An introductory review of the central ideas in the modern theory of dynamic critical phenomena is followed by a more detailed account of recent developments in the field. The concepts of the conventional theory, mode-coupling, scaling, universality, and the renormalization group are introduced and are illustrated in the context of a simple example---the phase separation of a symmetric binary fluid. The renormalization group is then developed in some detail, and applied to a variety of systems. The main dynamic universality classes are identified and characterized. It is found that the mode-coupling and renormalization group theories successfully explain available experimental data at the critical point of pure fluids, and binary mixtures, and at many magnetic phase transitions, but that a number of discrepancies exist with data at the superfluid transition of $^{4}\mathrm{He}$.

Theory of Dynamic Critical Phenomena

The d1 layer metals TaS2, TaSe2, NbSe2, in all their various polytypic modifications, acquire, below some appropriate temperature, phase conditions that their electromagnetic properties have previously revealed as ‘anomalous’. Our present electron-microscope studies indicate that this anomalous behaviour usually includes the adoption, at some stage, of a superlattice. The size of superlattice adopted often is forecast in the pattern of satellite spotting and strong diffuse scattering found above the transition. Our conclusions are that charge-density waves and their concomitant periodic structural distortions occur in all these 4d1/5d1 dichalcogenides. We have related the observed periodicities of these CDW states to the theoretical form of the parent Fermi surfaces. Particularly for the 1T octahedrally coordinated polytypes the Fermi surface is very simple and markedly two-dimensional in character, with large near-parallel walls. Such a situation is known theoretically to favour the formation of...

Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides

We report high transition temperature superconductivity in one unit-cell (UC) thick FeSe films grown on a Se-etched SrTiO3 (001) substrate by molecular beam epitaxy (MBE). A superconducting gap as large as 20 meV and the magnetic field induced vortex state revealed by in situ scanning tunneling microscopy (STM) suggest that the superconductivity of the 1 UC FeSe films could occur around 77 K. The control transport measurement shows that the onset superconductivity temperature is well above 50 K. Our work not only demonstrates a powerful way for finding new superconductors and for raising TC, but also provides a well-defined platform for systematic studies of the mechanism of unconventional superconductivity by using different superconducting materials and substrates.

https://iopscience.iop.org/article/10.1088/0256-307X/29/3/037402/pdf

Interface-Induced High-Temperature Superconductivity in Single Unit-Cell FeSe Films on SrTiO3

Monolayer iron selenide grown on SrTiO3 has recently gained attention due to suggestive evidence it superconducts at high temperature. In situ electrical transport measurements now reveal a transition temperature above 100 K.

Superconductivity above 100 K in single-layer FeSe films on doped SrTiO3

Recently, interface has been employed to enhance superconductivity in the single-layer FeSe films grown on SrTiO3(001)(STO) with a possible Tc of ~ 80 K, which is nearly ten times of the Tc of bulk FeSe and is above the Tc record of 56 K for the bulk Fe-based superconductors. This work together with those on superconducting oxides interfaces revives the long-standing idea that electron pairing at a two-dimensional (2D) interface between two different materials is a potential path to high transition temperature (Tc) superconductivity. Subsequent angle-resolved photoemission spectroscopy (ARPES) measurements revealed different electronic structure from those of bulk FeSe with a superconducting-like energy gap closing at around 65K. However, previous ex situ electrical transport measurements could only detect the zero-resistance below ~30 K. Here we report the observation of high Tc superconductivity in the FeSe/STO system. By in situ 4-point probe (4PP) electrical transport measurement that can be conducted at an arbitrary position of the FeSe film on STO, we detected superconductivity above 100 K. Our finding makes FeSe/STO the exciting and ideal research platform for higher Tc superconductivity.

Superconductivity in single-layer films of FeSe with a transition temperature above 100 K

We have observed large increases in the transition temperature ${T}_{c}$ of thin, quenched composite films consisting of alternating layers of dissimilar metals. We estimate the regions of the film which experience the greatest increase to be within a few monolayers of the metal-vacuum or metal-metal surfaces. A possible explanation for the increased ${T}_{c}'\mathrm{s}$ is the shift in the electron-phonon coupling to larger values due to the lowering of the phonon frequencies.

Enhanced Superconductivity in Layered Metallic Films

Temperature-dependent neutron-scattering experiments have been performed on a polycrystalline sample of ${\mathrm{Ce}}_{0.74}$${\mathrm{Th}}_{0.26}$ which undergoes a first-order $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\alpha}$ valence transition at ${T}_{V}\ensuremath{\sim}150$ K. By a measurement of the temperature dependence of the lattice parameter and use of V\'egard's law, we estimate the temperature behavior of the valence of Ce. The $Q$ dependence of the magnetic scattering is found to follow the form factor of the ${\mathrm{Ce}}^{3+}$ ion surprisingly well. In the inelastic scans, particular attention has been paid to the subtraction of the phonon background via an inelastic study of an identically sized and shaped sample of the nonmagnetic material ${\mathrm{La}}_{0.73}$${\mathrm{Th}}_{0.27}$. The corrected ${\mathrm{Ce}}_{0.74}$${\mathrm{Th}}_{0.26}$ spectra have then been expressed in the form of the imaginary part of the susceptibility ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},\ensuremath{\omega})$. The $\ensuremath{\gamma}$-phase dynamic susceptibility is a broad feature with significant intensity extending beyond 70.0 meV (the limit of our measurement) with a peak near \ensuremath{\sim}20.0 meV. On cooling below ${T}_{V}$ the susceptibility decreases in magnitude and broadens such that the peak is beyond 70.0 meV. These results are compared with the macroscopic magnetic susceptibility which exhibits behavior similar to $\ensuremath{\chi}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}})$ obtained from ${\ensuremath{\chi}}^{\ensuremath{'}\ensuremath{'}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},\ensuremath{\omega})$ by a Kramers-Kronig analysis.

Spin dynamics in the mixed valence alloy Ce 1-x Th x

Emphasis is placed on the temperature dependence of the magnetic resistance ${R}_{\mathrm{mag}}$ in the region ${10}^{\ensuremath{-}4}\ensuremath{\lesssim}|\ensuremath{\epsilon}|\ensuremath{\lesssim}{10}^{\ensuremath{-}2}$, where $\ensuremath{\epsilon}=\frac{(T\ensuremath{-}{T}_{c})}{{T}_{c}}$. The temperature dependence of $\frac{d{R}_{\mathrm{mag}}}{\mathrm{dT}}$ is found to be the same, within experimental error, as that of the specific heat, both above and below ${T}_{c}$. The anomalous behavior in the region $0\ensuremath{\lesssim}\ensuremath{\epsilon}\ensuremath{\lesssim}5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ reported by Craig, Goldburg, Kitchens, and Budnick is not observed.

Electrical Resistivity of Nickel Near the Curie Point

The isomorphic phase transition in the system ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Th}}_{x}$, which is characterized by a delocalization of the $4f$ electron and a concomitant volume change, has been studied through measurements of the density and resitivity. For samples near the critical composition ($x\ensuremath{\simeq}0.264$) derivatives of these quantities diverge with mean-field exponents. Such mean-field behavior implies a long-force range, and suggests that electroelastic coupling dominates the critical behavior.

R. D. Parks

Papers

Enhanced Superconductivity in Layered Metallic Films

Spin dynamics in the mixed valence alloy Ce 1-x Th x

Electrical Resistivity of Nickel Near the Curie Point

Critical Behavior at a Valence Instability

αβ transition in quartz: Classical versus critical behavior