arXiv subjects

N. Barišić

Publications and source records attributed to N. Barišić.

27 records · Page 2Linked to original sources

arXiv2012

Universal superconducting fluctuations and the implications for the phase diagram of the cuprates

Superconductivity in the cuprates emerges from an enigmatic metallic state. There remain profound open questions regarding the universality of observed phenomena and the character of precursor fluctuations above the superconducting (SC) transition temperature (T_c). For single-CuO_2-layer La_{2-x}Sr_xCuO_4 (LSCO) and Bi_2(Sr,La)_2CuO_{6+δ} (Bi2201), some experiments seem to indicate an onset of SC fluctuations at very high temperatures (2-3 times T_c^{max}, the T_c value at optimal hole concentration p), whereas other measurements suggest that fluctuations are confined to the immediate vicinity of T_c(p). Here we use torque magnetization to resolve this conundrum by systematically studying LSCO, Bi2201 and HgBa_2CuO_{4+δ} (Hg1201). The latter is a more ideal single-layer compound, featuring high structural symmetry, minimal disorder, and T_c^{max} = 97 K, a value more than twice those of LSCO and Bi2201. We find in all three cases that SC diamagnetism vanishes in an unusual exponential fashion above T_c, and at a rapid rate that is universal. Furthermore, the high characteristic fluctuation temperatures of LSCO and Bi2201 closely track T_c(p) of Hg1201. These observations suggest that, rather than being indicative of SC diamagnetism, the fluctuations at high temperatures in the low-T_c^{max} compounds are associated with a competing order. This picture is further supported by an analysis of available results for double-layer cuprates.

cond-mat.supr-con↗

arXiv2012

Evidence for a Fermi liquid in the pseudogap phase of high-Tc cuprates

Cuprate high-T_c superconductors on the Mott-insulating side of "optimal doping" (with respect to the highest T_c's) exhibit enigmatic behavior in the non-superconducting state. Near optimal doping the transport and spectroscopic properties are unlike those of a Landau-Fermi liquid. For carrier concentrations below optimal doping a pseudogap removes quasi-particle spectral weight from parts of the Fermi surface, and causes a break-up of the Fermi surface into disconnected nodal and anti-nodal sectors. Here we show that the near-nodal excitations of underdoped cuprates obey Fermi liquid behavior. Our optical measurements reveal that the dynamical relaxation rate 1/tau(omega,T) collapses on a universal function proportional to (hbar omega)^2+(1.5 pi k_B T)^2. Hints at possible Fermi liquid behavior came from the recent discovery of quantum oscillations at low temperature and high magnetic field in underdoped YBa2Cu3O6+d and YBa2Cu4O8, from the observed T^2-dependence of the DC (ω=0) resistivity for both overdoped and underdoped cuprates, and from the two-fluid analysis of nuclear magnetic resonance data. However, the direct spectroscopic determination of the energy dependence of the life-time of the excitations -provided by our measurements- has been elusive up to now. This observation defies the standard lore of non-Fermi liquid physics in high T_c cuprates on the underdoped side of the phase diagram.

cond-mat.supr-con↗

arXiv2010

High pressure study of transport properties in Co$_{1/3}$NbS$_2$

This is the first study of the effect of pressure on transition metal dichalcogenides intercalated by atoms that order magnetically. Co$_{1/3}$NbS$_2$ is a layered system where the intercalated Co atoms order antiferromagnetically at T$_N$ = 26 K at ambient pressure. We have conducted a detailed study of dc-resistivity ($ρ$), thermoelectric power (S) and thermal conductivity ($κ$). We found that at ambient pressure the magnetic transition corresponds to a well pronounced peak in dS/dT, as well as to a kink in the dc-resistivity. The effect of ordering on the thermal conductivity is rather small but, surprisingly, more pronounced in the lattice contribution than in the electronic contribution to $κ$. Under pressure, the resistivity increases in the high temperature range, contrary to all previous measurements in other layered transition metal dichalcogenides (TMD). In the low temperature range, the strong dependences of thermopower and resistivity on pressure are observed below TN, which, in turn, also depends on pressure at rate of dT$_N$/dp $\approx$ -1 K/kbar. Several possible microscopic explanations of the reduction of the ordering temperature and the evolution of the transport properties with pressure are discussed.

cond-mat.str-el↗

arXiv2010

Electrodynamics of electron doped iron-pnictide superconductors: Normal state properties

The electrodynamic properties of Ba(Fe$_{0.92}$Co$_{0.08})_2$As$_{2}$ and Ba(Fe$_{0.95}$Ni$_{0.05})_As$_{2}$ single crystals have been investigated by reflectivity measurements in a wide frequency range. In the metallic state, the optical conductivity consists of a broad incoherent background and a narrow Drude-like component which determines the transport properties; only the latter contribution strongly depends on the composition and temperature. This subsystem reveals a $T^2$ behavior in the dc resistivity and scattering rate disclosing a hidden Fermi-liquid behavior in the 122 iron-pnictide family. An extended Drude analysis yields the frequency dependence of the effective mass (with $m^*/m_b\approx 5$ in the static limit) and scattering rate that does not disclose a simple power law. The spectral weight shifts to lower energies upon cooling; a significant fraction is not recovered within the infrared range of frequencies.

cond-mat.supr-con↗

arXiv2009

Effects of the magnetic orderings on the dynamical conductivity: optical investigations of EuFe$_2$As$_2$ single crystals

The magnetic, transport and optical properties of EuFe$_2$As$_2$ single crystals have been investigated parallel and perpendicular to the $ab$-plane. The anisotropy $ρ_c/ρ_{ab}\approx 8$ depends only slightly on temperature. In both orientations, the spin-density wave transition at $T_{\rm SDW}=189$ K shows up as a considerable increase in the dc resistivity. Susceptibility measurements evidence the magnetic order of the Eu$^{2+}$ moments at $T_N=19$ K with little influence on the electronic transport taking place in the FeAs layers. Polarization-dependent infrared spectroscopy reveals strongly anisotropic optical properties and yields a carrier density of only $4.2\times 10^{21} {\rm cm}^{-3}$ and a bandmass of $m_b=2m_0$. A sizeable Drude contribution is present at all temperatures and narrows upon cooling. Below $T_{\rm SDW}$, the spin-density-wave gap develops in the in-plane optical conductivity; no appreciable change is detected for the perpendicular polarization. Modifications in the phonon features are associated with changes of the electronic properties at $T_{\rm SDW}$. The extended Drude analysis yields a linear behavior of the frequency-dependent scattering rate below $T_{\rm SDW}$, indicating an interaction between the charge carriers and spin fluctuations in the spin-density-wave state.

cond-mat.supr-con↗

arXiv2007

The evolution of the Non-Fermi Liquid behavior of BaVS$_3$ under high pressure

Temperature, pressure, and magnetic field dependencies of the resistivity of BaVS$_3$ were measured above the critical pressure of $p_{cr}$=2 GPa, which is associated with the zero temperature insulator-to-metal (MI) transition. The resistivity exhibits the $T^n$ temperature dependence below $T_g\approx$15 K, with $n$ of 1.5 at $p_{cr}$, which increases continuously with pressure towards 2. This is interpreted as a crossover from non-Fermi (NFL) to Fermi-liquid (FL) behavior. Although the spin configuration of the $e_g$ electrons influences the charge propagation, the NFL behavior is attributed to the pseudogap that appears in the single particle spectrum of the $d_z^2$ electrons related to large quasi-one dimensional (Q-1d) 2$k_F$-CDW fluctuations. The non-monotonic magnetic field dependence of $Δ$$ρ$/$ρ$ reveals a characteristic field $B_0\approx$12 T attributed to the full suppression of the pseudogap.

cond-mat.str-el↗

arXiv2007

Hysteretic behavior at the collapse of the metal-insulator transition in BaVS$_3$

Electrical resistivity as a function of temperature, pressure, and magnetic field was measured in high and low purity single crystals of BaVS$_3$ close to the critical pressure value $p_{cr}$$\approx$2 GPa, associated with the zero temperature insulator-to-metal (MI) transition. In the 1.8-2.0 GPa range, where the MI transition is below $\approx$20 K, one can observe a sudden collapse of the MI phase boundary upon increasing pressure and at fixed pressure a magnetic field induced insulator to metal transition. In high quality samples these features are accompanied by hysteresis in all measured physical quantities as a function of temperature and magnetic field. We ascribe these observations to the crossing of the MI and the magnetic phase boundaries upon increasing pressure.

cond-mat.str-el↗

arXiv2006

Quantum critical phase in BaVS$_3$

We study the high-pressure metallic phase of high-purity single crystals of BaVS$_3$ by measuring the temperature, pressure, and magnetic field dependence of the resistivity. Above the critical pressure of $p_{\rm cr}=1.97$GPa an extended non-Fermi liquid $p-T$ regime emerges with resistivity exponent $1.5 \le n<2$, crossing over to a FL only around $p=2.7$GPa. A hysteretic feature indicates that close to the insulator--metal boundary, the system is magnetically ordered. Our findings reveal a close analogy between the extended partially ordered NFL state of non-conventional itinerant magnets and the corresponding state of BaVS$_3

cond-mat.str-el↗

arXiv2003

Pressure induced suppression of the singlet insulator phase in BaVS$_3$: infrared optical study

The metal-insulator (MI) transition in BaVS$_3$ has been studied at ambient pressure and under hydrostatic pressure up to $p=26 $kbar in the frequency range of $20-3000 $cm$^{-1}$. The charge gap determined from the optical reflectivity is enhanced, $Δ_{ch}(p)/k_BT_{MI}(p)\sim 12$. This ratio is independent of pressure indicating that the character of the transition does not vary along the $p-T$ phase boundary. Above the critical pressure, $p_{cr}\sim20$ kbar, metallic spectra were recorded in the whole temperature range, as expected from the shape of the phase diagram. Our results exclude the opening of a pseudogap above $T_{MI}$ at any pressure. Below $T_{MI}$ an unusually strong temperature dependence of the charge gap was observed, resulting in a $Δ_{ch}(T)$ deviating strongly from the mean field-like variation of the structural order parameter.

cond-mat.str-el↗