromagnetic fluctuations as the physical origin of the $T-$ linear electronic scattering rate.   

Fermi arcs vs hole pockets: periodization of a cellular two-band model

It is still debated whether the low-doping Fermi surface of cuprates is composed of hole pockets or of disconnected Fermi arcs. Results from cellular dynamical mean field theory (c-DMFT) support the Fermi arcs hypothesis by predicting corresponding Fermi arcs for the Hubbard model. Here, we introduce a simple parametrization of the self-energy, in the spirit of Yang-Rice-Zhang theory, and show that state of the art c-CDMFT calculations cannot give a definitive answer to the question of Fermi arcs vs hole pockets, and this, independently of the periodization (cumulant or Green's function) used to display spectral weights of the infinite lattice.   

Pseudogap and Mott physics on a frustrated lattice

The pseudogap spans a large part of the phase diagram of hole-doped superconducting cuprates, so it is important to understand its origin. Cluster generalizations of dynamical mean-field theory suggest that it can be a consequence of Mott physics [1] and super-exchange without large antiferromagnetic correlation lengths. We explore this possibility by studying this phenomenon on the triangular lattice using Dynamical Cluster Approximation (DCA). On that lattice, frustration keeps the antiferromagnetic correlation length short. This approach allows us to obtain the size-converged results down to relatively low temperatures. We find that a pseudogap does appear and is separated from the correlated Fermi liquid by a first-order “Sordi” transition, a transition continuously connected to the Mott transition. We conclude that Mott physics and associated short-range anti-ferromagnetic correlations lead to a pseudogap.

 

[1] G. Sordi, et al. Scientific Reports, 2, 547 (2012)