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10 pages, 896 KiB

Open AccessArticle

Direct Visualization of Spatial Inhomogeneity of Spin Stripes Order in La_1.72Sr_0.28NiO₄

by Gaetano Campi, Nicola Poccia, Boby Joseph, Antonio Bianconi, Shrawan Mishra, James Lee, Sujoy Roy, Agustinus Agung Nugroho, Marcel Buchholz, Markus Braden, Christoph Trabant, Alexey Zozulya, Leonard Müller, Jens Viefhaus, Christian Schüßler-Langeheine, Michael Sprung and Alessandro Ricci

Condens. Matter 2019, 4(3), 77; https://doi.org/10.3390/condmat4030077 - 10 Aug 2019

Cited by 11 | Viewed by 5051

Abstract

In several strongly correlated electron systems, the short range ordering of defects, charge and local lattice distortions are found to show complex inhomogeneous spatial distributions. There is growing evidence that such inhomogeneity plays a fundamental role in unique functionality of quantum complex materials. [...] Read more.

In several strongly correlated electron systems, the short range ordering of defects, charge and local lattice distortions are found to show complex inhomogeneous spatial distributions. There is growing evidence that such inhomogeneity plays a fundamental role in unique functionality of quantum complex materials. La_1.72Sr_0.28NiO₄ is a prototypical strongly correlated perovskite showing spin stripes order. In this work we present the spatial distribution of the spin order inhomogeneity by applying micro X-ray diffraction to La_1.72Sr_0.28NiO₄, mapping the spin-density-wave order below the 120 K onset temperature. We find that the spin-density-wave order shows the formation of nanoscale puddles with large spatial fluctuations. The nano-puddle density changes on the microscopic scale forming a multiscale phase separation extending from nanoscale to micron scale with scale-free distribution. Indeed spin-density-wave striped puddles are disconnected by spatial regions with negligible spin-density-wave order. The present work highlights the complex spatial nanoscale phase separation of spin stripes in nickelate perovskites and opens new perspectives of local spin order control by strain. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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10 pages, 366 KiB

Open AccessArticle

Q Dependence of Magnetic Resonance Mode on FeTe_0.5Se_0.5 Studied by Inelastic Neutron Scattering

by Motoyuki Ishikado, Katsuaki Kodama, Ryoichi Kajimoto, Mitsutaka Nakamura, Yasuhiro Inamura, Kazuhiko Ikeuchi, Sungdae Ji, Masatoshi Arai and Shin-ichi Shamoto

Condens. Matter 2019, 4(3), 69; https://doi.org/10.3390/condmat4030069 - 12 Jul 2019

Viewed by 3376

Abstract

Inelastic neutron scattering measurements have been performed on a superconducting single crystal FeTe

_{0.5}

Se

_{0.5}

to examine the

Q

-dependent enhancement of the dynamical structure factor,

S (Q, E)

, from

Q

= (0, 0) to (

π

, [...] Read more.

Inelastic neutron scattering measurements have been performed on a superconducting single crystal FeTe

_{0.5}

Se

_{0.5}

to examine the

Q

-dependent enhancement of the dynamical structure factor,

S (Q, E)

, from

Q

= (0, 0) to (

π

,

π

), including (

π

, 0) in the superconducting state. In most of iron-based superconductors,

S (Q, E)

is enhanced at

Q

= (

π

, 0), where the “magnetic resonance mode” is commonly observed in the unfolded Brillouin zone. Constant-E cuts of

S (Q, E)

suggest that the enhancement is not uniform in the magnetic excitation, and limited around

Q

= (

π

, 0). This result is consistent with the theoretical simulation of the magnetic resonance mode due to the Bardeen–Cooper–Schrieffer coherence factor with the sign-reversing order parameter of s

_{\pm}

wave. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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8 pages, 10264 KiB

Open AccessArticle

Substrate-Induced Proximity Effect in Superconducting Niobium Nanofilms

by S.J. Rezvani, Andrea Perali, Matteo Fretto, Natascia De Leo, Luca Flammia, Milorad Milošević, Stefano Nannarone and Nicola Pinto

Condens. Matter 2019, 4(1), 4; https://doi.org/10.3390/condmat4010004 - 30 Dec 2018

Cited by 12 | Viewed by 4745

Abstract

Structural and superconducting properties of high-quality niobium nanofilms with different thicknesses are investigated on silicon oxide (SiO₂) and sapphire substrates. The role played by the different substrates and the superconducting properties of the Nb films are discussed based on the defectivity [...] Read more.

Structural and superconducting properties of high-quality niobium nanofilms with different thicknesses are investigated on silicon oxide (SiO₂) and sapphire substrates. The role played by the different substrates and the superconducting properties of the Nb films are discussed based on the defectivity of the films and on the presence of an interfacial oxide layer between the Nb film and the substrate. The X-ray absorption spectroscopy is employed to uncover the structure of the interfacial layer. We show that this interfacial layer leads to a strong proximity effect, especially in films deposited on a SiO₂ substrate, altering the superconducting properties of the Nb films. Our results establish that the critical temperature is determined by an interplay between quantum-size effects, due to the reduction of the Nb film thicknesses, and proximity effects. The detailed investigation here provides reference characterizations and has direct and important implications for the fabrication of superconducting devices based on Nb nanofilms. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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8 pages, 1017 KiB

Open AccessArticle

The Magnetic Properties of 1111-type Diluted Magnetic Semiconductor (La_1−xBa_x)(Zn_1−xMn_x)AsO in the Low Doping Regime

by Guoxiang Zhi, Kai Wang, Haojie Zhang, Cui Ding, Shengli Guo, Yilun Gu, Licheng Fu and F. L. Ning

Condens. Matter 2018, 3(4), 42; https://doi.org/10.3390/condmat3040042 - 29 Nov 2018

Viewed by 3205

Abstract

We investigated the magnetic properties of (La

_{1 - x}

Ba

_{x}

)(Zn

_{1 - x}

Mn

_{x}

)AsO with x varying from 0.005 to 0.05 at an external magnetic field of 1000 Oe. For doping levels of x ≤ 0.01, the system [...] Read more.

We investigated the magnetic properties of (La

_{1 - x}

Ba

_{x}

)(Zn

_{1 - x}

Mn

_{x}

)AsO with x varying from 0.005 to 0.05 at an external magnetic field of 1000 Oe. For doping levels of x ≤ 0.01, the system remains paramagnetic down to the lowest measurable temperature of 2 K. Only when the doping level increases to x = 0.02 does the ferromagnetic ordering appear. Our analysis indicates that antiferromagnetic exchange interactions dominate for x ≤ 0.01, as shown by the negative Weiss temperature fitted from the magnetization data. The Weiss temperature becomes positive, i.e., ferromagnetic coupling starts to dominate, for x ≥ 0.02. The Mn-Mn spin interaction parameter

∣ 2 J / k_{B} ∣

is estimated to be in the order of 10 K for both x ≤ 0.01 (antiferromagnetic ordered state) and x ≥ 0.02 (ferromagnetic ordered state). Our results unequivocally demonstrate the competition between ferromagnetic and antiferromagnetic exchange interactions in carrier-mediated ferromagnetic systems. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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24 pages, 633 KiB

Open AccessArticle

Negative Energy Antiferromagnetic Instantons Forming Cooper-Pairing ‘Glue’ and ‘Hidden Order’ in High-Tc Cuprates

by Sergei Mukhin

Condens. Matter 2018, 3(4), 39; https://doi.org/10.3390/condmat3040039 - 7 Nov 2018

Cited by 8 | Viewed by 3240

Abstract

An emergence of magnetic boson of instantonic nature, that provides a Cooper-‘pairing glue’, is considered in the repulsive ‘nested’ Hubbard model of superconducting cuprates. It is demonstrated that antiferromagnetic instantons of a spin density wave type may have negative energy due to coupling [...] Read more.

An emergence of magnetic boson of instantonic nature, that provides a Cooper-‘pairing glue’, is considered in the repulsive ‘nested’ Hubbard model of superconducting cuprates. It is demonstrated that antiferromagnetic instantons of a spin density wave type may have negative energy due to coupling with Cooper pair condensate. A set of Eliashberg like equations is derived and solved self-consistently, proving the above suggestion. An instantonic propagator plays the role of the Green function of the pairing ‘glue’ boson. Simultaneously, the instantons defy condensation of the mean-field spin-density wave (SDW) order. We had previously demonstrated in analytical form that periodic chain of instanton-anti-instanton pairs along the axis of Matsubara time has zero scattering cross section for weakly perturbing external probes, like neutrons, etc., thus representing a ‘hidden order’. Hence, the two competing orders, superconducting and antiferromagnetic, may coexist (below some

T_{c}

) in the form of the superconducting order coupled to ‘hidden’ instantonic one. This new picture is discussed in relation with the mechanism of high temperature superconductivity. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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21 pages, 2160 KiB

Open AccessArticle

Majorana Fermions in One-Dimensional Structures at LaAlO₃/SrTiO₃ Oxide Interfaces

by Maria Vittoria Mazziotti, Niccolò Scopigno, Marco Grilli and Sergio Caprara

Condens. Matter 2018, 3(4), 37; https://doi.org/10.3390/condmat3040037 - 29 Oct 2018

Cited by 21 | Viewed by 4439

Abstract

We study one-dimensional structures that may be formed at the LaAlO

_{3}

/SrTiO

_{3}

oxide interface by suitable top gating. These structures are modeled via a single-band model with Rashba spin-orbit coupling, superconductivity and a magnetic field along the one-dimensional chain. We first [...] Read more.

We study one-dimensional structures that may be formed at the LaAlO

_{3}

/SrTiO

_{3}

oxide interface by suitable top gating. These structures are modeled via a single-band model with Rashba spin-orbit coupling, superconductivity and a magnetic field along the one-dimensional chain. We first discuss the conditions for the occurrence of a topological superconducting phase and the related formation of Majorana fermions at the chain endpoints, highlighting a close similarity between this model and the Kitaev model, which also reflects in a similar condition the formation of a topological phase. Solving the model in real space, we also study the spatial extension of the wave function of the Majorana fermions and how this increases with approaching the limit condition for the topological state. Using a scattering matrix formalism, we investigate the stability of the Majorana fermions in the presence of disorder and discuss the evolution of the topological phase with increasing disorder. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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11 pages, 367 KiB

Open AccessArticle

Crossover Induced Electron Pairing and Superconductivity by Kinetic Renormalization in Correlated Electron Systems

by Takashi Yanagisawa, Mitake Miyazaki and Kunihiko Yamaji

Condens. Matter 2018, 3(3), 26; https://doi.org/10.3390/condmat3030026 - 6 Sep 2018

Cited by 1 | Viewed by 3021

Abstract

We investigate the ground state of strongly correlated electron systems based on an optimization variational Monte Carlo method to clarify the mechanism of high-temperature superconductivity. The wave function is optimized by introducing variational parameters in an exponential-type wave function beyond the Gutzwiller function. [...] Read more.

We investigate the ground state of strongly correlated electron systems based on an optimization variational Monte Carlo method to clarify the mechanism of high-temperature superconductivity. The wave function is optimized by introducing variational parameters in an exponential-type wave function beyond the Gutzwiller function. The many-body effect plays an important role as an origin of superconductivity in a correlated electron system. There is a crossover between weakly correlated region and strongly correlated region, where two regions are characterized by the strength of the on-site Coulomb interaction U. We insist that high-temperature superconductivity occurs in the strongly correlated region. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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10 pages, 272 KiB

Open AccessArticle

On the Evaluation of the Spin Galvanic Effect in Lattice Models with Rashba Spin-Orbit Coupling

by Götz Seibold, Sergio Caprara, Marco Grilli and Roberto Raimondi

Condens. Matter 2018, 3(3), 22; https://doi.org/10.3390/condmat3030022 - 24 Jul 2018

Viewed by 3644

Abstract

The spin galvanic effect (SGE) describes the conversion of a non-equilibrium spin polarization into a charge current and has recently attracted renewed interest due to the large conversion efficiency observed in oxide interfaces. An important factor in the SGE theory is disorder which [...] Read more.

The spin galvanic effect (SGE) describes the conversion of a non-equilibrium spin polarization into a charge current and has recently attracted renewed interest due to the large conversion efficiency observed in oxide interfaces. An important factor in the SGE theory is disorder which ensures the stationarity of the conversion. Through this paper, we propose a procedure for the evaluation of the SGE on disordered lattices which can also be readily implemented for multiband systems. We demonstrate the performance of the method for a single-band Rashba model and compare our results with those obtained within the self-consistent Born approximation for a continuum model. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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Review

Jump to: Research

17 pages, 722 KiB

Open AccessReview

Anomalous Transport Behavior in Quantum Magnets

by Dietrich Belitz and Theodore R. Kirkpatrick

Condens. Matter 2018, 3(4), 30; https://doi.org/10.3390/condmat3040030 - 10 Oct 2018

Cited by 2 | Viewed by 2853

Abstract

Transport behavior that is characterized by a low-temperature electrical resistivity that displays a power law behavior (

ρ (T \to 0) \propto T^{s}

) with an exponent of

s < 2

is commonly observed in magnetic materials in both the [...] Read more.

Transport behavior that is characterized by a low-temperature electrical resistivity that displays a power law behavior (

ρ (T \to 0) \propto T^{s}

) with an exponent of

s < 2

is commonly observed in magnetic materials in both the magnetic and non-magnetic phases. We give a pedagogical overview of this phenomenon that summarizes both the experimental situation and the state of its theoretical understanding. We also put it in context by drawing parallels with unusual power law transport behavior in other systems. Full article

(This article belongs to the Special Issue Selected Papers from Quantum Complex Matter 2018)

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