Short-range Fluctuations Research Articles

Nanoscale polar regions embedded within ferroelectric domains in Na1/2Bi1/2TiO3−BaTiO3

Relaxor ferroelectrics are an eminent group of functional materials, characterized by complex micro- and nanoscale structures, accounting for their enhanced piezoelectric properties. In the $(1\ensuremath{-}x){\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_{3}\text{\ensuremath{-}}x{\mathrm{BaTiO}}_{3}$ (NBT-BT) solid solution, the evolution of nanoscale domains and their hierarchical association with ferroelectric domains is investigated using conventional and scanning transmission electron microscopy on compositions with 6, 9, and 12 mol % BT. Short-range fluctuations in the local polar displacement (polar nanoregions, PNRs) account for a heterogeneous nanostructure at the morphotropic phase boundary (6 mol % BT). Platelike nanodomains of tetragonal $P4bm$ symmetry coexist with a minor volume fraction of rhombohedral $R3c$ nanodomains. Their overall population decreases with increasing BT content. However, ferroelectric $P4mm$ domains in the composition with 12 mol % BT still exhibit nanoscale regions, which deviate from the average polarization. Small volume fractions of both $P4bm$ and $R3c$ nanodomains remain embedded within the ferroelectric domains. This hierarchical domain configuration underpins the complex structural characteristics of NBT-based relaxor ferroelectrics.

Degenerate plaquette physics as key ingredient of high-temperature superconductivity in cuprates

We study the physics of high-temperature cuprate superconductors starting from the highly degenerate four-site plaquette of the t-t^{prime} -U Hubbard model as a reference system. The degeneracy causes strong fluctuations when a lattice of plaquettes is constructed. We show that there is a large binding energy between holes when a set of four plaquettes is considered. The next-nearest-neighbour hopping t^{prime} plays a crucial role in the formation of these strongly bound electronic bipolarons whose coherence at lower temperature could be the explanation for superconductivity. A complementary approach is cluster dual fermion starting from a single degenerate plaquette, which contains the relevant short-ranged fluctuations from the beginning. It gives d-wave superconductivity as the leading instability under a reasonably broad range of parameters. The origin of the pseudogap is also discussed in terms of the coupling of degenerate plaquettes. Thus, some of the essential elements of cuprate superconductivity appear from the local plaquette physics.

Short length scale fluctuations in lattice growth models

Fluctuations of interfaces produced by lattice growth models scale as those of stochastic equations at distances r much larger than the lattice constant a. However, those equations may be derived from the short range interactions through renormalization, which suggests that universal properties may also be observed in short scale fluctuations of the lattice models. We first investigate this question in interfaces with preset exact power law structure factors by expanding the autocovariance function, which is shown to scale as r2α+constant at distances as small as r∼5a (α is the roughness exponent). Next we perform numerical simulations of lattice models in five universality classes, in one and two dimensions, and calculate the autocovariance function and the fluctuation of a local average height in their growth regimes, where finite-size effects are negligible. In cases of normal roughening with α>0, those quantities also scale as affine functions of r2α in distances from a to ∼10a, in contrast with the usual expectation that such relation is applicable only in the hydrodynamic limit. In a model with super-roughening in the Mullins–Herring class, similar relation is applicable with the local roughness exponent in one and two dimensions. In cases with α≤0, the fits of those functions diverge in the zero distance limit, in the same form as the one-point fluctuations of the corresponding stochastic equations. Finally, we study competitive models with crossovers in the roughness evolution and show that, at any given time, short and long range fluctuations scale with the exponent α of the dominant universality class at that time. Thus, short range correlations at long times do not keep a memory on the short time kinetics. These results reinforce the connection between discrete and continuous growth models by showing that their short range fluctuations have related properties.

Short-Range Backbone Dihedral Rotations Modulate Internal Friction in Intrinsically Disordered Proteins.

Protein folding and dynamics are governed by an intricate interplay of thermal and viscosity-mediated effects. The solvent viscosity contributes to the frictional drag in protein dynamics. In addition to this viscosity-dependent effect, there is also an intriguing viscosity-independent component that represents the intrinsic resistance of the polypeptide chain to changing its conformation. This solvent-independent component is termed internal friction. A longstanding question is what is the fundamental molecular origin of internal friction in highly solvated and rapidly fluctuating intrinsically disordered proteins (IDPs) devoid of any persistent intrachain interactions? Here, we present a unique case to directly demonstrate that sequence-specific backbone dihedral barriers control local internal friction in an archetypal IDP, namely, α-synuclein. We performed site-directed fluorescence depolarization kinetics using picosecond time-resolved fluorescence anisotropy measurements to directly observe the directional decorrelation arising due to short-range backbone torsional fluctuations in the dihedral space. A linear viscosity-dependent model of the dihedral relaxation time yielded a finite zero-viscosity intercept that corresponds to internal friction. Our site-specific dynamic readouts were able to detect localized sequence-specific frictional components that are otherwise skewed in viscosity-dependent long-range chain fluctuations. Our results revealed the presence of low internal friction in nonproline sequence segments. In contrast, a proline introduces torsional stiffness in the segment exhibiting high internal friction that can be compensated by a conformationally flexible glycine. Such an intriguing interplay of local dihedral dynamics can modulate sequence-dependent internal friction in a wide range of IDPs involved in a myriad of important events including folding, binding, assembly, and phase transitions.

Cu9O2(SeO3)4Cl6 revisited: Crystal structure, Raman scattering and first-principles calculations

Chloromenite, Cu9O2(SeO3)4Cl6, is the product of volcanic activity. To conduct various physical studies both natural and synthetic samples of chloromenite were used which resulted in a long-standing controversy regarding its crystal structure. We carried out the temperature dependent single crystal X-ray diffraction study in the range 100 – 370 K and established that the reflection conditions are consistent with a primitive monoclinic cell with the only possible P21/n space group. Raman scattering evidences a complex interplay of crystal lattice and spin degrees of freedom. The observation of a sequence of short range fluctuation effects points to the existence of competing interactions or nearly degenerate energy scales. To gain microscopic understanding of the magnetic properties of Cu9O2(SeO3)4Cl6 the first principles density functional theory calculations have been performed. Basing on the new data on magnetization and the hierarchy of exchange interaction parameters a model of Cu9O2(SeO3)4Cl6 magnetic structure is suggested.

Short-Range Nematic Fluctuations in Sr_{1-x}Na_{x}Fe_{2}As_{2} Superconductors.

Interactions between nematic fluctuations, magnetic order and superconductivity are central to the physics of iron-based superconductors. Here we report on in-plane transverse acoustic phonons in hole-doped Sr_{1-x}Na_{x}Fe_{2}As_{2} measured via inelastic x-ray scattering, and extract both the nematic susceptibility and the nematic correlation length. By a self-contained method of analysis, for the underdoped (x=0.36) sample, which harbors a magnetically ordered tetragonal phase, we find it hosts a short nematic correlation length ξ∼10 Å and a large nematic susceptibility χ_{nem}. The optimal-doped (x=0.55) sample exhibits weaker phonon softening effects, indicative of both reduced ξ and χ_{nem}. Our results suggest short-range nematic fluctuations may favor superconductivity, placing emphasis on the nematic correlation length for understanding the iron-based superconductors.

Crystal Lattice Design of H2O-Tolerant n-Type Semiconducting Dianionic Naphthalenediimide Derivatives.

Dianionic bis(propionate)-naphthalenediimide (PCNDI2-) formed simple 2:1 cation-anion salts of (M+)2(PCNDI2-)·(H2O)n (M+ = Li+, Na+, K+, Rb+, and Cs+), which exhibited reversible H2O adsorption-desorption behavior because of the presence of their electrostatically binding crystal lattices. The maximum H2O adsorption amounts (n) for M+ = Li+, Na+, K+, Rb+, and Cs+ were 0.25, 6.0, 4.0, 6.0, and 2.0, respectively, whereas the reversible gate-opening (gate-closing) H2O adsorption-desorption isotherms were observed at 273 and 298 K, except for M+ = Li+. High ionic conductivities of around 10-4-10-5 S cm-1 were observed in M+ = Na+ and K+ salts, whereas short-range thermal fluctuations occurred in large cations of M+ = Rb+ and Cs+. The change in the electrostatic lattice energy for M+ = Na+ and K+ salts during the H2O adsorption-desorption cycles was significantly larger than those for M+ = Rb+ and Cs+. Therefore, the Na+ and K+ salts had a considerably flexible electrostatic crystal lattice with a large amplitude of lattice modulation during the H2O sorption cycle. In contrast, the lattice modulation for M+ = Rb+ and Cs+ salts involved a low magnitude of ion displacements, forming a relatively rigid cation-anion electrostatic crystal lattice. The flash-photolysis time-resolved microwave conductivity and transition absorption spectroscopy results revealed the high electron mobility of H2O-adsorbed thin films, wherein the crystallized H2O molecules did not act as electron-trapping sites. The values of electron mobility increased in the order of Cs+ ≈ Rb+ > K+ > Na+ > Li+.

Hund-Heisenberg model in superconducting infinite-layer nickelates

We theoretically investigate the unconventional superconductivity in the newly discovered infinite-layer nickelates Nd$_{1-x}$Sr$_{x}$NiO$_{2}$ based on a two-band model. By analyzing the transport experiments, we propose that the doped holes dominantly enter the Ni $d_{xy}$ or/and $d_{3z^{2}-r^{2}}$ orbitals as charged carriers, and form a conducting band. Via the onsite Hund coupling, the doped holes are coupled to the Ni localized holes in the $d_{x^{2}-y^{2}}$ orbital band. We demonstrate that this two-band model could be further reduced to a Hund-Heisenberg model. Using the reduced model, we show the non-Fermi liquid state above the critical $T_{c}$ could stem from the carriers coupled to the spin fluctuations of the localized holes. In the superconducting phase, the short-range spin fluctuations mediate the carriers into Cooper pairs and establish $d_{x^{2}-y^{2}}$-wave superconductivity. We further predict that the doped holes ferromagnetically coupled with the local magnetic moments remain itinerant even at very low temperature, and thus the pseudogap hardly emerges in nickelates. Our work provides a new superconductivity mechanism for strongly correlated multi-orbital systems and paves a distinct way to exploring new superconductors in transition or rare-earth metal oxides.

Single Crystal Heat Capacity Measurement of Charge Glass Compound θ-(BEDT-TTF)2CsZn(SCN)4 Performed under Current and Voltage Application

Heat capacity measurements of θ-(BEDT-TTF)2CsZn(SCN)4 in its non-equilibrium electronic states induced by applying electric currents and voltages were performed by a modified relaxation calorimetry technique. We developed a single crystal heat capacity measurements system by which the Joule heating produced in samples by external currents and voltages can be balanced with the cooling power to make a non-equilibrium steady state. Although temperature versus time profiles in the relaxation process in calorimetry can be obtained as exponential curves as in the usual relaxation technique, we found that the change of resistivity that occurs during the heating and relaxation process should be taken into account in analyzing the data. By correcting this factor in the analyses, we succeeded in evaluating absolute values of Cp(I) and Cp(V) in these non-equilibrium states. The experiments up to 150 μA and the constant voltage of 20 mV do not induce visible change in the structure of the Boson peak in CpT−3 vs. T suggestive of the glassy ground state of phonons. Although the suppression of the short-range fluctuations of the charge density has been reported, it does not seriously affect the glassy phonons in this current range.

Spectroscopic evidence of bilayer splitting and strong interlayer pairing in the superconductor KCa2Fe4As4F2

In high temperature cuprate superconductors, the interlayer coupling between the CuO$_2$ planes plays an important role in dictating superconductivity, as indicated by the sensitive dependence of the critical temperature (T$_C$) on the number of CuO$_2$ planes in one structural unit. In Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ superconductor with two CuO$_2$ planes in one structural unit, the interaction between the two CuO$_2$ planes gives rise to band splitting into two Fermi surface sheets (bilayer splitting) that have distinct superconducting gap. The iron based superconductors are composed of stacking of the FeAs/FeSe layers; whether the interlayer coupling can cause similar band splitting and its effect on superconductivity remain unclear. Here we report high resolution laser-based angle-resolved photoemission spectroscopy (ARPES) measurements on a newly discovered iron based superconductor, KCa$_2$Fe$_4$As$_4$F$_2$ (T$_C$=33.5\,K) which consists of stacking FeAs blocks with two FeAs layers separated by insulating Ca$_2$F$_2$ blocks. Bilayer splitting effect is observed for the first time that gives rise to totally five hole-like Fermi surface sheets around the Brilliouin zone center. Band structure calculations reproduce the observed bilayer splitting by identifying interlayer interorbital interaction between the two FeAs layers within one FeAs block. All the hole-like pockets around the zone center exhibit Fermi surface-dependent and nodeless superconducting gap. The gap functions with short-range antiferromagetic fluctuations are proposed and the gap symmetry can be well understood when the interlayer pairing is considered. The particularly strong interlayer pairing is observed for one of the bands. Our observations provide key information on the interlayer coupling and interlayer pairing in understanding superconductivity in iron based superconductors.

Magnetic, thermal, and electronic-transport properties of EuMg2Bi2 single crystals

The trigonal compound EuMg2Bi2 has recently been discussed in terms of its topological band properties. These are intertwined with its magnetic properties. Here detailed studies of the magnetic, thermal, and electronic transport properties of EuMg2Bi2 single crystals are presented. The Eu{+2} spins-7/2 in EuMg2Bi2 exhibit an antiferromagnetic (AFM) transition at a temperature TN = 6.7 K, as previously reported. By analyzing the anisotropic magnetic susceptibility chi data below TN in terms of molecular-field theory (MFT), the AFM structure is inferred to be a c-axis helix, where the ordered moments in the hexagonal ab-plane layers are aligned ferromagnetically in the ab plane with a turn angle between the moments in adjacent moment planes along the c axis of about 120 deg. The magnetic heat capacity exhibits a lambda anomaly at TN with evidence of dynamic short-range magnetic fluctuations both above and below TN. The high-T limit of the magnetic entropy is close to the theoretical value for spins-7/2. The in-plane electrical resistivity rho(T) data indicate metallic character with a mild and disorder-sensitive upturn below Tmin = 23 K. An anomalous rapid drop in rho(T) on cooling below TN as found in zero field is replaced by a two-step decrease in magnetic fields. The rho(T) measurements also reveal an additional transition below TN in applied fields of unknown origin that is not observed in the other measurements and may be associated with an incommensurate to commensurate AFM transition. The dependence of TN on the c-axis magnetic field Hperp was derived from the field-dependent chi(T), Cp(T), and rho(T) measurements. This TN(Hperp) was found to be consistent with the prediction of MFT for a c-axis helix with S = 7/2 and was used to generate a phase diagram in the Hperp-T plane.

Short-range charge fluctuations in the two-dimensional Hubbard model

We investigate charge fluctuations in the two-dimensional Hubbard model as a function of doping, interaction strength, next-nearest-neighbor hopping, and temperature within the eight-site dynamical cluster approximation. In the regime of intermediate interaction strengths, we find that $d$ density wave fluctuations, which had previously been postulated using analytical arguments, are present and strong but cannot be interpreted as the cause of the pseudogap in the model, due to their evolution with doping and interaction strength. For all parameters away from half filling, the charge fluctuations investigated, including $d$ density wave fluctuations, are weaker than $d$-wave superconducting fluctuations.

Coupled Sublattice Melting and Charge-Order Transition in Two Dimensions.

Two-dimensional melting is one of the most fascinating and poorly understood phase transitions in nature. Theoretical investigations often point to a two-step melting scenario involving unbinding of topological defects at two distinct temperatures. Here, we report on a novel melting transition of a charge-ordered K-Sn alloy monolayer on a silicon substrate. Melting starts with short-range positional fluctuations in the K sublattice while maintaining long-range order, followed by longer-range K diffusion over small domains, and ultimately resulting in a molten sublattice. Concomitantly, the charge order of the Sn host lattice collapses in a multistep process with both displacive and order-disorder transition characteristics. Our combined experimental and theoretical analysis provides a rare insight into the atomistic processes of a multistep melting transition of a two-dimensional materials system.

Dynamical charge density fluctuations pervading the phase diagram of a Cu-based high-T c superconductor.

Charge density modulations have been observed in all families of high-critical temperature (T c) superconducting cuprates. Although they are consistently found in the underdoped region of the phase diagram and at relatively low temperatures, it is still unclear to what extent they influence the unusual properties of these systems. Using resonant x-ray scattering, we carefully determined the temperature dependence of charge density modulations in YBa2Cu3O7-δ and Nd1+ x Ba2- x Cu3O7-δ for several doping levels. We isolated short-range dynamical charge density fluctuations in addition to the previously known quasi-critical charge density waves. They persist up to well above the pseudogap temperature T*, are characterized by energies of a few milli-electron volts, and pervade a large area of the phase diagram.

Nonequilibrium Casimir pressures in liquids under shear.

In stationary nonequilibrium states coupling between hydrodynamic modes causes thermal fluctuations to become long ranged inducing nonequilibrium Casimir pressures. Here we consider nonequilibrium Casimir pressures induced in liquids by a velocity gradient. Specifically, we have obtained explicit expressions for the magnitude of the shear-induced pressure enhancements in a liquid layer between two horizontal plates that complete and correct results previously presented in the literature. In contrast to nonequilibrium Casimir pressures induced by a temperature or concentration gradient, we find that in shear nonequilibrium contributions from short-range fluctuations are no longer negligible. In addition, it is noted that currently available computer simulations of model fluids in shear observe effects from molecular correlations at nanoscales that have a different physical origin and do not probe shear-induced pressures resulting from coupling of long-wavelength hydrodynamic modes. Even more importantly, we find that in actual experimental conditions, shear-induced pressure enhancements are caused by viscous heating and not by thermal velocity fluctuations. Hence, isothermal computer simulations are irrelevant for the interpretation of experimental shear-induced pressure enhancements.

Exactly solvable Kondo lattice model in the anisotropic limit

In this paper we introduce an exactly solvable Kondo lattice model without any fine-tuning local gauge symmetry. This model describes itinerant electrons interplaying with a localized magnetic moment via only longitudinal Kondo exchange. Its solvability results from conservation of the localized moment at each site, and is valid for arbitrary lattice geometry and electron filling. A case study on square lattice shows that the ground state is a N\'{e}el antiferromagnetic insulator at half-filling. At finite temperature, paramagnetic phases including a Mott insulator and correlated metal are found. The former is a melting antiferromagnetic insulator with a strong short-range magnetic fluctuation, while the latter corresponds to a Fermi liquid-like metal. Monte Carlo simulation and theoretical analysis demonstrate that the transition from paramagnetic phases into the antiferromagnetic insulator is a continuous $2D$ Ising transition. Away from half-filling, patterns of spin stripes (inhomogeneous magnetic order) at weak coupling, and phase separation at strong coupling are predicted. With established Ising antiferromagnetism and spin stripe orders, our model may be relevant to a heavy fermion compound CeCo(In$_{1-x}$Hg$_{x}$)$_{5}$ and novel quantum liquid-crystal order in a hidden order compound URu$_{2}$Si$_{2}$.

Quaternary antiferromagnetic Ba2BiFeS5 with isolated FeS4 tetrahedra**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300504), the National Natural Science Foundation of China (Grant Nos. 11574394, 11774423, and 11822412), the Fundamental Research Funds for the Central Universities, and the Research Funds of Renmin University of China (RUC) (Grant Nos. 15XNLQ07, 18XNLG14, and 19XNLG17).

We report the detailed physical properties of quaternary compound Ba2BiFeS5 with the key structural ingredient of isolated FeS4 tetrahedra. Magnetization and heat capacity measurements clearly indicate that Ba2BiFeS5 has a paramagnetic to antiferromagnetic transition at about 30 K. The calculated magnetic entropy above ordering temperature is much smaller than theoretical value for high-spin Fe3+ ion with S = 5/2, implying the possible short-range antiferromagnetic fluctuation in Ba2BiFeS5.

Quantitative characterization of short-range orthorhombic fluctuations in FeSe through pair distribution function analysis

Neutron and x-ray total scattering measurements have been performed on powder samples of the iron chalcogenide superconductor FeSe. Using pair distribution function (PDF) analysis of the total scattering data to investigate short-range atomic correlations, we establish the existence of an instantaneous, local orthorhombic structural distortion attributable to nematic fluctuations that persists well into the high-temperature tetragonal phase, at least up to 300 K and likely to significantly higher temperatures. This short-range orthorhombic distortion is correlated over a length scale of about 1 nm at 300 K and grows to several nm as the temperature is lowered toward the long-range structural transition temperature. In the low-temperature nematic state, the local instantaneous structure exhibits an enhanced orthorhombic distortion relative to the average structure with a typical relaxation length of 3 nm. The quantitative characterization of these orthorhombic fluctuations sheds light on nematicity in this canonical iron-based superconductor.

Finite-temperature charge dynamics and the melting of the Mott insulator

The Mott insulator is the quintessential strongly correlated electronic state. We obtain complete insight into the physics of the two-dimensional Mott insulator by extending the slave-fermion (holon-doublon) description to finite temperatures. We first benchmark its predictions against state-of-the-art quantum Monte Carlo simulations, demonstrating quantitative agreement. Qualitatively, the short-ranged spin fluctuations both induce holon-doublon bound states and renormalize the charge sector to form the Hubbard bands. The Mott gap is understood as the charge gap renormalized downwards by these spin fluctuations. As temperature increases, the Mott gap closes before the charge gap, causing a pseudogap regime to appear naturally during the melting of the Mott insulator.

Antiferromagnetism and the emergence of frustration in the sawtooth lattice chalcogenide olivines Mn2SiS4−xSex ( x=0–4 )

The magnetism in the sawtooth lattice of Mn in the olivine chalcogenides, ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$ ($x=1--4$), is studied in detail by analyzing their magnetization, specific heat, and thermal conductivity properties and complemented with density functional theory calculations. The air-stable chalcogenides are antiferromagnets and show a linear trend in the transition temperature ${T}_{N}$ as a function of Se content ($x$), which shows a decrease from ${T}_{N}\ensuremath{\approx}86\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ to 66 K for ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$. Additional magnetic anomalies are revealed at low temperatures for all the compositions. Magnetization irreversibilities are observed as a function of $x$. The specific heat and the magnetic entropy indicate the presence of short-range spin fluctuations in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$. A spin-flop antiferromagnetic phase transition in the presence of applied magnetic field is present in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4\ensuremath{-}x}{\mathrm{Se}}_{x}$, where the critical field for the spin flop increases from $x=0$ towards 4 in a nonlinear fashion. Density functional theory calculations show that an overall antiferromagnetic structure with ferromagnetic coupling of the spins in the $ab$ plane minimizes the total energy. The band structures calculated for ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ and ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$ reveal features near the band edges similar to those reported for Fe-based olivines suggested as thermoelectrics; however the experimentally determined thermal transport data do not support superior thermoelectric features. The transition from long-range magnetic order in ${\mathrm{Mn}}_{2}{\mathrm{SiS}}_{4}$ to short-range order and spin fluctuations in ${\mathrm{Mn}}_{2}{\mathrm{SiSe}}_{4}$ is explained using the variation of the Mn-Mn distances in the triangle units that constitutes the sawtooth lattice upon progressive replacement of sulfur with selenium. Overall, the results presented here point towards the role played by magnetic anisotropy and geometric frustration in the antiferromagnetic state of the sawtooth olivines.