Optimising conditions for the growth of nanocrystalline ZnS thin films from acidic chemical baths

https://www.research.manchester.ac.uk/portal/files/49131114/porous_ZnS_accepted_manuscript.pdf

Surfactant and template free synthesis of porous ZnS nanoparticles

The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in a magnetic material, which can be detected electrically via the inverse spin Hall effect in a metallic contact. Since the discovery of SSE in 2008, intensive studies on the SSE have been conducted to elucidate its origin. SSEs appear in a wide range of magnetic materials including ferro-, ferri-, and antiferromagnets and also paramagnets with classical or quantum spin fluctuation. SSE voltage reflects fundamental properties of a magnet, such as elementary excitation, static magnetic order, spin correlation, and spin transport. In this article, we review recent progress on the SSEs in various systems, with particular emphasis on its emerging role as a probe of these magnetic properties in solids. We also briefly discuss the recently discovered nuclear SSE. Expected final online publication date for the Annual Review of Condensed Matter Physics, Volume 14 is March 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

/pdf/spin-seebeck-effect-sensitive-probe-for-elementary-9c38ji9q.pdf

Spin Seebeck Effect: Sensitive Probe for Elementary Excitation, Spin Correlation, Transport, Magnetic Order, and Domains in Solids

Flat-band materials such as the kagome metals or moiré superlattices are of intense current interest. Flat bands can result from the electron motion on numerous (special) lattices and usually exhibit topological properties. Their reduced bandwidth proportionally enhances the effect of Coulomb interaction, even when the absolute magnitude of the latter is relatively small. Seemingly unrelated to these materials is the large family of strongly correlated electron systems, which include the heavy-fermion compounds, and cuprate and pnictide superconductors. In addition to itinerant electrons from large, strongly overlapping orbitals, they frequently contain electrons from more localized orbitals, which are subject to a large Coulomb interaction. The question then arises as to what commonality in the physical properties and microscopic physics, if any, exists between these two broad categories of materials. A rapidly increasing body of strikingly similar phenomena across the different platforms — from electronic localization–delocalization transitions to strange-metal behaviour and unconventional superconductivity — suggests that similar underlying principles could be at play. Indeed, it has recently been suggested that flat-band physics can be understood in terms of Kondo physics. Inversely, the concept of electronic topology from lattice symmetry, which is fundamental in flat-band systems, is enriching the field of strongly correlated electron systems, in which correlation-driven topological phases are increasingly being investigated. In this Perspective article, we elucidate this connection, survey the new opportunities for cross-fertilization across platforms and assess the prospect for new insights that may be gained into correlation physics and its intersection with electronic topology. Flat-band materials such as kagome and moiré lattices and strongly correlated electron systems including heavy-fermion compounds exhibit strikingly similar phenomena of topology and strong correlations. This Perspective article discusses Kondo physics as the underlying theme and a route to a unified understanding.

Flat bands, strange metals and the Kondo effect

We have theoretically investigated the spin Seebeck effect (SSE) in a normal metal (NM)/paramagnetic insulator (PI) bilayer system. Through a linear response approach, we calculated the thermal spin pumping from PI to NM and backflow spin current from NM to PI, where the spin-flip scattering via the interfacial exchange coupling between conduction-electron spin in NM and localized spin in PI is taken into account. We found a finite spin current appears at the interface under the difference in the effective temperatures between spins in NM and PI, and its intensity increases by increasing the density of the localized spin $S$. Our model well reproduces the magnetic-field-induced reduction of the paramagnetic SSE in Pt/${\mathrm{Gd}}_{3}{\mathrm{Ga}}_{5}{\mathrm{O}}_{12}$ experimentally observed when the Zeeman energy is comparable to the thermal energy, which can be interpreted as the suppression of the interfacial spin-flip scattering. The present finding provides an insight into the mechanism of paramagnetic SSEs and the thermally induced spin-current generation in magnetic materials. 

/pdf/mechanism-of-paramagnetic-spin-seebeck-effect-1pvfxiq1.pdf

Mechanism of paramagnetic spin Seebeck effect

As spin caloritronic measurements become increasingly common techniques for characterizing material properties, it is important to quantify potentially confounding effects. We report measurements of the Nernst–Ettingshausen response from room temperature to 5 K in thin film wires of Pt and W, metals commonly used as inverse spin Hall detectors in spin Seebeck characterization. Johnson–Nyquist noise thermometry is used to assess the temperature change in the metals with heater power at low temperatures, and the thermal path is analyzed via finite-element modeling. The Nernst–Ettingshausen response of W is found to be approximately temperature-independent, while the response of Pt increases at low temperatures. These results are discussed in the context of theoretical expectations and the possible role of magnetic impurities in Pt.

https://pubs.aip.org/aip/apl/article-pdf/doi/10.1063/5.0146427/17336459/182405_1_5.0146427.pdf

Nernst–Ettingshausen effect in thin Pt and W films at low temperatures

Zinc sulfide (ZnS) quantum thin films have been deposited from precursors with a variety of OH- concentration onto microscope glass substrates by chemical bath deposition method. The growth and structural properties have been investigated. XRD patterns of ZnS films obtained from acidic solution showed a favorable wurtzite structure, while for those obtained from alkaline solution, showed a sphalerite structure. The growth studied of the deposited films has also shown that the OH- played a vital role in nucleation and the film growth.

OH- Effect on the Growth and Structural Properties of Chemical Bath Deposited ZnS Quantum Thin Films

Shot noise measures out-of-equilibrium current ﬂuctuations and is a powerful tool to probe the nature of current-carrying excitations in quantum systems. Recent shot noise measurements in the heavy fermion strange metal YbRh 2 Si 2 exhibit a strong suppression of the Fano factor ( F ) – the ratio of the current noise to the average current in the DC limit. This system is representative of metals in which electron correlations are extremely strong. Here we carry out the ﬁrst theoretical study on the shot noise of diﬀusive metals in the regime of strong correlations. A Boltzmann-Langevin equation formulation is constructed in a quasiparticle description in the presence of strong correlations. We ﬁnd that F = √ 3 / 4 in such a correlation regime. Hence, the Fano factor suppression observed in experiments on YbRh 2 Si 2 necessitates a loss of the quasiparticles. Our work opens the door to systematic theoretical studies of shot noise as a means of characterizing strongly correlated metallic phases and materials.

pdf/shot-noise-as-a-characterization-of-strongly-correlated-9xmhaenk.pdf

Shot noise as a characterization of strongly correlated metals

The low temperature monoclinic, insulating phase of vanadium dioxide is ordinarily considered nonmagnetic, with dimerized vanadium atoms forming spin singlets, though paramagnetic response is seen at low temperatures. We find a nonlocal spin Seebeck signal in VO2 films that appears below 30 K and that increases with a decrease in temperature. The spin Seebeck response has a nonhysteretic dependence on the in-plane external magnetic field. This paramagnetic spin Seebeck response is discussed in terms of prior findings on paramagnetic spin Seebeck effects and expected magnetic excitations of the monoclinic ground state.

/pdf/spin-seebeck-effect-at-low-temperatures-in-the-nominally-2omtp77q.pdf

Spin Seebeck effect at low temperatures in the nominally paramagnetic insulating state of vanadium dioxide

The spin Seebeck effect (SSE) is sensitive to thermally driven magnetic excitations in magnetic insulators. Vanadium dioxide in its insulating low temperature phase is expected to lack magnetic degrees of freedom, as vanadium atoms are thought to form singlets upon dimerization of the vanadium chains. Instead, we find a paramagnetic SSE response in VO2 films that grows as the temperature decreases below 50 K. The field and temperature dependent SSE voltage is qualitatively consistent with a general model of paramagnetic SSE response and inconsistent with triplet spin transport. The microscopic nature of the magnetic excitations in VO2 requires further examination.

Low temperature spin Seebeck effect in non-magnetic vanadium dioxide

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