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arXiv2017

Heavy-Fermion Superconductivity in CeAg$_2$Si$_2$ --Interplay of Spin and Valence Fluctuations--

We present the pressure-temperature phase diagram of the antiferromagnet CeAg$_2$Si$_2$ established via resistivity and calorimetry measurements under quasi-hydrostatic conditions up to 22.5~GPa. With increasing pressure, the Néel temperature [$T_{\mathrm{N}}(p=0)=8.6$~K] slowly increases up to $T_{\mathrm{N}}=13.4$~K at 9.4~GPa and then vanishes abruptly at the magnetic critical pressure $p_{\mathrm{c}}\sim13$~GPa. For the first time, heavy fermion superconductivity is observed in CeAg$_2$Si$_2$ . Superconductivity emerges at $\sim 11$~GPa and persists over roughly 10~GPa. Partial- and bulk-transition temperatures are highest at $p=16$~GPa, with a maximal $T_{\mathrm{c}^{\rm bulk}}=1.25$~K. In the pressure region of superconductivity, Kondo and crystal-field splitting energies become comparable and resistivity exhibits clear signatures of a Ce-ion valence crossover. The crossover line is located at a rapid collapse in resistivity as function of pressure and extrapolates to a valence transition critical endpoint at critical pressure and temperature of $p_{\rm cr}\sim 17$~GPa and $T_{\rm cr}\sim-13~K$, respectively. Both critical spin and valence fluctuations may build up superconductivity in CeAg$_2$Si$_2$.

cond-mat.str-el↗

arXiv2018

Electronic properties of the ternary system YbPd$_2 $Ge$_{2-x}$

The data on the electronic and magnetic properties of the ternary ytterbium-based alloy YbPd$_2 $Ge$_{1.7}$ are presented and compared with those for YbPd$_2 $Ge$_{2}$ and YbPd$_2 $Ge compounds. YbPd$_2 $Ge$_{1.7}$ demonstrates a moderate heavy-fermion behavior with linear specific heat coefficient $γ\simeq $ 100 mJ/mol K$^2$. Magnetic susceptibility is large and demonstrates nearly Curie behavior. Resistivity shows a power-law $T^{1.2}$-dependence at not too low temperatures $T$. Thermoelectric power has non-monotonous behavior and changes its sign at low $T$.

cond-mat.str-el↗

arXiv2010

$c$-axis charge gap and its critical point in the heavily doped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$

Temperature-dependent inter-plane resistivity, $ρ_c(T)$, was used to characterise the normal state of the iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ over a broad doping range $0\leq x<0.50$. The data were compared with in-plane resistivity, $ρ_a(T)$, and magnetic susceptibility, $χ(T)$, taken in $H \bot c$, as well as Co NMR Knight shift, $^{59}K$, and spin relaxation rate, $1/T_1T$. The inter-plane resistivity data show a clear correlation with the NMR Knight shift, assigned to the formation of the pseudo-gap. Evolution of $ρ_c(T)$ with doping reveals two characteristic energy scales. The temperature of the cross-over from non-metallic, increasing on cooling, behavior of $ρ_c(T)$ at high-temperatures to metallic behavior at low temperatures, $T^*$, correlates well with an anomaly in all three magnetic measurements. This characteristic temperature, equal to approximately 200~K in the parent compound, $x$=0, decreases with doping and vanishes near $x^* \approx$0.25. For doping levels $x \geq 0.166$, an additional feature appears above $T^*$, with metallic behavior of $ρ_c (T)$ found above the low-temperature resistivity increase. The characteristic temperature of this charge-gap formation, $T_{\rm CG}$, vanishes at $x_{\rm CG} \simeq$0.30, paving the way to metallic, $T$-linear, $ρ_c (T)$ close to $x_{\rm CG}$ and super-linear $T$-dependence for $x>x_{\rm CG}$. None of these features are evident in the in-plane resistivity $ρ_a(T)$. For doping levels $x x_{\rm CG}$.

cond-mat.supr-con↗

arXiv2013

Unconventional superconductivity in a two-dimensional repulsive gas of fermions with spin-orbit coupling

We investigate the superconducting instability of a two-dimensional repulsive fermion gas with Rashba spin-orbit coupling $\al_R$. Using renormalization group approach, we find the superconducting transition temperature as a function of the dimensionless ratio $Θ={1}{2}m\al_R^2/E_F$ where $E_F=0$ when the smaller Fermi surface shrinks to a (Dirac) point. The general trend is that superconductivity is enhanced as $Θ$ increases, but in an intermediate regime $Θ\sim0.1$, a dome-like behavior appears. At a very small value of $Θ$, the angular momentum channel $j_z$ in which superconductivity occurs is quite high. With increasing $Θ$, $j_z$ decreases with a step of 2 down to $j_z=6$, after which we find the sequence $j_z=6, 4, 6, 2$, the last value of which continues to $Θ\rightarrow\infty$. In an extended range of $Θ$, the superconducting gap predominantly resides on the large Fermi surface, while Josephson coupling induces a much smaller gap on the small Fermi surface. Below the superconducting transition temperature, we apply mean field theory to derive the self-consistent equations and find the condensation energies. The state with the lowest condensation energy is an unconventional superconducting state which breaks time reversal symmetry, and in which singlet and triplet pairings are mixed. In general, these states are topologically nontrivial, and the Chern number of the state with total angular momentum $j_z$ is $C=2j_z$.

cond-mat.supr-con↗

arXiv2013

Pseudogap and charge dynamics in doped cuprates

Within the microscopic theory of the normal-state pseudogap state, the doping and temperature dependence of the charge dynamics in doped cuprates is studied in the whole doping range from the underdoped to heavily overdoped. The conductivity spectrum in the underdoped and optimally doped regimes contains the low-energy non-Drude peak and unusual midinfrared band. However, the position of the midinfrared band shifts towards to the low-energy non-Drude peak with increasing doping. In particular, the low-energy non-Drude peak incorporates with the midinfrared band in the heavily overdoped regime, and then the low-energy Drude behavior recovers. It is shown that the striking behavior of the low-energy non-Drude peak and unusual midinfrared band in the underdoped and optimally doped regimes is closely related to the emergence of the doping and temperature dependence of the normal-state pseudogap.

cond-mat.str-el↗

arXiv2012

Quantification of Entanglement from Magnetic Susceptibility for a Heisenberg Spin 1/2 system

Temperature and magnetic field dependent magnetization and extraction of entanglement from the experimental data is reported for dichloro(thiazole)copper(II), a Heisenberg spin chain system. The magnetic susceptibility vs. temperature plot exhibits signature of infinite spin chain. Isothermal magnetization measurements (as a function of magnetic field) were performed at various temperatures below the antiferromagnetic (AFM) ordering, where the AFM correlations persist significantly. These magnetization curves are fitted to the theoretically generated Bonner-Fisher model. Magnetic susceptibility can be used as a macroscopic witness of entanglement; using which entanglement is extracted from the experimentally measured magnetic data.

cond-mat.str-el↗

arXiv2013

Flat Bands and Enigma of Metamagnetic Quantum Critical Regime in Sr3Ru2O7

Understanding the nature of field-tuned metamagnetic quantum criticality in the ruthenate Sr3Ru2O7 has presented a significant challenge within condensed matter physics. It is known from experiments that the entropy within the ordered phase forms a peak, and is unexpectedly higher than that outside, while the magnetoresistivity experiences steep jumps near the ordered phase. We find a challenging connection between Sr3Ru2O7 and heavy-fermion metals expressing universal physics that transcends microscopic details. Our construction of the T-B phase diagram of Sr3Ru2O7 permits us to explain main features of the experimental one, and unambiguously implies an interpretation of its extraordinary low-temperature thermodynamic in terms of fermion condensation quantum phase transition leading to the formation of a flat band at the restricted range of magnetic fields B. We show that it is the flat band that generates both the entropy peak and the resistivity jumps at the QCPs.

cond-mat.str-el↗

arXiv2014

Cumulative quantum work-deficit versus entanglement in the dynamics of an infinite spin chain

We find that the dynamical phase transition (DPT) in nearest-neighbor bipartite entanglement of time-evolved states of the anisotropic infinite quantum XY spin chain, in a transverse time-dependent magnetic field, can be quantitatively characterized by the dynamics of an information-theoretic quantum correlation measure, namely, quantum work-deficit (QWD). We show that only those nonequilibrium states exhibit entanglement resurrection after death, on changing the field parameter during the DPT, for which the cumulative bipartite QWD is above a threshold. The results point to an interesting inter-relation between two quantum correlation measures that are conceptualized from different perspectives.

quant-ph↗

arXiv2014

Renormalization group approach to a $p$-wave superconducting model

We present in this work an exact renormalization group (RG) treatment of a one-dimensional $p$-wave superconductor. The model proposed by Kitaev consists of a chain of spinless fermions with a $p$-wave gap. It is a paradigmatic model of great actual interest since it presents a weak pairing superconducting phase that has Majorana fermions at the ends of the chain. Those are predicted to be useful for quantum computation. The RG allows to obtain the phase diagram of the model and to study the quantum phase transition from the weak to the strong pairing phase. It yields the attractors of these phases and the critical exponents of the weak to strong pairing transition. We show that the weak pairing phase of the model is governed by a chaotic attractor being non-trivial from both its topological and RG properties. In the strong pairing phase the RG flow is towards a conventional strong coupling fixed point. Finally, we propose an alternative way for obtaining $p$-wave superconductivity in a one-dimensional system without spin-orbit interaction.

cond-mat.supr-con↗

arXiv2018

Characterizing quantum phase transition by teleportation

In this paper we provide a novel way to explore the relation between quantum teleportation and quantum phase transition. We construct a quantum channel with a mixed state which is made from one dimensional quantum Ising chain with infinite length, and then consider the teleportation with the use of entangled Werner states as input qubits. The fidelity as a figure of merit to measure how well the quantum state is transferred is studied numerically. Remarkably we find the first-order derivative of the fidelity with respect to the parameter in quantum Ising chain exhibits a logarithmic divergence at the quantum critical point. The implications of this phenomenon and possible applications are also briefly discussed.

quant-ph↗

arXiv2018

Benford analysis of quantum critical phenomena: First digit provides high finite-size scaling exponent while first two and further are not much better

Benford's law is an empirical edict stating that the lower digits appear more often than higher ones as the first few significant digits in statistics of natural phenomena and mathematical tables. A marked proportion of such analyses is restricted to the first significant digit. We employ violation of Benford's law, up to the first four significant digits, for investigating magnetization and correlation data of paradigmatic quantum many-body systems to detect cooperative phenomena, focusing on the finite-size scaling exponents thereof. We find that for the transverse field quantum XY model, behavior of the very first significant digit of an observable, at an arbitrary point of the parameter space, is enough to capture the quantum phase transition in the model with a relatively high scaling exponent. A higher number of significant digits do not provide an appreciable further advantage, in particular, in terms of an increase in scaling exponents. Since the first significant digit of a physical quantity is relatively simple to obtain in experiments, the results have potential implications for laboratory observations in noisy environments.

quant-ph↗

arXiv2014

Mimic the optical conductivity in disordered solids via gauge/gravity duality

We study the optical conductivity in a (2+1)-dimensional non-relativistic field theory holographically dual to a (3+1)-dimensional charged Lifshitz black brane with the Einstein-Maxwell-dilaton theory. Surprisingly, we find that the optical AC conductivity satisfies the nontrivial (non-)power law scaling in the high frequency regime rather than approaching to a constant when the dynamical critical exponent $z>1$, which is qualitatively similar to those in various disordered solids in condensed matter systems. Besides, this (non-)power law scaling behavior shows some universality, which is robust against the temperatures. We argue that the peculiar scaling behavior of AC conductivity may stem from the couplings of the dilaton field with the gauge fields and also the logarithmic behavior near the boundary in the Lifshitz spacetime.

hep-th↗

arXiv2012

Virial expansion for a strongly correlated Fermi system and its application to ultracold atomic Fermi gases

Strongly correlated Fermi system plays a fundamental role in very different areas of physics, from neutron stars, quark-gluon plasmas, to high temperature superconductors. Despite the broad applicability, it is notoriously difficult to be understood theoretically because of the absence of a small interaction parameter. Recent achievements of ultracold trapped Fermi atoms near a Feshbach resonance have ushered in enormous changes. The unprecedented control of interaction, geometry and purity in these novel systems has led to many exciting experimental results, which are to be urgently understood at both low and finite temperatures. Here we review the latest developments of virial expansion for a strongly correlated Fermi gas and their applications on ultracold trapped Fermi atoms. We show remarkable, quantitative agreements between virial predictions and various recent experimental measurements at about the Fermi degenerate temperature. For equation of state, we discuss a practical way of determining high-order virial coefficients and use it to calculate accurately the long-sought third-order virial coefficient, which is now verified firmly in experiments at ENS and MIT. We discuss also virial expansion of a new many-body paramter - Tan's contact. We then turn to less widely discussed issues of dynamical properties. For dynamic structure factor, the virial prediction agrees well with the measurement at the Swinburne University of Technology. For single-particle spectral function, we show that the expansion up to the second order accounts for the main feature of momentum-resolved rf-spectroscopy for a resonantly interacting Fermi gas, as recently reported by JILA. In the near future, more practical applications with virial expansion are possible, owing to the ever-growing power in computation.

cond-mat.quant-gas↗

arXiv2012

Rare-earth mononitrides

When the rare earth mononitrides (RENs) first burst onto the scientific scene in the middle of last century, there were feverish dreams that their strong magnetic moment would afford a wide range of applications. For decades research was frustrated by poor stoichiometry and the ready reaction of the materials in ambient conditions, and only recently have these impediments finally been overcome by advances in thin film fabrication with ultra-high vacuum based growth technology. Currently, the field of research into the RENs is growing rapidly, motivated by the materials demands of proposed electronic and spintronic devices. Both semiconducting and ferromagnetic properties have been established in some of the RENs which thus attract interest for the potential to exploit the spin of charge carriers in semiconductor technologies for both fundamental and applied science. In this review, we take stock of where progress has occurred within the last decade in both theoretical and experimental fields, and which has led to the point where a proof-of-concept spintronic device based on RENs has already been demonstrated. The article is organized into three major parts. First, we describe the epitaxial growth of REN thin films and their structural properties, with an emphasis on their prospective spintronic applications. Then, we conduct a critical review of the different advanced theoretical calculations utilised to determine both the electronic structure and the origins of the magnetism in these compounds. The rest of the review is devoted to the recent experimental results on optical, electrical and magnetic properties and their relation to current theoretical descriptions. These results are discussed particularly with regard to the controversy about the exact nature of the magnetic state and conduction processes in the RENs.

cond-mat.str-el↗

arXiv2013

Superconductivity in Ba1-xKxTi2Sb2O (0 < x < 1) controlled by charge doping

The solid solution of antimonide-oxides Ba1-xKxTi2Sb2O (0 < x < 1) has been synthesized by solid-state reactions and characterized by X-ray powder diffraction (CeCr2Si2C-type structure; P4/mmm, Z = 1). The crystal structure consists of Ti2Sb2O-layers that are stacked with layers of barium atoms along the c-axis. BaTi2Sb2O is a known superconductor with a critical temperature (Tc) of 1.2 K. Substitution of barium through potassium raises Tc up to 6.1 K at 12 % potassium, while no superconductivity emerges with concentrations higher than 20 %. Anomalies in electrical transport and magnetic susceptibility indicate charge density wave (CDW) instabilities. The CDW transition temperatures (Ta) decrease from 50 K in the parent compound to 28 K at 10 % potassium substitution. No CDW transition was detected at higher concentrations, and no evidence for a reduction of the lattice symmetry below Ta was found. The lattice parameters vary linearly while the unit cell volume increases with higher potassium concentrations. The phase diagrams Tc(x) and Ta(x) of Ba1-xKxTi2Sb2O are remarkably similar to the known series Ba1-xNaxTi2Sb2O (0 < x < 0.33) in spite of the reverse volume effect. From this we conclude that the charge and not the volume determines the phase diagrams of these superconducting antimony oxides.

cond-mat.supr-con↗

arXiv2013

Phase diagram of Ca$_{1-x}$Ce$_x$MnO$_3$ thin films studied by X-ray magnetic circular dichroism

In the perovskite-type Ca$_{1-x}$Ce$_{x}$MnO$_{3}$ (CCMO), one can control the transport and magnetic properties through varying Ce content. In the case of thin films, the properties can also be controlled by epitaxial strain from the substrate through changing it such as YAlO$_{3}$ (YAO), NdAlO$_{3}$ (NAO), and LaSrAlO$_{4}$ (LSAO). However, one cannot measure the magnetization of thin films on NAO substrates by conventional magnetization measurements because of the strong paramagnetic signals from the Nd$^{3+}$ ions. In order to eliminate the influence of Nd$^{3+}$ and to identify magnetic phases of the CCMO thin films, we have performed element-selective X-ray magnetic circular dichroism (XMCD) measurements of the Mn 2{\it p} core level. By studying the anisotropy of the XMCD intensity, we could unambiguously determine the magnetic phase diagram of the CCMO thin films.

cond-mat.str-el↗

arXiv2013

Phase diagram of spin-$\frac 12$ quantum Heisenberg $J_1-J_2$ antiferromagnet on the body-centered-cubic lattice in random phase approximation

Magnetic properties of spin $\frac 12$ $J_1-J_2$ Heisenberg antiferromagnet on body centered cubic lattice are investigated. By using two-time temperature Green's functions, sublattice magnetization and critical temperature depending on the frustration ratio $p=J_2/J_1$ are obtained in both stripe and Néel phase. The analysis of ground state sublattice magnetization and phase diagram indicates the critical end point at $J_2/J_1 =0.714$, in agreement with previous studies.

cond-mat.str-el↗

arXiv2011

Magnetism in superconducting EuFe$_2$As$_{1.4}$P$_{0.6}$ single crystals studied by local probes

We have studied the magnetism in superconducting single crystals of EuFe2 As1.4 P0.6 by using the local probe techniques of zero-field muon spin rotation/relaxation and 151 Eu/57 Fe Mössbauer spec- troscopy. All of these measurements reveal magnetic hyperfine fields below the magnetic ordering temperature TM = 18 K of the Eu2+ moments. The analysis of the data shows that there is a coexistence of ferromagnetism, resulting from Eu2+ moments ordered along the crystallographic c-axis, and superconductivity below TSC \approx 15 K. We find indications for a change in the dynamics of the small Fe magnetic moments (\sim 0.07 μB) at the onset of superconductivity: below TSC the Fe magnetic moments seem to be "frozen" within the ab-plane.

cond-mat.str-el↗