Condensed Matter

Research

22 pages, 4334 KiB

Open AccessArticle

Measuring the Electron–Phonon Interaction in Two-Dimensional Superconductors with He-Atom Scattering

by Giorgio Benedek, Joseph R. Manson, Salvador Miret-Artés, Adrian Ruckhofer, Wolfgang E. Ernst, Anton Tamtögl and Jan Peter Toennies

Condens. Matter 2020, 5(4), 79; https://doi.org/10.3390/condmat5040079 - 3 Dec 2020

Cited by 13 | Viewed by 4931 | Correction

Abstract

Helium-atom scattering (HAS) spectroscopy from conducting surfaces has been shown to provide direct information on the electron–phonon interaction, more specifically the mass-enhancement factor λ from the temperature dependence of the Debye–Waller exponent, and the mode-selected electron–phonon coupling constants λ_Q_ν from the [...] Read more.

Helium-atom scattering (HAS) spectroscopy from conducting surfaces has been shown to provide direct information on the electron–phonon interaction, more specifically the mass-enhancement factor λ from the temperature dependence of the Debye–Waller exponent, and the mode-selected electron–phonon coupling constants λ_Q_ν from the inelastic HAS intensities from individual surface phonons. The recent applications of the method to superconducting ultra-thin films, quasi-1D high-index surfaces, and layered transition-metal and topological pnictogen chalcogenides are briefly reviewed. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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12 pages, 6127 KiB

Open AccessFeature PaperArticle

First-Principles Calculation of Copper Oxide Superconductors That Supports the Kamimura-Suwa Model

by Hiroshi Kamimura, Masaaki Araidai, Kunio Ishida, Shunichi Matsuno, Hideaki Sakata, Kenji Shiraishi, Osamu Sugino and Jaw-Shen Tsai

Condens. Matter 2020, 5(4), 69; https://doi.org/10.3390/condmat5040069 - 2 Nov 2020

Cited by 4 | Viewed by 3841

Abstract

In 1986 Bednorz and Műller discovered high temperature superconductivity in copper oxides by chemically doping holes into La₂CuO₄ (LCO), the antiferromagnetic insulator. Despite intense experimental and theoretical research during the past 34 years, no general consensus on the electronic-spin structures [...] Read more.

In 1986 Bednorz and Műller discovered high temperature superconductivity in copper oxides by chemically doping holes into La₂CuO₄ (LCO), the antiferromagnetic insulator. Despite intense experimental and theoretical research during the past 34 years, no general consensus on the electronic-spin structures and the origin of pseudogap has been obtained. In this circumstance, we performed a first-principles calculation of underdoped cuprate superconductors La_2-xSr_xCuO₄ (LSCO) within the meta-generalized gradient approximation of the density functional theory. Our calculations clarify first the important role of the anti Jahn-Teller (JT) effect, the backward deformation against the JT distortion in La₂CuO₄ by doping extra holes. The resulting electronic structure agrees with the two-component theory provided by the tight-binding model of Kamimura and Suwa (K-S), which has been also used to elucidate the d-wave superconductivity. Our first-principles calculation thus justifies the K-S model and demonstrates advanced understanding of cuprates. For example, the remarkable feature of our calculations is the appearance of the spin-polarized band with a nearly flat-band character, showing the peaky nature in the density of states at the Fermi level. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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27 pages, 2188 KiB

Open AccessArticle

Perovskite Crystals: Unique Pseudo-Jahn–Teller Origin of Ferroelectricity, Multiferroicity, Permittivity, Flexoelectricity, and Polar Nanoregions

by Isaac B. Bersuker and Victor Polinger

Condens. Matter 2020, 5(4), 68; https://doi.org/10.3390/condmat5040068 - 2 Nov 2020

Cited by 27 | Viewed by 4363

Abstract

In a semi-review paper, we show that the local pseudo-Jahn–Teller effect (PJTE) in transition metal B ion center of ABO₃ perovskite crystals, notably BaTiO₃, is the basis of all their main properties. The vibronic coupling between the ground and excited [...] Read more.

In a semi-review paper, we show that the local pseudo-Jahn–Teller effect (PJTE) in transition metal B ion center of ABO₃ perovskite crystals, notably BaTiO₃, is the basis of all their main properties. The vibronic coupling between the ground and excited electronic states of the local BO₆ center results in dipolar distortions, leading to an eight-well adiabatic potential energy surface with local tunneling or over-the-barrier transitions between them. The intercenter interaction between these dipolar dynamic units results in the formation of the temperature-dependent three ferroelectric and one paraelectric phases with order–disorder phase transitions. The local PJTE dipolar distortion is subject to the presence of sufficiently close in energy local electronic states with opposite parity but the same spin multiplicity, thus limiting the electronic structure and spin of the B(

d^{n}

) ions that can trigger ferroelectricity. This allowed us to formulate the necessary conditions for the transition metal perovskites to possess both ferroelectric and magnetic (multiferroic) properties simultaneously. It clarifies the role of spin in the spontaneous polarization. We also show that the interaction between the independently rotating dipoles in the paraelectric phase may lead to a self-assembly process resulting in polar nanoregions and relaxor properties. Exploring interactions of PJTE ferroelectrics with external perturbations, we revealed a completely novel property—orientational polarization in solids—a phenomenon first noticed by P. Debye in 1912 as a possibility, which was never found till now. The hindered rotation of the local dipole moments and their ordering along an external field is qualitatively similar to the behavior of polar molecules in liquids, thus adding a new dimension to the properties of solids—notably, the perovskite ferroelectrics. We estimated the contribution of the orientational polarization to the permittivity and flexoelectricity of perovskite crystals in different limiting conditions. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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19 pages, 1338 KiB

Open AccessFeature PaperArticle

Temperature-Independent Cuprate Pseudogap from Planar Oxygen NMR

by Jakob Nachtigal, Marija Avramovska, Andreas Erb, Danica Pavićević, Robin Guehne and Jürgen Haase

Condens. Matter 2020, 5(4), 66; https://doi.org/10.3390/condmat5040066 - 21 Oct 2020

Cited by 10 | Viewed by 3569

Abstract

Planar oxygen nuclear magnetic resonance (NMR) relaxation and shift data from all cuprate superconductors available in the literature are analyzed. They reveal a temperature-independent pseudogap at the Fermi surface, which increases with decreasing doping in family-specific ways, i.e., for some materials, the pseudogap [...] Read more.

Planar oxygen nuclear magnetic resonance (NMR) relaxation and shift data from all cuprate superconductors available in the literature are analyzed. They reveal a temperature-independent pseudogap at the Fermi surface, which increases with decreasing doping in family-specific ways, i.e., for some materials, the pseudogap is substantial at optimal doping while for others it is nearly closed at optimal doping. The states above the pseudogap, or in its absence are similar for all cuprates and doping levels, and Fermi liquid-like. If the pseudogap is assumed exponential it can be as large as about 1500 K for the most underdoped systems, relating it to the exchange coupling. The pseudogap can vary substantially throughout a material, being the cause of cuprate inhomogeneity in terms of charge and spin, so consequences for the NMR analyses are discussed. This pseudogap appears to be in agreement with the specific heat data measured for the YBaCuO family of materials, long ago. Nuclear relaxation and shift show deviations from this scenario near

T_{c}

, possibly due to other in-gap states. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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23 pages, 787 KiB

Open AccessFeature PaperArticle

Phase Separation and Pairing Fluctuations in Oxide Materials

by Andreas Bill, Vladimir Hizhnyakov, Reinhard K. Kremer, Götz Seibold, Aleksander Shelkan and Alexei Sherman

Condens. Matter 2020, 5(4), 65; https://doi.org/10.3390/condmat5040065 - 19 Oct 2020

Cited by 1 | Viewed by 3107

Abstract

The microscopic mechanism of charge instabilities and the formation of inhomogeneous states in systems with strong electron correlations is investigated. We demonstrate that within a strong coupling expansion the single-band Hubbard model shows an instability towards phase separation and extend the approach also [...] Read more.

The microscopic mechanism of charge instabilities and the formation of inhomogeneous states in systems with strong electron correlations is investigated. We demonstrate that within a strong coupling expansion the single-band Hubbard model shows an instability towards phase separation and extend the approach also for an analysis of phase separation in the Hubbard-Kanamori hamiltonian as a prototypical multiband model. We study the pairing fluctuations on top of an inhomogeneous stripe state where superconducting correlations in the extended s-wave and d-wave channels correspond to (anti)bound states in the two-particle spectra. Whereas extended s-wave fluctuations are relevant on the scale of the local interaction parameter U, we find that d-wave fluctuations are pronounced in the energy range of the active subband which crosses the Fermi level. As a result, low energy spin and charge fluctuations can transfer the d-wave correlations from the bound states to the low energy quasiparticle bands. Our investigations therefore help to understand the coexistence of stripe correlations and d-wave superconductivity in cuprates. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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9 pages, 1097 KiB

Open AccessArticle

Unconventional Transport Properties of Reduced Tungsten Oxide WO_2.9

by Alexander Shengelaya, Fabio La Mattina and Kazimierz Conder

Condens. Matter 2020, 5(4), 63; https://doi.org/10.3390/condmat5040063 - 16 Oct 2020

Cited by 3 | Viewed by 3287

Abstract

The temperature and magnetic field dependence of resistivity in WO_2.9 was investigated. The variation of resistivity with temperature displayed unusual features, such as a broad maximum around 230 K and a logarithmic increase of resistivity below 16 K. In the temperature range [...] Read more.

The temperature and magnetic field dependence of resistivity in WO_2.9 was investigated. The variation of resistivity with temperature displayed unusual features, such as a broad maximum around 230 K and a logarithmic increase of resistivity below 16 K. In the temperature range 16–230 K, we observed metallic-like behavior with a positive temperature coefficient. The combined analysis of resistivity and magnetoresistance (MR) data shows that these unusual transport properties of WO_2.9 can be understood by considering the (bi)polaronic nature of charge carriers. In contrast to magnetization data, superconducting transition below T_c = 80 K was not detected in resistivity measurements, indicating that the superconductivity is localized in small regions that do not percolate. We found a strong increase in positive MR below 80 K. This effect is similar to that observed in underdoped cuprates, where the substantial increase of MR is attributed to superconducting fluctuations in small clusters. Therefore, the temperature dependence of MR indicates the presence of non-percolating superconducting clusters in WO_2.9 below 80 K in agreement with magnetization data. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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10 pages, 1678 KiB

Open AccessArticle

Ferroelectricity, Superconductivity, and SrTiO₃—Passions of K.A. Müller

by Gernot Scheerer, Margherita Boselli, Dorota Pulmannova, Carl Willem Rischau, Adrien Waelchli, Stefano Gariglio, Enrico Giannini, Dirk van der Marel and Jean-Marc Triscone

Condens. Matter 2020, 5(4), 60; https://doi.org/10.3390/condmat5040060 - 15 Oct 2020

Cited by 15 | Viewed by 6985

Abstract

SrTiO

_{3}

is an insulating material which, using chemical doping, pressure, strain or isotope substitution, can be turned into a ferroelectric material or into a superconductor. The material itself, and the two aforementioned phenomena, have been subjects of intensive research of Karl Alex [...] Read more.

SrTiO

_{3}

is an insulating material which, using chemical doping, pressure, strain or isotope substitution, can be turned into a ferroelectric material or into a superconductor. The material itself, and the two aforementioned phenomena, have been subjects of intensive research of Karl Alex Müller and have been a source of inspiration, among other things, for his Nobel prize-winning research on high temperature superconductivity. An intriguing outstanding question is whether the occurrence of ferroelectricity and superconductivity in the same material is just a coincidence, or whether a deeper connection exists. In addition there is the empirical question of how these two phenomena interact with each other. Here we show that it is possible to induce superconductivity in a two-dimensional layer at the interface of SrTiO

_{3}

and LaAlO

_{3}

when we make the SrTiO

_{3}

ferroelectric by means of

^{18}

O substitution. Our experiments indicate that the ferroelectricity is perfectly compatible with having a superconducting two-dimensional electron system at the interface. This provides a promising avenue for manipulating superconductivity in a non centrosymmetric environment. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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11 pages, 1247 KiB

Open AccessCommunication

Detection of Two Phenomena Opposite to the Expected Ones

by Boris I. Kochelaev

Condens. Matter 2020, 5(4), 56; https://doi.org/10.3390/condmat5040056 - 24 Sep 2020

Cited by 1 | Viewed by 2309

Abstract

Both phenomena mentioned in the title were revealed by the electron paramagnetic resonance (EPR) method. The first phenomenon was found in superconducting La metal with Er impurities—the spin relaxation rate of the erbium impurities was sharply decreasing after transition into the superconducting state [...] Read more.

Both phenomena mentioned in the title were revealed by the electron paramagnetic resonance (EPR) method. The first phenomenon was found in superconducting La metal with Er impurities—the spin relaxation rate of the erbium impurities was sharply decreasing after transition into the superconducting state instead of the expected, i.e., the well-known Hebel–Slichter peak. The second unexpected phenomenon was discovered in the YbRh₂Si₂ compound—an excellent EPR signal from the Yb ions was observed at temperatures below the Kondo temperature determined thermodynamically, while according to the existing belief the EPR signal should not be observed at these temperatures due to the Kondo effect. In this tribute to K. Alex Müller, I describe the nature of the detected phenomena. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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11 pages, 551 KiB

Open AccessFeature PaperArticle

Suppression of the s-Wave Order Parameter Near the Surface of the Infinite-Layer Electron-Doped Cuprate Superconductor Sr_0.9La_0.1CuO₂

by Rustem Khasanov, Alexander Shengelaya, Roland Brütsch and Hugo Keller

Condens. Matter 2020, 5(3), 50; https://doi.org/10.3390/condmat5030050 - 3 Aug 2020

Cited by 5 | Viewed by 2543

Abstract

The temperature dependencies of the in-plane (

λ_{a b}

) and out-of-plane (

λ_{c}

) components of the magnetic field penetration depth were investigated near the surface and in the bulk of the electron-doped superconductor Sr

_{0.9}

La

_{0.1}

CuO

_{2}

[...] Read more.

The temperature dependencies of the in-plane (

λ_{a b}

) and out-of-plane (

λ_{c}

) components of the magnetic field penetration depth were investigated near the surface and in the bulk of the electron-doped superconductor Sr

_{0.9}

La

_{0.1}

CuO

_{2}

by means of magnetization measurements. The measured

λ_{a b} (T)

and

λ_{c} (T)

were analyzed in terms of a two-gap model with mixed

s + d

-wave symmetry of the order parameter.

λ_{a b} (T)

is well described by an almost pure anisotropic d-wave symmetry component (

≃ 96

%), mainly reflecting the surface properties of the sample. In contrast,

λ_{c} (T)

exhibits a mixed

s + d

-wave order parameter with a substantial s-wave component of more than 50%. The comparison of

λ_{a b}^{- 2} (T)

measured near the surface with that determined in the bulk by means of the muon-spin rotation/relaxation technique demonstrates that the suppression of the s-wave component of the order parameter near the surface is associated with a reduction of the superfluid density by more than a factor of two. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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22 pages, 7601 KiB

Open AccessFeature PaperArticle

A Lattice Litany for Transition Metal Oxides

by Alan R. Bishop

Condens. Matter 2020, 5(3), 46; https://doi.org/10.3390/condmat5030046 - 13 Jul 2020

Cited by 3 | Viewed by 3242

Abstract

In this tribute to K Alex Müller, I describe how his early insights have influenced future decades of research on perovskite ferroelectrics and more broadly transition metal oxides (TMOs) and related quantum materials. I use his influence on my own research journey to [...] Read more.

In this tribute to K Alex Müller, I describe how his early insights have influenced future decades of research on perovskite ferroelectrics and more broadly transition metal oxides (TMOs) and related quantum materials. I use his influence on my own research journey to discuss impacts in three areas: structural phase transitions, precursor structure, and quantum paraelectricity. I emphasize materials functionality in ground, metastable, and excited states arising from competitions among lattice, charge, and spin degrees of freedom, which results in highly tunable landscapes and complex networks of multiscale configurations controlling macroscopic functions. I discuss competitions between short- and long-range forces as particularly important in TMOs (and related materials classes) because of their localized and directional metal orbitals and the polarizable oxygen ions. I emphasize crucial consequences of elasticity and metal–oxygen charge transfer. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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7 pages, 1264 KiB

Open AccessFeature PaperCommunication

Polaronic States and Superconductivity in WO_3-x

by Ekhard K. H. Salje

Condens. Matter 2020, 5(2), 32; https://doi.org/10.3390/condmat5020032 - 1 May 2020

Cited by 11 | Viewed by 3549

Abstract

Superconducting domain boundaries were found in WO_3-x and doped WO₃. The charge carriers in WO₃-type materials were identified by Schirmer and Salje as bipolarons. Several previous attempts to determine the electronic properties of polarons in WO₃ failed [...] Read more.

Superconducting domain boundaries were found in WO_3-x and doped WO₃. The charge carriers in WO₃-type materials were identified by Schirmer and Salje as bipolarons. Several previous attempts to determine the electronic properties of polarons in WO₃ failed until Bousque et al. (2020) reported a full first principle calculation of free polarons in WO₃. They confirmed the model of Schirmer and Salje that each single polaron is centred around one tungsten position with surplus charges smeared over the adjacent eight tungsten positions. Small additional charges are distributed further apart. Further calculations to clarify the coupling mechanism between polaron to form bipolarons are not yet available. These calculations would help to identify the carrier distribution in Magneli clusters, which were shown recently to contain high carrier concentrations and may indicate totally localized superconductivity in non-percolating clusters. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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Review

Jump to: Research, Other

25 pages, 3689 KiB

Open AccessFeature PaperReview

From SrTiO₃ to Cuprates and Back to SrTiO₃: A Way Along Alex Müller’s Scientific Career

by Annette Bussmann-Holder and Hugo Keller

Condens. Matter 2021, 6(1), 2; https://doi.org/10.3390/condmat6010002 - 31 Dec 2020

Cited by 4 | Viewed by 4983

Abstract

K.A. Müller took a long route in science leaving many traces and imprints, which have been and are still today initiations for further research activities. We “walk” along this outstanding path but are certainly not able to provide a complete picture of it, [...] Read more.

K.A. Müller took a long route in science leaving many traces and imprints, which have been and are still today initiations for further research activities. We “walk” along this outstanding path but are certainly not able to provide a complete picture of it, since the way was not always straight, often marked by unintended detours, which had novel impact on the international research society. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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16 pages, 5882 KiB

Open AccessReview

Revival of Charge Density Waves and Charge Density Fluctuations in Cuprate High-Temperature Superconductors

by Carlo Di Castro

Condens. Matter 2020, 5(4), 70; https://doi.org/10.3390/condmat5040070 - 2 Nov 2020

Cited by 7 | Viewed by 4162

Abstract

I present here a short memory of my scientific contacts with K.A. Müller starting from the Interlaken Conference (1988), Erice (1992 and 1993), and Cottbus (1994) on the initial studies on phase separation (PS) and charge inhomogeneity in cuprates carried out against the [...] Read more.

I present here a short memory of my scientific contacts with K.A. Müller starting from the Interlaken Conference (1988), Erice (1992 and 1993), and Cottbus (1994) on the initial studies on phase separation (PS) and charge inhomogeneity in cuprates carried out against the view of the majority of the scientific community at that time. Going over the years and passing through the charge density wave (CDW) instability of the correlated Fermi liquid (FL) and to the consequences of charge density fluctuations (CDFs), I end with a presentation of my current research activity on CDWs and the related two-dimensional charge density fluctuations (2D-CDFs). A scenario follows of the physics of cuprates, which includes the solution of the decades-long problem of the strange metal (SM) state. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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12 pages, 5451 KiB

Open AccessReview

SrTiO₃—Glimpses of an Inexhaustible Source of Novel Solid State Phenomena

by Wolfgang Kleemann, Jan Dec, Alexander Tkach and Paula M. Vilarinho

Condens. Matter 2020, 5(4), 58; https://doi.org/10.3390/condmat5040058 - 4 Oct 2020

Cited by 13 | Viewed by 3523

Abstract

The purpose of this selective review is primarily to demonstrate the large versatility of the insulating quantum paraelectric perovskite SrTiO₃ explained in “Introduction” part, and “Routes of SrTiO₃ toward ferroelectricity and other collective states” part. Apart from ferroelectricity under various boundary [...] Read more.

The purpose of this selective review is primarily to demonstrate the large versatility of the insulating quantum paraelectric perovskite SrTiO₃ explained in “Introduction” part, and “Routes of SrTiO₃ toward ferroelectricity and other collective states” part. Apart from ferroelectricity under various boundary conditions, it exhibits regular electronic and superconductivity via doping or external fields and is capable of displaying diverse coupled states. “Magnetoelectric multiglass (Sr,Mn)TiO₃” part, deals with mesoscopic physics of the solid solution SrTiO₃:Mn²⁺. It is at the origin of both polar and spin cluster glass forming and is altogether a novel multiferroic system. Independent transitions at different glass temperatures, power law dynamic criticality, divergent third-order susceptibilities, and higher order magneto-electric interactions are convincing fingerprints. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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15 pages, 1216 KiB

Open AccessReview

Unconventional Magnetism in Layered Transition Metal Dichalcogenides

by Zurab Guguchia

Condens. Matter 2020, 5(2), 42; https://doi.org/10.3390/condmat5020042 - 20 Jun 2020

Cited by 6 | Viewed by 4629

Abstract

In this contribution to the MDPI Condensed Matter issue in Honor of Nobel Laureate Professor K.A. Müller I review recent experimental progress on magnetism of semiconducting transition metal dichalcogenides (TMDs) from the local-magnetic probe point of view such as muon-spin rotation and discuss [...] Read more.

In this contribution to the MDPI Condensed Matter issue in Honor of Nobel Laureate Professor K.A. Müller I review recent experimental progress on magnetism of semiconducting transition metal dichalcogenides (TMDs) from the local-magnetic probe point of view such as muon-spin rotation and discuss prospects for the creation of unique new device concepts with these materials. TMDs are the prominent class of layered materials, that exhibit a vast range of interesting properties including unconventional semiconducting, optical, and transport behavior originating from valley splitting. Until recently, this family has been missing one crucial member: magnetic semiconductor. The situation has changed over the past few years with the discovery of layered semiconducting magnetic crystals, for example CrI

_{3}

and VI

_{2}

. We have also very recently discovered unconventional magnetism in semiconducting Mo-based TMD systems 2H-MoTe

_{2}

and 2H-MoSe

_{2}

[Guguchia et. al., Science Advances 2018, 4(12)]. Moreover, we also show the evidence for the involvement of magnetism in semiconducting tungsten diselenide 2H-WSe

_{2}

. These results open a path to studying the interplay of 2D physics, semiconducting properties and magnetism in TMDs. It also opens up a host of new opportunities to obtain tunable magnetic semiconductors, forming the basis for spintronics. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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Other

Jump to: Research, Review

4 pages, 185 KiB

Open AccessPerspective

The Role of the Short Coherence Length in Unconventional Superconductors

by Guy Deutscher

Condens. Matter 2020, 5(4), 77; https://doi.org/10.3390/condmat5040077 - 1 Dec 2020

Cited by 4 | Viewed by 2865

Abstract

A short coherence length is a distinctive feature of many cases of unconventional superconductivity. While in conventional superconductors, it is many orders of magnitude larger than the basic inter-particle distance, a short coherence length is common to superconductors as diverse as the cuprates, [...] Read more.

A short coherence length is a distinctive feature of many cases of unconventional superconductivity. While in conventional superconductors, it is many orders of magnitude larger than the basic inter-particle distance, a short coherence length is common to superconductors as diverse as the cuprates, the picnites and granular superconductors. We dwell particularly on the last, because their simple chemical structure makes them a favorable material for exploring fundamental phenomena such as the Bardeen-Cooper Schrieffer (BCS)-to-Bose–Einstein condensation cross-over and the effect of the vicinity of a Mott metal-to-insulator transition. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

9 pages, 67006 KiB

Open AccessCreative

Color Centers and Jahn-Teller Effect in Ionic Crystals—My Scientific Encounters with Alex Müller

by Hans Bill

Condens. Matter 2020, 5(4), 59; https://doi.org/10.3390/condmat5040059 - 15 Oct 2020

Viewed by 2496

Abstract

This contribution presents a personal account of the influence Karl Alex Müller had on the early stages of my career and the scientific questions about which we exchanged our views over the years. While both our research branched into a variety of topics, [...] Read more.

This contribution presents a personal account of the influence Karl Alex Müller had on the early stages of my career and the scientific questions about which we exchanged our views over the years. While both our research branched into a variety of topics, the common experimental technique, Electron Paramagnetic Resonance, and the Jahn-Teller effect led to fruitful exchanges of ideas on these matters in semiconducting, metallic and ionic crystals. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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9 pages, 1060 KiB

Open AccessCreative

A Retrospective of Materials Synthesis at the Paul Scherrer Institut (PSI)

by Kazimierz Conder, Albert Furrer and Ekaterina Pomjakushina

Condens. Matter 2020, 5(4), 55; https://doi.org/10.3390/condmat5040055 - 23 Sep 2020

Cited by 1 | Viewed by 2914

Abstract

The availability of high-quality and well characterized materials is a key factor for condensed-matter research [...] Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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11 pages, 2144 KiB

Open AccessCreative

Meetings with a Remarkable Man, Alex Müller—The Professor of SrTiO3

by Thomas W. Kool

Condens. Matter 2020, 5(3), 44; https://doi.org/10.3390/condmat5030044 - 2 Jul 2020

Cited by 1 | Viewed by 2521

Abstract

After my bachelor degree in chemistry with physics and mathematics (in Dutch kandidaatsexamen) at the University of Amsterdam, I chose to study for my master degree (in Dutch doctoraal) a physical chemistry direction [...] Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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9 pages, 1958 KiB

Open AccessCreative

The Crucial Things in Science Often Happen Quite Unexpectedly—Das Entscheidende in der Wissenschaft geschieht oft ganz unerwartet (K. Alex Müller)

by Reinhard K. Kremer, Annette Bussmann-Holder, Hugo Keller and Robin Haunschild

Condens. Matter 2020, 5(3), 43; https://doi.org/10.3390/condmat5030043 - 1 Jul 2020

Cited by 2 | Viewed by 2854

Abstract

We analyzed the publication output of one of the 1987 Nobel Prize awardees, K. Alex Müller, using bibliometric methods. The time-dependent number of publications and citations and the network with respect to the coauthors and their affiliations was studied. Specifically, the citation history [...] Read more.

We analyzed the publication output of one of the 1987 Nobel Prize awardees, K. Alex Müller, using bibliometric methods. The time-dependent number of publications and citations and the network with respect to the coauthors and their affiliations was studied. Specifically, the citation history of the Nobel Prize awarded 1986 article on “Possible high-temperature superconductivity in the Ba-La-Cu-O system” has been evaluated in terms of the overall number of articles on superconductivity and the corresponding citations of other most frequently referenced articles. Thereby, a publication with “delayed recognition” was identified. Full article

(This article belongs to the Special Issue From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework)

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From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework

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