arXiv subjects

Ece Uykur

Publications and source records attributed to Ece Uykur.

23 records · Page 2Linked to original sources

arXiv2020

Two linear regimes in optical conductivity of a Type-I Weyl semimetal: the case of elemental tellurium

Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa. This semimetallic Weyl phase can be tuned by increasing pressure further: a kink separates two linear regimes in the optical conductivity (at 3.7 GPa), a signature proposed for Type-II Weyl semimetals with tilted cones; this however reveals a different origin in trigonal tellurium. Our density-functional calculations do not reveal any significant tilting and suggest that Te-I remains in the Type-I Weyl phase, but with two valence bandsin the vicinity of the Fermi level. Their interplay giving rise to the peculiar optical conductivity behavior with more than one linear regime. Pressure above 4.3 GPa stabilizes the more complex Te-II and Te-III polymorphs, which are robust metals.

cond-mat.mes-hall↗

arXiv2020

Optical signatures of phase transitions and structural modulation in elemental tellurium under pressure

A room-temperature infrared spectroscopy study of elemental tellurium at pressures up to 8.44 GPa in the energy range 0.015-2 eV is reported. Optical signatures of the high-pressure polymorphs are investigated and compared to the results of density-functional band-structure calculations. A Drude peak is first seen in the optical conductivity around 3.5 GPa indicating a semiconductor-to-metal transition within trigonal Te-I. A sharp increase in the Drude spectral weight and dc-conductivity around 4.3 GPa signals the transformation toward the triclinic Te-II polymorph. An absorption peak around 0.15 eV appears above 5 GPa concomitant with the gradual transformation of Te-II into the structurally similar but incommensurately modulated Te-III. Microscopically, this peak can only be reproduced within a sufficiently large commensurate approximant, suggesting the low-energy optical response as a fingerprint of the structural modulation.

cond-mat.mtrl-sci↗

arXiv2018

Electronic Correlations Among the Dirac Electrons in $α$-(BEDT-TTF)$_2$I$_3$ Unveiled by High-Pressure Optical Spectroscopy

The charge-ordered insulator $α$-(BEDT-TTF)$_2$I$_3$ gradually evolves to a metal when pressure is applied, and at low temperatures the electronic bands form tilted Dirac-like cones. A metallic state with a frequency-independent optical conductivity indicates the coexistence of the trivial and massless Dirac electrons in the system. Our infrared investigations also reveal that at the boundary between insulating and metallic states an energy gap opens due to correlated massive Dirac fermions, which is gradually suppressed when pressure increases.

cond-mat.str-el↗

arXiv2014

Persistence of superconducting condensate far above the critical temperature in YBa$_2$(Cu,Zn)$_3$O$_y$ revealed by $c$-axis optical conductivity measurements for several Zn concentrations and carrier doping levels

The superconductivity precursor phenomena in high temperature cuprate superconductors was studied by direct measurements of superconducting condensate with the use of the $c$-axis optical conductivity of YBa$_{2}$(Cu$_{1-x}$Zn$_{x}$)$_{3}$O$_{y}$ for several doping levels ($p$) as well as for several Zn-concentrations. Both of the real and imaginary part of the optical conductivity clearly showed that the superconducting carriers persist up to the high temperatures $T$$_{p}$ that is higher than the critical temperature $T$$_{c}$ but lower than the pseudogap temperature $T$$^{\ast}$. $T$$_{p}$ increases with reducing doping level like $T$$^{\ast}$, but decreases with Zn-substitution unlike $T$$^{\ast}$.

cond-mat.supr-con↗

arXiv2013

Coexistence of the Pseudogap and the Superconducting Gap Revealed by the \textit{c}-axis Optical Study of YBa$_{2}$(Cu$_{1-x}$Zn$_{x}$)$_{3}$O$_{7-δ}$

To address the issue of the pseudogap in the superconducting state of the high temperature superconducting cuprates, we studied the temperature dependent redistribution of the spectral weight in the \textit{c}-axis polarized optical spectra for underdoped YBa$_{2}$(Cu$_{1-x}$Zn$_{x}$)$_{3}$O$_{7-δ}$ single crystals with $x=0,0.007,$ 0.012, and 0.04. It was found that for Zn substituted samples, the spectral weight transfer to the high energy region, which characterizes the pseudogap state, continues even below the superconducting transition temperature (\textit{T}$_{c}$), indicating the coexistence of the pseudogap and the superconducting gap. Moreover, the observation of the pseudogap in the Zn-doped non-superconducting sample gives evidence that the pseudogap is not a precursor of superconductivity.

cond-mat.supr-con↗