Magnetocaloric effect: From materials research to refrigeration devices

A topological superconductor A promising path toward topological quantum computing involves exotic quasiparticles called the Majorana bound states (MBSs). MBSs have been observed in heterostructures that require careful nanofabrication, but the complexity of such systems makes further progress tricky. Zhang et al. identified a topological superconductor in which MBSs may be observed in a simpler way by looking into the cores of vortices induced by an external magnetic field. Using angle-resolved photoemission, the researchers found that the surface of the iron superconductor FeTe0.55Se0.45 satisfies the required conditions for topological superconductivity. Science, this issue p. 182 Angle-resolved photoemission spectroscopy indicates that FeTe0.55Se0.45 harbors Dirac-cone–type spin-helical surface states. Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe1–xSex (x = 0.45; superconducting transition temperature Tc = 14.5 kelvin) hosts Dirac-cone–type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below Tc. Our study shows that the surface states of FeTe0.55Se0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.

Observation of topological superconductivity on the surface of iron-based superconductor

Review on magnetic and related properties of RTX compounds

This article reviews the 40+ year old spin-glass field and one of its earliest model interpretations as a spin density wave. Our description is from an experimental phenomenological point of view with emphasis on new spin glass materials and their relation to topical problems and strongly correlated materials in condensed matter physics. We first simply define a spin glass (SG), give its basic ingredients and explain how the spin glasses enter into the statistical mechanics of classical phase transitions. We then consider the four basic experimental properties to solidly characterize canonical spin glass behavior and introduce the early theories and models. Here the spin density wave (SDW) concept is used to explain the difference between a short-range SDW, i.e. a SG and, in contrast, a long-range SDW, i.e. a conventional magnetic phase transition. We continue with the present state of SG, its massive computer simulations and recent proposals of chiral glasses and quantum SG. We then collect and mention the various SG 'spin-off's'. A major section uncovers the fashionable unconventional materials that display SG-like freezing and glassy ground states, such as (high temperature) superconductors, heavy fermions, intermetallics and Heuslers, pyrochlor and spinels, oxides and chalogenides and exotics, e.g. quasicrystals. Some conclusions and future directions complete the review.

https://iopscience.iop.org/article/10.1088/0034-4885/78/5/052501/pdf

Spin glasses: redux: an updated experimental/materials survey

Dielectrics and Waves.

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Electronic, magnetic, and electric transport properties of Ce3Rh4Sn13 and Ce3Co4Sn13: A comparative study

The structural and electronic properties of the skutterudite-related Ce${}_{3}$$M$${}_{4}$Sn${}_{13}$ and La${}_{3}$$M$${}_{4}$Sn${}_{13}$ intermetallic compounds with $M=\text{Co}$, Ru, and Rh have been investigated. A subtle structural transition from a simple cubic phase (Yb${}_{3}$Rh${}_{4}$Sn${}_{13}$-type structure) to the superlattice variant at ${T}_{D}\ensuremath{\sim}160$ K has been evidenced by x-ray diffraction, resistivity, and specific-heat data. Recently, a high charge density between metal $M$ and Sn atoms was calculated for Ce${}_{3}$Rh${}_{4}$Sn${}_{13}$ and Ce${}_{3}$Co${}_{4}$Sn${}_{13}$. The strong charge accumulation implies a strong $M$-Sn2 covalent bonding interaction. In consequence, a local distortion of the trigonal Sn prisms around metal $M$ can modify the electronic structure of the system. The main goal of this report is to study the electronic structure in the family of RE${}_{3}$$M$${}_{4}$Sn${}_{13}$, where $\text{RE}=\text{Ce}$ or La above and below ${T}_{D}$. We show that the Sn $4d$ x-ray photoelectron spectroscopy spectra have the complex structure, which can be interpreted as a result of different charge distribution around Sn atoms. The shape of the $4d$ Sn lines is also temperature dependent, which we attribute to the local distortion of the Sn2 cages in RE${}_{3}$$M$${}_{4}$Sn${}_{13}$.

Electronic structure and crystallographic properties of skutterudite-related Ce 3 M 4 Sn 13 and La 3 M 4 Sn 13 ( M = Co , Ru, and Rh)

We discuss and interpret the properties of the $\mathrm{Ce}\mathrm{Rh}{\mathrm{Sb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}$ systems in the concentration interval $0\ensuremath{\leqslant}xl0.2$. For $x\ensuremath{\cong}0.13$ a quantum critical point has been observed by us recently and separates the Kondo-insulator (KI) from the metallic non-Fermi-liquid (NFL) state. We present the temperature dependence of the resistivity, the ac and dc magnetic susceptibilities, the specific heat, and the magnetization data through the critical concentration regime $x\ensuremath{\sim}0.12--0.13$, as well as provide their discussion in terms of the transition from the nonmagnetic (Kondo-lattice) insulator to a weakly magnetic and singular non-Fermi liquid. The difference between the Kondo-lattice and the Mott-Hubbard semiconductors at temperature $Tg0$ is emphasized. Also, the difference between those two types of insulating states is discussed. On the basis of the experimental results we propose a schematic phase diagram on the plane $T\text{\ensuremath{-}}x$ and demonstrate the rationale for the existence of the quantum critical point separating KI and NFL phases. Namely, we propose that the metallization of the insulating Kondo lattice in terms of the collective spin-singlet Kondo-lattice state destruction and a concomitant delocalization of the $4f$ electrons, within an effective $f$-band model of correlated electrons undergoing the phase transition to the insulating state.

From Kondo semiconductor to a singular non-Fermi liquid via a quantum critical point: The case of Ce Rh Sb 1 − x Sn x

The aim of this work is to investigate electronic structure, magnetic properties and electrical resistivity of Fe2V1?xTixAl Heusler alloys. Numerical calculations give a pseudogap at the Fermi level for the majority-spin band of Fe2TiAl and a magnetic moment larger than 0.9??B, whereas the ground state of Fe2VAl is calculated as a nonmagnetic semimetal with a very low total density of states at the Fermi level. In our calculations the remaining alloys of the Fe2V1?xTixAl series are nonmagnetic for x<0.1 and weakly magnetic for 0.1<x?1. The magnetic moment ? of the series of Fe2V1?xTixAl compounds scales with the number of valence electrons and fits well to the Slater?Pauling curve. We also present a study of the electronic transport properties and magnetic susceptibility. The resistivities ?(T) of Fe2VAl and Fe2V0.9Ti0.1Al are large and exhibit a negative temperature coefficient d?/dT of the resistivity between 2 and 300?K. Below 20?K, ?(T) also shows an activated character. The magnetic susceptibility of Fe2VAl and Fe2V0.9Ti0.1Al shows a maximum at ~2?K which could reflect either the disorder effect or the hybridization gap, characteristic of Kondo insulators.

http://iopscience.iop.org/article/10.1088/0953-8984/18/46/002/pdf

Electronic structure, magnetic properties and electrical resistivity of the Fe(2)V(1-x)Ti(x)Al Heusler alloys: experiment and calculation.

We investigate the electronic structure of the skutterudite-related ${\mathrm{Ce}}_{3}{M}_{4}{\mathrm{Sn}}_{13}$ compounds, where metal $M$ is Co, Ru, or Rh, using x-ray photoemission spectroscopy (XPS) and band structure calculations, both within local density approximation scheme and within scheme including additional Coulomb correlations (LDA+$U$). The $d$-electron states located near the Fermi level slightly change the intensity with applying the correlation energy $U$. We note the large impact of $d$-electron correlation, which is clearly observed in the change of Ce $5p$ states located about 18 eV in the valence band. Namely, for ${\mathrm{Ce}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ and ${\mathrm{Ce}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Sn}}_{13}$ the $d$-electron correlations significantly shift the $5p$ states towards higher binding energies, while for ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ the energy of the Ce $5p$ states almost does not depend on $U$. The influence of the $d$-electron correlations is also manifested by the positive magnetoresistivity (MR) of ${\mathrm{Ce}}_{3}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ and the ${\mathrm{Ce}}_{3\ensuremath{-}x}{\mathrm{La}}_{x}{\mathrm{Rh}}_{4}{\mathrm{Sn}}_{13}$ alloys that reaches 40% at the highest magnetic field ($B=9$ T) for the sample $x=0.6$, whereas only a small positive $\text{MR}\ensuremath{\sim}0.5%$ is observed in ${\mathrm{Ce}}_{3}{\mathrm{Ru}}_{4}{\mathrm{Sn}}_{13}$. The magnetoresistivity of ${\mathrm{Ce}}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ exhibits quite different behavior and is negative.

Jerzy Goraus

Papers

Electronic, magnetic, and electric transport properties of Ce3Rh4Sn13 and Ce3Co4Sn13: A comparative study

Electronic structure and crystallographic properties of skutterudite-related Ce 3 M 4 Sn 13 and La 3 M 4 Sn 13 ( M = Co , Ru, and Rh)

From Kondo semiconductor to a singular non-Fermi liquid via a quantum critical point: The case of Ce Rh Sb 1 − x Sn x

Electronic structure, magnetic properties and electrical resistivity of the Fe(2)V(1-x)Ti(x)Al Heusler alloys: experiment and calculation.

Study of d -electron correlations in skutterudite-related Ce 3 M 4 Sn 13 ( M = Co , Ru, and Rh)