arXiv subjects

A. Stucky

Publications and source records attributed to A. Stucky.

5 recordsLinked to original sources

arXiv2021

Isotope tuning of the superconducting dome of strontium titanate

Doped strontium titanate SrTiO$_3$ (STO) is one of the most dilute superconductors known today. The fact that superconductivity occurs at very low carrier concentrations is one of the two reasons that the pairing mechanism is not yet understood, the other is the role played by the proximity to a ferroelectric instability. In undoped STO, ferroelectric order can in fact be stabilized by substituting $^{16}$O with its heavier isotope $^{18}$O. Here we explore the superconducting properties of doped and isotope-substituted SrTi$(^{18}$O$_{y}^{16}$O$_{1-y})_{3-δ}$ for $0\le y \le 0.81$ and carrier concentrations between $6\times 10^{17}$ and $2\times 10^{20}$ cm$^{-3}$ ($δ<0.02$). We show that the superconducting $T_c$ increases when the $^{18}$O concentration is increased. For carrier concentrations around $5\times 10^{19}$~cm$^{-3}$ this $T_c$ increase amounts to almost a factor $3$, with $T_c$ as high as 580~mK for $y=0.74$. When approaching SrTi$^{18}$O$_3$ the maximum $T_c$ occurs at a much smaller carrier densities than for pure SrTi$^{16}$O$_3$. Our observations agree qualitatively with a scenario where superconducting pairing is mediated by fluctuations of the ferroelectric soft mode.

cond-mat.supr-con↗

arXiv2020

Manifestion of structural Higgs and Goldstone modes in the hexagonal manganites

Structural phase transitions described by Mexican hat potentials should in principle exhibit aspects of Higgs and Goldstone physics. Here, we investigate the relationship between the phonons that soften at such structural phase transitions and the Higgs- and Goldstone-boson analogues associated with the crystallographic Mexican hat potential. We show that, with the exception of systems containing only one atom type, the usual Higgs and Goldstone modes are represented by a combination of several phonon modes, with the lowest energy phonons of the relevant symmetry having substantial contribution. Taking the hexagonal manganites as a model system, we identify these modes using Landau theory, and predict the temperature dependence of their frequencies using parameters obtained from density functional theory. Separately, we calculate the additional temperature dependence of all phonon mode frequencies arising from thermal expansion within the quasi-harmonic approximation. We predict that Higgs-mode softening will dominate the low-frequency vibrational spectrum of InMnO$_3$ between zero kelvin and room-temperature, whereas the behavior of ErMnO$_3$ will be dominated by lattice expansion effects. We present temperature-dependent Raman scattering data that support our predictions, in particular confirming the existence of the Higgs mode in InMnO$_3$.

cond-mat.mtrl-sci↗

arXiv2018

Cooling a polaronic liquid: Phase mixture and pseudogap-like spectra in superconducting $Ba_{1-x}K_{x}BiO_{3}$

Many complex electronic systems exhibit so-called pseudogaps, which are poorly-understood suppression of low-energy spectral intensity in the absence of an obvious gap-inducing symmetry. Here we investigate the superconductor $Ba_{1-x}K_{x}BiO_{3}$ near optimal doping, where unconventional transport behavior and evidence of pseudogap(s) have been observed above the superconducting transition temperature $T_{c}$, and near an insulating phase with long-range lattice distortions. Angle-resolved photoemission spectroscopy (ARPES) reveals a dispersive band with vanishing quasiparticle weight and "tails" of deep-energy intensity that strongly decay approaching the Fermi level. Upon cooling below a transition temperature $T_{p} > T_{c}$, which correlates with a change in the slope of the resistivity vs. temperature, a partial transfer of spectral weight near $E_{F}$ into the deep-binding energy tails is found to result from metal-insulator phase separation. Combined with simulations and Raman scattering, our results signal that insulating islands of ordered bipolarons precipitate out of a disordered polaronic liquid and provide evidence that this process is regulated by a crossover in the electronic mean free path.

cond-mat.supr-con↗

arXiv2016

Isotope effect in superconducting n-doped SrTiO$_3$

We report the influence on the superconducting critical temperature $T_c$ in doped SrTiO$_3$ of the substitution of the natural $^{16}$O atoms by the heavier isotope $^{18}$O. We observe that for a wide range of doping this substitution causes a strong ($\sim 50 \%$) enhancement of $T_c$. Also the magnetic critical field $H_{c2}$ is increased by a factor $\sim 2$. Such a strong impact on $T_c$ and $H_{c2}$, with a sign opposite to conventional superconductors, is unprecedented. The observed effect could be the consequence of strong coupling of the doped electrons to lattice vibrations (phonons), a notion which finds support in numerous optical and photo-emission studies. The unusually large size of the observed isotope effect supports a recent model for superconductivity in these materials based on strong coupling to the ferroelectric soft modes of SrTiO$_{3}$.

cond-mat.supr-con↗

arXiv2016

Jahn-Teller induced nematic orbital order in tetragonal Sr$_2$VO$_4$

Using high resolution X-Ray diffraction (XRD) on high purity powders, we resolved the structure and $ab$ symmetry of the intriguing compound \svo$ $ from room temperature down to 20 K to an unprecedented level of accuracy. Upon cooling, this new set of data unambiguously reveals a second order phase transition lowering the symmetry from tetragonal to orthorhombic at a temperature $T_{c2}=136$ K. The observation of an orthorhombic distortion of the $ab$-plane is attributed to nematic phase formation supported by local Jahn-Teller (JT) dynamical instability. At $T_{N}=105$ K, spins order and at $T_{c1}=100$ K the tetragonal structure is recovered with an elongated c-axis.

cond-mat.str-el↗