Session C25: Optical Spectroscopy of Superconductors

Ultrafast studies of coexisting electronic order in cuprate superconductors

The cuprate family of high temperature superconductors displays a variety of electronic phases which emerge when charge carriers are added to the antiferromagnetic parent compound. These electronic phases are characterized by subtle differences in the low energy electronic excitations. Ultrafast time-resolved reflectivity (TRR) provides an ideal tool for investigating the cuprate phase diagram, as small changes in the electronic structure can produce significant contrast in the non-equilibrium reflectivity. Here we present TRR measurements of cuprate superconductors, focusing on the model single-layer cuprate HgBa$_{2}$CuO$_{4+\delta}$. We observe a cusp-like feature in the quasiparticle lifetime near the superconducting transition temperature Tc. This feature can be understood using a model of coherently-mixed charge-density wave and superconducting pairing. We propose extending this technique to the nanoscale using ultrafast scattering scanning near-field microscopy (u-SNOM). This will allow us to explore how these electronic phases coexist and compete in real-space.   

Probing broken symmetry states in cuprate superconductors with polarization-sensitive infrared spectroscopy

The nature of the pseudogap state in high-temperature superconducting (HTS) cuprates has drawn a lot of attention in the past two decades. A fundamental question is whether the pseudogap is a distinct phase with its own broken symmetries. Recent optical studies in the near-IR (800 meV) [1] and THz (2-6 meV) [2] ranges have observed symmetry breaking in the pseudogap state of HTS cuprates, suggesting that the pseudogap is a distinct phase. To probe the spectral character of this broken symmetry, we have performed infrared/visible Faraday and Kerr effect measurements at zero magnetic field and various temperatures on a series of HTS cuprate thin films, grown epitaxially by pulsed laser-ablated deposition. We will present and discuss our data, primarily complex Faraday/Kerr angle as a function of energy (0.1-3 eV), temperature (10-300K) and sample orientation with respect to the incident light polarization. 1. Xia, J et.al PRL 100, 127002 (2008). 2. Lubashevsky, Y et.al PRL 112, 147001 (2014) .   

Optical anisotropy of a cuprate high Tc superconductor

Evidence for a symmetry breaking phase transition across the pseudogap temperature T* of cuprate high-Tc superconductors has been reported in several experiments. For example, resonant ultrasound spectroscopy reveals a discontinuous change in the normal mode frequencies of the lattice across T* while spin-flip neutron scattering shows time reversal symmetry breaking across T*. Recent THz spectroscopy measurements also suggest the loss of mirror symmetries in the vicinity of T*. Optical anisotropy is in principle sensitive to the point group symmetries of a crystal and is highly complementary to the aforementioned techniques. I will present our progress in measuring and understanding the optical anisotropy of YBa$_{2}$Cu$_{3}$O$_{y}$ in the vicinity of its pseudogap transition temperature.   

Elucidating the driving force of superconductivity increase in compressed optimally doped Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+x}}$

An optimally doped cuprate Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+x\thinspace }}$is as a perfect model system to explore the mechanism of superconductivity by applying pressure as one can avoid complicated competing orders in the underdoped regime and explore pure intrinsic effects rather than secondary effects related to change in the carrier concentration. Here, by carefully examining the collected high-pressure Raman spectra at low temperatures, we have observed an enhanced two-magnon mode and connected this to the observed 10 K increase in $T_{\mathrm{c}}$ (reaching more than 100 K for the first time) in the optimally doped Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+x\thinspace }}$upon compression clearly delineating the effect of pressure-induced charge transfer that must suppress $T_{\mathrm{c}}$ for this optimally doped sample. Our finely designed experiments offer the direct and convincing evidence for identifying the magnetic fluctuations as the pairing interaction in cuprate superconductors.   

Time domain THz studies of thin film spinel superconductor LiTi$_2$O$_4$

Recent advances in growth of high-quality crystalline thin films of the only known spinel superconductor (SC) LiTi$_2$O$_4$ have allowed the discovery of an anomalous anisotropic magnetoresistance in its normal state [1]. We have used time domain terahertz spectroscopy, a contactless transport measurement, to determine the ac conductivity of LiTi$_2$O$_4$ and to examine the BCS nature of the superconducting state. We obtained the temperature dependence of the SC gap as well as the London penetration depth, and also found a hint of a second gap. We will discuss and compare these results in terms of the known dc transport properties [1].\newline [1] K. Jin, et al. Nature Communications, 6, 7183, 2015.   

Superconductivity-induced changes in density-density correlation function enabled by Umklapp processes

Motivated by the midinfrared scenario for high-temperature superconductivity proposed by Leggett, the effects of Umklapp processes on the density-density correlation function in the presence of long-range Coulomb interaction have been investigated. We show that because Umklapp processes enable scatterings that conserve total momentum only up to an integer times the reciprocal wave vector, significant amounts of spectral weight in the plasmonic excitations at long wavelength are transferred into lower frequency around the midinfrared regime. We further find that regardless of the gap symmetry, superconductivity generally suppresses the Umklapp scatterings due to the nature of Cooper pairs. This suppression is unique for the superconductivity due to the interplay between electron pairing and the odd parity of the matrix elements associated with Umklapp channels, which usually does not occur in other known competing orders. Specific predictions for the experimental signatures in optical conductivity and electron energy loss spectroscopy will be discussed.   

New Method for Imaging Gap Nodal Structure of Unconventional Superconductors through the Anisotropic Nonlinear Meissner Effect

We present a new measurement method which can be used to image gap nodal structure of superconductors whose pairing symmetry is unknown. This method utilizes photoresponse from a microwave resonance of the superconducting sample perturbed by a scanned laser spot. For an epitaxial or single crystal sample, the anisotropy of this photoresponse is directly related to that of gap function via the non-linear Meissner coefficient, so the gap nodal directions can be inferred from the photoresponse image. The significant advantage of the presented method over previous spiral or lumped circuit resonator methods is that it does not require a complicated lithographic patterning process which often degrades superconductivity or introduces defect-dominant photoresponse and hence limits one from testing various kinds of materials. The validity of the method is confirmed both by HFSS simulation and experiments on unpatterned superconducting thin films. Photoresponse images from example unconventional superconductors will be also presented and discussed.   

Imaging the Anisotropic Non-linear Meissner Effect in the Millikelvin Range

The anisotropic non-linear Meissner effect of unconventional superconductors, which gives information about gap nodal structure, has been observed by measurements of anisotropic photoresponse for temperatures down to about 3 Kelvin.[1] Since the anisotropy of photoresponse originates from the non-linear Meissner effect coefficient, and theory predicts a strong temperature dependence for the photoresponse at temperatures below $0.1T_c$,[2] it will be interesting to measure photoresponse in that range. We are building a laser scanning microscope around a dilution refrigerator, and plan to measure photoresponse of unconventional superconductors in the millikelvin temperature range to test the predictions. Reference: $[1]$ A. P. Zhuravel, B. G. Ghamsari, C. Kurter, P. Jung, S. Remillard, J. Abrahams, A. V. Lukashenko, A. V. Ustinov, and S. M. Anlage, "Imaging the Anisotropic Nonlinear Meissner Effect in Nodal $\text{YBa}_2\text{Cu}_3\text{O}_{7-\delta}$ Thin-Film Superconductors" Phys. Rev. Lett. 110, 087002, 2013. $[2]$ T. Dahm and D.J. Scalapino, "Theory of Intermodulation in a Superconducting Microstrip Resonator", Journal of Applied Physics, vol. 81, pp. 2002-2009, 1997.   

Examining the low energy electrodynamics of the superconductor-insulator transition in the potential topological superconductor Tl$_4$(Tl$_{1-x}$Sn$_x$)Te$_3$

The search for an intrinsic single crystal topological superconductor is one of the most dynamic areas of modern condensed matter physics. One of the best candidates of such a material is Tl$_5$Te$_3$ (T$_c = 2.3$K), which previous ARPES measurements have shown possesses a Dirac cone within its superconducting gap. However, the fundamental nature of superconductivity, i.e. the superconducting order parameter, in Tl$_5$Te$_3$ remains unknown. Additionally, it has been shown that Tl$_5$Te$_3$ undergoes a superconducting-insulator transition upon doping with Sn. With no band parity inversion expected in the fully Sn doped compound one expects a topological supercondutor - trivial insulator transition, the nature of which is also unknown. In this work we use highly sensitive microwave cavity perturbation measurements, a direct probe of the superfluid density, to study the low energy electrodynamics of superconductivity in Tl$_5$Te$_3$ and its corresponding superconductor-insulator transition upon Sn doping.   

Characterizing phase transitions in known materials with Magnetic Field Modulated Microwave Spectroscopy (MFMMS)

We have previously introduced Magnetic Field Modulated Microwave Spectroscopy (MFMMS), a sensitive and selective technique used to identify electromagnetic phase transitions in homogenous and inhomogeneous materials. By scanning the temperature, we can detect the phase transitions of a material. In standard operation, samples are placed in a microwave cavity with a resonance frequency of 9.4 GHz. A 100 kHz modulation field with 15 Oe amplitude and an optional DC field are applied while temperature is scanned. [1,2,3]. Here we will discuss different methods to further characterize phase transitions by scanning DC field while temperature is fixed. Since the response of different phase transitions to the applied field is varied, DC field scans can help to distinguish and reveal the origin of the transition. We have investigated many known superconducting and other reference materials and will compare the results in these different materials. 1. J. G. Ram\'{\i}rez, A. C. Basaran \textit{et al.}, \textit{Rep. Prog. Phys.} \textbf{77}, 093902 (2014). 2. S. Gu\'{e}non \textit{et al.}, \textit{Scientific Reports} \textbf{4}, 7333 (2014). 3. S. Gu\'{e}non\textit{ et al., arxiv:1509.04452,} manuscript submitted.   

Progress towards a Hybrid Superconducting Microwave Cavity for Axion Searches

Axions are a well motivated dark matter candidate and can be detected by their resonant conversion into photons using a microwave resonant cavity in an axial magnetic field. This is the basis of both the ADMX and ADMX-HF experiments. The predicted axion-photon conversion power is extremely small ($< 10^{-22}$ W) and is directly related to the quality factor (Q = resonant frequency over bandwidth) of the microwave cavity. To date copper cavities have been used with Q $\sim 10^5$ at frequencies of 1 GHz. As one scales to higher frequencies this Q degrades substantially. Superconducting cavities can regularly be made with Q $> 10^9$ but would in general be driven normal in the high magnetic field of ADMX and ADMX-HF ($> 8$ T). Here we describe progress of R\&D efforts to make and test hybrid cavities with regular copper endcaps and thin-film superconducting barrels, produced with NbTiN RF sputtering, which are designed to maintain RF superconducting properties in the presence of a strong axial magnetic field at low temperatures ($< 1$ K).   

Anomalous gap edge dissipation in disordered superconductors at the brink of localization

In highly disordered conventional superconductors, it is frequently found that the optical conductivity presents an anomalous additional conductivity below the superconducting gap 2$\Delta$ even as T approaches zero. According to Bardeen-Cooper-Schrieffer theory and Matthis-Bardeen (MB) formula, no dissipation state should exist below 2$\Delta$ at T=0 K. To resolve this problem, we studied a number of NbN superconducting films by time-domain terahertz spectroscopy. We found an extra conductivity beyond the predictions of MB theory begin to show up even at medium disorder level. With increasing disorder level, more and more optical spectral weights are moved to in-gap region ($\omega$ $<$ 2$\Delta$). By using a self-consistent Abrikosov-Gorkov model, we found, disorder acts as a pairing breaking factor, which blurs the region around the gap edge and introduces dissipative states into the original gap region ($\omega$ $<$ 2$\Delta$) in the optical conductivity. Our results show that the collective modes of superconductivity are not necessary to explain the extra dissipative states in disordered superconductors.   

Extreme and Local 3rd Harmonic Response of Niobium (Nb) Superconductor

Superconducting Radio Frequency (SRF) cavities are being widely used in new generation particle accelerators. These SRF cavities are based on bulk Nb. Based on the needs of the SRF community to identify defects on Nb surfaces, a novel near-field magnetic microwave microscope was successfully built using a magnetic writer from a conventional magnetic recording hard-disk drive1. This magnetic writer can create an RF magnetic field, localized and strong enough to drive Nb into the vortex state. This probe enables us to locate defects through scanning and mapping of the local electrodynamic response in the multi-GHz frequency range. Recent measurements have shown that 3rd harmonic nonlinear response is far more sensitive to variations in input power and temperature then linear response, thus we mainly study the 3rd harmonic response. Moreover, the superconductor is usually the only source for nonlinear response in our setup, thus there is less chance of having noise or background signal. Understanding the mechanism responsible for this non-linear response is important for improving the performance of SRF cavities. Besides Nb we also study various other superconductors such as MgB2 and the cuprate Bi-Sr-Ca-Cu-O (BSCCO) for potential applications in SRF cavities.   

Observation of Raman active phonon with Fano lineshape in quasi-one-dimensional superconductor K$_2$Cr$_3$As$_3$

We study the polarization-resolved phononic Raman scattering in the recent discovered quasi-one-dimensional superconductor K$_2$Cr$_3$As$_3$. With support from first-principles calculations, we characterize several phonons, among which one mode has a Fano lineshape, indicative of an electron-phonon coupling. While the common expectation of an electron-phonon coupling is the conventional superconducting mechanism, we show that this mode is related to the in-plane Cr vibration, which modulates the exchange coupling between the first nearest Cr neighbors. Our result support the presence of magnetic fluctuations coupled to the electrons $via$ the lattice. We acknowledge MOST (2010CB923000, 2011CBA001000, 2011CBA00102, 2012CB821403 and 2013CB921703), NSFC (11004232, 11034011/A0402, 11234014, 11274362 and 11474330) of China and by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, Grant No. XDB07020100.   

Superconducting pairing in resonant inelastic X-ray scattering

We develop a method to study the effect of the superconducting transition on resonant inelastic X-ray scattering (RIXS) signal in superconductors with an order parameter with an arbitrary symmetry within a quasiparticle approach. As an example, we compare the direct RIXS signal below and above the superconducting transition for $p$-wave type order parameters. For a $p$-wave order parameter with a nodal line, we show that, counterintuitively, the effect of the gap is most noticeable for momentum transfers in the nodal direction. This phenomenon may be naturally explained as a type of nesting effect.