Editorial

2 pages, 176 KiB

Open AccessEditorial

by Andrej Pustogow

Crystals 2023, 13(1), 64; https://doi.org/10.3390/cryst13010064 - 30 Dec 2022

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Metal-insulator transitions (MITs) constitute a core subject of fundamental condensed-matter research [...] Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

Research

Jump to: Editorial, Review, Other

20 pages, 9783 KiB

Open AccessArticle

A New Charge-Ordered Molecular Conductor: κ-(BEDT-TTF)₂K⁺(18-crown-6)[Co^II(NCS)₄]∙(H₂O)

by Andrei A. Bardin, Tatiana G. Prokhorova and Lev I. Buravov

Crystals 2023, 13(10), 1504; https://doi.org/10.3390/cryst13101504 - 16 Oct 2023

Viewed by 819

Abstract

A new molecular conductor, i.e., κ-(BEDT-TTF)₂K⁺(18-crown-6)[Co^II(NCS)₄]∙(H₂O), is semiconductive with substantial charge gap values (ΔE) of 0.57 eV (measured) and 0.37 eV (calculated). There is a full band separation despite formal [...] Read more.

A new molecular conductor, i.e., κ-(BEDT-TTF)₂K⁺(18-crown-6)[Co^II(NCS)₄]∙(H₂O), is semiconductive with substantial charge gap values (ΔE) of 0.57 eV (measured) and 0.37 eV (calculated). There is a full band separation despite formal average charge on BEDT-TTF of +0.5 and κ(kappa)-type packing of BEDT-TTF dimers that favors high conductivity. X-ray crystal structure analysis reveals complete charge ordering with full Coulomb charge on unique BEDT-TTF radical cations A (Q_A = +1), while unique molecules B are uncharged (Q_B = 0). Geometries of A (flat) and B (bent) differ considerably and are in accordance with the ascribing charges. Charge segregation is enhanced by forming tight face-to-face BEDT-TTF dimers AA (Q_AA = +2) and BB (Q_BB = 0). Strongly interacting double-charged dimers AA form “superstripes” running along a that are interleaved along b with chains of neutral dimers BB. Peculiar extremely thick (13.7 Å) four-decker insulating anion layers cast strong Coulomb potential onto the conductive layers predetermining charge localization in the latter. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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13 pages, 2400 KiB

Open AccessEditor’s ChoiceArticle

Neural Network Solver for Small Quantum Clusters

by Nicholas Walker, Samuel Kellar, Yi Zhang, Ka-Ming Tam and Juana Moreno

Crystals 2022, 12(9), 1269; https://doi.org/10.3390/cryst12091269 - 06 Sep 2022

Cited by 2 | Viewed by 1979

Abstract

Machine learning approaches have recently been applied to the study of various problems in physics. Most of these studies are focused on interpreting the data generated by conventional numerical methods or the data on an existing experimental database. An interesting question is whether [...] Read more.

Machine learning approaches have recently been applied to the study of various problems in physics. Most of these studies are focused on interpreting the data generated by conventional numerical methods or the data on an existing experimental database. An interesting question is whether it is possible to use a machine learning approach, in particular a neural network, for solving the many-body problem. In this paper, we present a neural network solver for the single impurity Anderson model, the paradigm of an interacting quantum problem in small clusters. We demonstrate that the neural-network-based solver provides quantitative accurate results for the spectral function as compared to the exact diagonalization method. This opens the possibility of utilizing the neural network approach as an impurity solver for other many-body numerical approaches, such as the dynamical mean field theory. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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24 pages, 5774 KiB

Open AccessArticle

How to Recognize the Universal Aspects of Mott Criticality?

by Yuting Tan, Vladimir Dobrosavljević and Louk Rademaker

Crystals 2022, 12(7), 932; https://doi.org/10.3390/cryst12070932 - 30 Jun 2022

Cited by 4 | Viewed by 1981

Abstract

In this paper we critically discuss several examples of two-dimensional electronic systems displaying interaction-driven metal-insulator transitions of the Mott (or Wigner–Mott) type, including dilute two-dimension electron gases (2DEG) in semiconductors, Mott organic materials, as well as the recently discovered transition-metal dichalcogenide (TMD) moiré [...] Read more.

In this paper we critically discuss several examples of two-dimensional electronic systems displaying interaction-driven metal-insulator transitions of the Mott (or Wigner–Mott) type, including dilute two-dimension electron gases (2DEG) in semiconductors, Mott organic materials, as well as the recently discovered transition-metal dichalcogenide (TMD) moiré bilayers. Remarkably similar behavior is found in all these systems, which is starting to paint a robust picture of Mott criticality. Most notable, on the metallic side a resistivity maximum is observed whose temperature scale vanishes at the transition. We compare the available experimental data on these systems to three existing theoretical scenarios: spinon theory, Dynamical Mean Field Theory (DMFT) and percolation theory. We show that the DMFT and percolation pictures for Mott criticality can be distinguished by studying the origins of the resistivity maxima using an analysis of the dielectric response. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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11 pages, 3547 KiB

Open AccessArticle

A Database for Crystalline Organic Conductors and Superconductors

by Owen Ganter, Kevin Feeny, Morgan Brooke-deBock, Stephen M. Winter and Charles C. Agosta

Crystals 2022, 12(7), 919; https://doi.org/10.3390/cryst12070919 - 28 Jun 2022

Cited by 2 | Viewed by 2256

Abstract

We present a prototype database for quasi two-dimensional crystalline organic conductors and superconductors based on molecules related to bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, ET). The database includes crystal structures, calculated electronic structures, and experimentally measured properties such as the superconducting transition temperature and critical magnetic fields. [...] Read more.

We present a prototype database for quasi two-dimensional crystalline organic conductors and superconductors based on molecules related to bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF, ET). The database includes crystal structures, calculated electronic structures, and experimentally measured properties such as the superconducting transition temperature and critical magnetic fields. We obtained crystal structures from the Cambridge Structural Database and created a crystal structure analysis algorithm to identify cation molecules and execute tight binding electronic structure calculations. We used manual data entry to encode experimentally measured properties reported in publications. Crystalline organic conductors and superconductors exhibit a wide variety of electronic ground states, particularly those with correlations. We hope that this database will ultimately lead to a better understanding of the fundamental mechanisms of such states. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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10 pages, 2327 KiB

Open AccessArticle

Optical Conductivity Spectra of Charge-Crystal and Charge-Glass States in a Series of θ-Type BEDT-TTF Compounds

by Kenichiro Hashimoto, Ryota Kobayashi, Satoshi Ohkura, Satoru Sasaki, Naoki Yoneyama, Masayuki Suda, Hiroshi M. Yamamoto and Takahiko Sasaki

Crystals 2022, 12(6), 831; https://doi.org/10.3390/cryst12060831 - 12 Jun 2022

Cited by 2 | Viewed by 2283

Abstract

In the 3/4-filled band system

θ

-(BEDT-TTF)

_{2} X

with a two-dimensional triangular lattice, charge ordering (CO) often occurs due to strong inter-site Coulomb repulsion. However, the strong geometrical frustration of the triangular lattice can prohibit long-range CO, resulting in a charge-glass state [...] Read more.

In the 3/4-filled band system

θ

-(BEDT-TTF)

_{2} X

with a two-dimensional triangular lattice, charge ordering (CO) often occurs due to strong inter-site Coulomb repulsion. However, the strong geometrical frustration of the triangular lattice can prohibit long-range CO, resulting in a charge-glass state in which the charge configurations are randomly distributed. Here, we investigate the charge-glass states of orthorhombic and monoclinic

θ

-type BEDT-TTF salts by measuring the electrical resistivity and optical conductivity spectra. We find a substantial difference between the charge-glass states of the orthorhombic and monoclinic systems. The charge-glass state in the orthorhombic system with an isotropic triangular lattice exhibits larger low-energy excitations than that in the monoclinic one with an anisotropic triangular lattice and becomes more metallic as the isotropy of the triangular lattice increases. These results can be understood by the different charge-glass formation mechanisms in the two systems: in the orthorhombic system, the charge-glass state originates from geometric frustration due to the equilateral triangular lattice, leading to metallic 3-fold COs, whereas in the monoclinic system, the charge-glass formation originates from geometric frustration of the isosceles triangular lattice, in which the charge-glass state is described by the superposition of insulating 2-fold stripe COs. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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22 pages, 7726 KiB

Open AccessArticle

Grain-Size-Induced Collapse of Variable Range Hopping and Promotion of Ferromagnetism in Manganite La_0.5Ca_0.5MnO₃

by Nikolina Novosel, David Rivas Góngora, Zvonko Jagličić, Emil Tafra, Mario Basletić, Amir Hamzić, Teodoro Klaser, Željko Skoko, Krešimir Salamon, Ivna Kavre Piltaver, Mladen Petravić, Bojana Korin-Hamzić, Silvia Tomić, Boris P. Gorshunov, Tao Zhang, Tomislav Ivek and Matija Čulo

Crystals 2022, 12(5), 724; https://doi.org/10.3390/cryst12050724 - 19 May 2022

Cited by 5 | Viewed by 2285

Abstract

Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) [...] Read more.

Among transition metal oxides, manganites have attracted significant attention because of colossal magnetoresistance (CMR)—a magnetic field-induced metal–insulator transition close to the Curie temperature. CMR is closely related to the ferromagnetic (FM) metallic phase which strongly competes with the antiferromagnetic (AFM) charge ordered (CO) phase, where conducting electrons localize and create a long range order giving rise to insulator-like behavior. One of the major open questions in manganites is the exact origin of this insulating behavior. Here we report a dc resistivity and magnetization study on manganite La

_{1 - x}

Ca

_{x}

MnO

_{3}

ceramic samples with different grain size, at the very boundary between CO/AFM insulating and FM metallic phases

x = 0.5

. Clear signatures of variable range hopping (VRH) are discerned in resistivity, implying the disorder-induced (Anderson) localization of conducting electrons. A significant increase of disorder associated with the reduction in grain size, however, pushes the system in the opposite direction from the Anderson localization scenario, resulting in a drastic decrease of resistivity, collapse of the VRH, suppression of the CO/AFM phase and growth of an FM contribution. These contradictory results are interpreted within the standard core-shell model and recent theories of Anderson localization of interacting particles. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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13 pages, 10543 KiB

Open AccessEditor’s ChoiceArticle

Fingerprints of Topotactic Hydrogen in Nickelate Superconductors

by Liang Si, Paul Worm and Karsten Held

Crystals 2022, 12(5), 656; https://doi.org/10.3390/cryst12050656 - 04 May 2022

Cited by 9 | Viewed by 2545

Abstract

Superconductivity has entered the nickel age marked by enormous experimental and theoretical efforts. Notwithstanding, synthesizing nickelate superconductors remains extremely challenging, not least due to incomplete oxygen reduction and topotactic hydrogen. Here, we present density-functional theory calculations for nickelate superconductors with additional topotactic hydrogen [...] Read more.

Superconductivity has entered the nickel age marked by enormous experimental and theoretical efforts. Notwithstanding, synthesizing nickelate superconductors remains extremely challenging, not least due to incomplete oxygen reduction and topotactic hydrogen. Here, we present density-functional theory calculations for nickelate superconductors with additional topotactic hydrogen or oxygen, namely La

_{1 - x}

Sr

_{x}

NiO

_{2}

H

_{δ}

and LaNiO

_{2 + δ}

. We identify a phonon mode as a possible indication for topotactic hydrogen and discuss the charge redistribution patterns around oxygen and hydrogen impurities. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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8 pages, 2018 KiB

Open AccessArticle

A Discrepancy in Thermal Conductivity Measurement Data of Quantum Spin Liquid β′-EtMe₃Sb[Pd(dmit)₂]₂ (dmit = 1,3-Dithiol-2-thione-4,5-dithiolate)

by Reizo Kato, Masashi Uebe, Shigeki Fujiyama and Hengbo Cui

Crystals 2022, 12(1), 102; https://doi.org/10.3390/cryst12010102 - 13 Jan 2022

Cited by 12 | Viewed by 2583

Abstract

A molecular Mott insulator β′-EtMe₃Sb[Pd(dmit)₂]₂ is a quantum spin liquid candidate. In 2010, it was reported that thermal conductivity of β′-EtMe₃Sb[Pd(dmit)₂]₂ is characterized by its large value and gapless behavior (a finite temperature-linear [...] Read more.

A molecular Mott insulator β′-EtMe₃Sb[Pd(dmit)₂]₂ is a quantum spin liquid candidate. In 2010, it was reported that thermal conductivity of β′-EtMe₃Sb[Pd(dmit)₂]₂ is characterized by its large value and gapless behavior (a finite temperature-linear term). In 2019, however, two other research groups reported opposite data (much smaller value and a vanishingly small temperature-linear term) and the discrepancy in the thermal conductivity measurement data emerges as a serious problem concerning the ground state of the quantum spin liquid. Recently, the cooling rate was proposed to be an origin of the discrepancy. We examined effects of the cooling rate on electrical resistivity, low-temperature crystal structure, and ¹³C-NMR measurements and could not find any significant cooling rate dependence. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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8 pages, 3185 KiB

Open AccessEditor’s ChoiceArticle

Metallic Conduction and Carrier Localization in Two-Dimensional BEDO-TTF Charge-Transfer Solid Crystals

by Hiroshi Ito, Motoki Matsuno, Seiu Katagiri, Shinji K. Yoshina, Taishi Takenobu, Manabu Ishikawa, Akihiro Otsuka, Hideki Yamochi, Yukihiro Yoshida, Gunzi Saito, Yongbing Shen and Masahiro Yamashita

Crystals 2022, 12(1), 23; https://doi.org/10.3390/cryst12010023 - 24 Dec 2021

Cited by 2 | Viewed by 2802

Abstract

Charge-transfer salts based on bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF or BO for short) provide a stable two-dimensional (2D) metallic state, while the electrical resistance often shows an upturn at low temperatures below ~10 K. Such 2D weak carrier localization was first recognized for BO salts in [...] Read more.

Charge-transfer salts based on bis(ethylenedioxy)tetrathiafulvalene (BEDO-TTF or BO for short) provide a stable two-dimensional (2D) metallic state, while the electrical resistance often shows an upturn at low temperatures below ~10 K. Such 2D weak carrier localization was first recognized for BO salts in the Langmuir–Blodgett films fabricated with fatty acids; however, it has not been characterized in charge-transfer solid crystals. In this paper, we discuss the carrier localization of two crystalline BO charge-transfer salts with or without magnetic ions at low temperatures through the analysis of the weak negative magnetoresistance. The phase coherence lengths deduced with temperature dependence are largely dominated by the electron–electron scattering mechanism. These results indicate that the resistivity upturn at low temperatures is caused by the 2D weak localization. Disorders causing elastic scattering within the metallic domains, such as those of terminal ethylene groups, should be suppressed to prevent the localization. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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12 pages, 2019 KiB

Open AccessArticle

Electronic Heat Capacity and Lattice Softening of Partially Deuterated Compounds of κ-(BEDT-TTF)₂Cu[N(CN)₂]Br

by Yuki Matsumura, Shusaku Imajo, Satoshi Yamashita, Hiroki Akutsu and Yasuhiro Nakazawa

Crystals 2022, 12(1), 2; https://doi.org/10.3390/cryst12010002 - 21 Dec 2021

Cited by 3 | Viewed by 2396

Abstract

Thermodynamic investigation by calorimetric measurements of the layered organic superconductors, κ-(BEDT-TTF)₂Cu[N(CN)₂]Br and its partially deuterated compounds of κ-(d[2,2]-BEDT-TTF)₂Cu[N(CN)₂]Br and κ-(d[3,3]-BEDT-TTF)₂Cu[N(CN)₂]Br, performed in a wide temperature range is reported. The latter two [...] Read more.

Thermodynamic investigation by calorimetric measurements of the layered organic superconductors, κ-(BEDT-TTF)₂Cu[N(CN)₂]Br and its partially deuterated compounds of κ-(d[2,2]-BEDT-TTF)₂Cu[N(CN)₂]Br and κ-(d[3,3]-BEDT-TTF)₂Cu[N(CN)₂]Br, performed in a wide temperature range is reported. The latter two compounds were located near the metal–insulator boundary in the dimer-Mott phase diagram. From the comparison of the temperature dependences of their heat capacities, we indicated that lattice heat capacities of the partially deuterated compounds were larger than that of the pristine compound below about 40 K. This feature probably related to the lattice softening was discussed also by the sound velocity measurement, in which the dip-like structures of the Δv/v were observed. We also discussed the variation of the electronic heat capacity under magnetic fields. From the heat capacity data at magnetic fields up to 6 T, we evaluated that the normal-state γ value of the partially deuterated compound, κ-(d[3,3]-BEDT-TTF)₂Cu[N(CN)₂]Br, was about 3.1 mJ K⁻² mol⁻¹. Under the magnetic fields higher than 3.0 T, we observed that the magnetic-field insulating state was induced due to the instability of the mid-gap electronic state peculiar for the two-dimensional dimer-Mott system. Even though the volume fraction was much reduced, the heat capacity of κ-(d[3,3]-BEDT-TTF)₂Cu[N(CN)₂]Br showed a small hump structure probably related to the strong coupling feature of the superconductivity near the boundary. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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10 pages, 739 KiB

Open AccessArticle

Tuning Charge Order in (TMTTF)₂X by Partial Anion Substitution

by Andrej Pustogow, Daniel Dizdarevic, Sebastian Erfort, Olga Iakutkina, Valentino Merkl, Gabriele Untereiner and Martin Dressel

Crystals 2021, 11(12), 1545; https://doi.org/10.3390/cryst11121545 - 10 Dec 2021

Cited by 2 | Viewed by 2265

Abstract

In the quasi-one-dimensional (TMTTF)

_{2}

X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions [...] Read more.

In the quasi-one-dimensional (TMTTF)

_{2}

X compounds with effectively quarter-filled bands, electronic charge order is stabilized from the delicate interplay of Coulomb repulsion and electronic bandwidth. The correlation strength is commonly tuned by physical pressure or chemical substitution with stoichiometric ratios of anions and cations. Here, we investigate the charge-ordered state through partial substitution of the anions in (TMTTF)

_{2}

[AsF

_{6}

]

_{1 - x}

[SbF

_{6}

]

_{x}

with

x \approx 0.3

, determined from the intensity of infrared vibrations, which is sufficient to suppress the spin-Peierls state. Our dc transport experiments reveal a transition temperature

T_{CO}

= 120 K and charge gap

Δ_{CO}

= 430

K between the values of the two parent compounds (TMTTF)

_{2}

AsF

_{6}

and (TMTTF)

_{2}

SbF

_{6}

. Upon plotting the two parameters for different (TMTTF)

_{2}

X, we find a universal relationship between

T_{CO}

and

Δ_{CO}

yielding that the energy gap vanishes for transition temperatures

T_{CO}

\leq 60

K. While these quantities indicate that the macroscopic correlation strength is continuously tuned, our vibrational spectroscopy results probing the local charge disproportionation suggest that

2 δ

is modulated on a microscopic level. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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10 pages, 3836 KiB

Open AccessArticle

Fermi Surface Structure and Isotropic Stability of Fulde-Ferrell-Larkin-Ovchinnikov Phase in Layered Organic Superconductor β″-(BEDT-TTF)₂SF₅CH₂CF₂SO₃

by Shiori Sugiura, Hiroki Akutsu, Yasuhiro Nakazawa, Taichi Terashima, Syuma Yasuzuka, John A. Schlueter and Shinya Uji

Crystals 2021, 11(12), 1525; https://doi.org/10.3390/cryst11121525 - 07 Dec 2021

Cited by 1 | Viewed by 2646

Abstract

The Fermi surface structure of a layered organic superconductor

β

^{″}

-(BEDT-TTF)

_{2}

SF

_{5}

CH

_{2}

CF

_{2}

SO

_{3}

was determined by angular-dependent magnetoresistance oscillations measurements and band-structure calculations. This salt was found to have two small pockets with the same [...] Read more.

The Fermi surface structure of a layered organic superconductor

β

^{″}

-(BEDT-TTF)

_{2}

SF

_{5}

CH

_{2}

CF

_{2}

SO

_{3}

was determined by angular-dependent magnetoresistance oscillations measurements and band-structure calculations. This salt was found to have two small pockets with the same area: a deformed square hole pocket and an elliptic electron pocket. Characteristic corrugations in the field dependence of the interlayer resistance in the superconducting phase were observed at any in-plane field directions. The features were ascribed to the commensurability (CM) effect between the Josephson vortex lattice and the periodic nodal structure of the superconducting gap in the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase. The CM effect was observed in a similar field region for various in-plane field directions, in spite of the anisotropic nature of the Fermi surface. The results clearly showed that the FFLO phase stability is insensitive to the in-plane field directions. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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11 pages, 3223 KiB

Open AccessArticle

The FFLO State in the Dimer Mott Organic Superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br

by Shusaku Imajo and Koichi Kindo

Crystals 2021, 11(11), 1358; https://doi.org/10.3390/cryst11111358 - 08 Nov 2021

Cited by 4 | Viewed by 2127

Abstract

The superconducting phase diagram for a quasi-two-dimensional organic superconductor, κ-(BEDT-TTF)₂Cu[N(CN)₂]Br, was studied using pulsed magnetic field penetration depth measurements under rotating magnetic fields. At low temperatures, H_c2 was abruptly suppressed even by small tilts of the applied fields [...] Read more.

The superconducting phase diagram for a quasi-two-dimensional organic superconductor, κ-(BEDT-TTF)₂Cu[N(CN)₂]Br, was studied using pulsed magnetic field penetration depth measurements under rotating magnetic fields. At low temperatures, H_c2 was abruptly suppressed even by small tilts of the applied fields owing to the orbital pair-breaking effect. In magnetic fields parallel to the conducting plane, the temperature dependence of the upper critical field H_c2 exhibited an upturn and exceeded the Pauli limit field H_P in the lower temperature region. Further analyses with the second derivative of the penetration depth showed an anomaly at 31–32 T, which roughly corresponded to H_P. The origin of the anomaly should not be related to the orbital effect, but the paramagnetic effect, which is almost isotropic in organic salts, because it barely depends on the field angle. Based on these results, the observed anomaly is most likely due to the transition between the Bardeen-Cooper-Schrieffer (BCS) and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states. Additionally, we discuss the phase diagram and physical parameters of the transition by comparing them with other FFLO candidates. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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16 pages, 744 KiB

Open AccessArticle

Real Space Quantum Cluster Formulation for the Typical Medium Theory of Anderson Localization

by Ka-Ming Tam, Hanna Terletska, Tom Berlijn, Liviu Chioncel and Juana Moreno

Crystals 2021, 11(11), 1282; https://doi.org/10.3390/cryst11111282 - 22 Oct 2021

Cited by 1 | Viewed by 1802

Abstract

We develop a real space cluster extension of the typical medium theory (cluster-TMT) to study Anderson localization. By construction, the cluster-TMT approach is formally equivalent to the real space cluster extension of the dynamical mean field theory. Applying the developed method to the [...] Read more.

We develop a real space cluster extension of the typical medium theory (cluster-TMT) to study Anderson localization. By construction, the cluster-TMT approach is formally equivalent to the real space cluster extension of the dynamical mean field theory. Applying the developed method to the 3D Anderson model with a box disorder distribution, we demonstrate that cluster-TMT successfully captures the localization phenomena in all disorder regimes. As a function of the cluster size, our method obtains the correct critical disorder strength for the Anderson localization in 3D, and systematically recovers the re-entrance behavior of the mobility edge. From a general perspective, our developed methodology offers the potential to study Anderson localization at surfaces within quantum embedding theory. This opens the door to studying the interplay between topology and Anderson localization from first principles. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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14 pages, 1507 KiB

Open AccessArticle

Dielectric Anomaly and Charge Fluctuations in the Non-Magnetic Dimer Mott Insulator λ-(BEDT-STF)₂GaCl₄

by Olga Iakutkina, Roland Rosslhuber, Atsushi Kawamoto and Martin Dressel

Crystals 2021, 11(9), 1031; https://doi.org/10.3390/cryst11091031 - 27 Aug 2021

Cited by 2 | Viewed by 1815

Abstract

The dimer Mott insulator

λ

-(BEDT-STF)

_{2}

GaCl

_{4}

undergoes no magnetic order down to the lowest temperatures, suggesting the formation of a novel quantum disordered state. Our frequency and temperature-dependent investigations of the dielectric response reveal a relaxor-like behavior below [...] Read more.

The dimer Mott insulator

λ

-(BEDT-STF)

_{2}

GaCl

_{4}

undergoes no magnetic order down to the lowest temperatures, suggesting the formation of a novel quantum disordered state. Our frequency and temperature-dependent investigations of the dielectric response reveal a relaxor-like behavior below

T \approx 100

K for all three axes, similar to other spin liquid candidates. Optical measurement of the charge-sensitive vibrational mode

ν_{27} (b_{1 u})

identifies a charge disproportionation

Δ ρ \approx 0.04 e

on the dimer that exists up to room temperature and originates from inequivalent molecules in the weakly coupled dimers. The linewidth of the charge sensitive mode is broader than that of typical organic conductors, supporting the existence of a disordered electronic state. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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Review

Jump to: Editorial, Research, Other

28 pages, 12776 KiB

Open AccessReview

Ingredients for Generalized Models of κ-Phase Organic Charge-Transfer Salts: A Review

by Kira Riedl, Elena Gati and Roser Valentí

Crystals 2022, 12(12), 1689; https://doi.org/10.3390/cryst12121689 - 22 Nov 2022

Cited by 3 | Viewed by 2216

Abstract

The families of organic charge-transfer salts

κ

-(BEDT-TTF)

_{2} X

and

κ

-(BETS)

_{2} X

, where BEDT-TTF and BETS stand for the organic donor molecules C

_{10}

H

_{8}

S

_{8}

and C

_{10}

H

_{8}

S

_{4}

Se

_{4}

, respectively, [...] Read more.

The families of organic charge-transfer salts

κ

-(BEDT-TTF)

_{2} X

and

κ

-(BETS)

_{2} X

, where BEDT-TTF and BETS stand for the organic donor molecules C

_{10}

H

_{8}

S

_{8}

and C

_{10}

H

_{8}

S

_{4}

Se

_{4}

, respectively, and X for an inorganic electron acceptor, have been proven to serve as a powerful playground for the investigation of the physics of frustrated Mott insulators. These materials have been ascribed a model character, since the dimerization of the organic molecules allows to map these materials onto a single band Hubbard model, in which the dimers reside on an anisotropic triangular lattice. By changing the inorganic unit X or applying physical pressure, the correlation strength and anisotropy of the triangular lattice can be varied. This has led to the discovery of a variety of exotic phenomena, including quantum-spin liquid states, a plethora of long-range magnetic orders in proximity to a Mott metal-insulator transition, and unconventional superconductivity. While many of these phenomena can be described within this effective one-band Hubbard model on a triangular lattice, it has become evident in recent years that this simplified description is insufficient to capture all observed magnetic and electronic properties. The ingredients for generalized models that are relevant include, but are not limited to, spin-orbit coupling, intra-dimer charge and spin degrees of freedom, electron-lattice coupling, as well as disorder effects. Here, we review selected theoretical and experimental discoveries that clearly demonstrate the relevance thereof. At the same time, we outline that these aspects are not only relevant to this class of organic charge-transfer salts, but are also receiving increasing attention in other classes of inorganic strongly correlated electron systems. This reinforces the model character that the

κ

-phase organic charge-transfer salts have for understanding and discovering novel phenomena in strongly correlated electron systems from a theoretical and experimental point of view. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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15 pages, 4461 KiB

Open AccessReview

Superconductivity and Charge Ordering in BEDT-TTF Based Organic Conductors with β″-Type Molecular Arrangement

by Yoshihiko Ihara and Shusaku Imajo

Crystals 2022, 12(5), 711; https://doi.org/10.3390/cryst12050711 - 17 May 2022

Cited by 5 | Viewed by 1859

Abstract

Exotic superconductivity that appears near the charge ordering instability has attracted significant interest since the beginning of superconducting study. The discovery of possible coexistence of charge ordering and superconductivity in cuprates and kagome metals has further fascinated researchers in recent years. In this [...] Read more.

Exotic superconductivity that appears near the charge ordering instability has attracted significant interest since the beginning of superconducting study. The discovery of possible coexistence of charge ordering and superconductivity in cuprates and kagome metals has further fascinated researchers in recent years. In this review, we focus on the BEDT-TTF-based organic superconductor with β″-type molecular packing sequence, which shows the charge ordering transition in the very vicinity of superconducting transition, and summarize the experimental results reported up to the present. At the charge ordering temperature, ultrasonic measurement detects the softening of the crystal lattice, and ¹³C-NMR measurement shows an increase in nuclear spin-lattice relaxation rate divided by temperature 1/T₁T. These results suggest that low-energy dynamics are activated near the charge ordering transition, leading us to invoke the charge-fluctuation mediated superconducting pairing mechanism. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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15 pages, 5569 KiB

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Simultaneous Control of Bandfilling and Bandwidth in Electric Double-Layer Transistor Based on Organic Mott Insulator κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl

by Yoshitaka Kawasugi and Hiroshi M. Yamamoto

Crystals 2022, 12(1), 42; https://doi.org/10.3390/cryst12010042 - 28 Dec 2021

Cited by 6 | Viewed by 2300

Abstract

The physics of quantum many-body systems have been studied using bulk correlated materials, and recently, moiré superlattices formed by atomic bilayers have appeared as a novel platform in which the carrier concentration and the band structures are highly tunable. In this brief review, [...] Read more.

The physics of quantum many-body systems have been studied using bulk correlated materials, and recently, moiré superlattices formed by atomic bilayers have appeared as a novel platform in which the carrier concentration and the band structures are highly tunable. In this brief review, we introduce an intermediate platform between those systems, namely, a band-filling- and bandwidth-tunable electric double-layer transistor based on a real organic Mott insulator κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. In the proximity of the bandwidth-control Mott transition at half filling, both electron and hole doping induced superconductivity (with almost identical transition temperatures) in the same sample. The normal state under electric double-layer doping exhibited non-Fermi liquid behaviors as in many correlated materials. The doping levels for the superconductivity and the non-Fermi liquid behaviors were highly doping-asymmetric. Model calculations based on the anisotropic triangular lattice explained many phenomena and the doping asymmetry, implying the importance of the noninteracting band structure (particularly the flat part of the band). Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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18 pages, 4808 KiB

Open AccessPerspective

Are Heavy Fermion Strange Metals Planckian?

by Mathieu Taupin and Silke Paschen

Crystals 2022, 12(2), 251; https://doi.org/10.3390/cryst12020251 - 12 Feb 2022

Cited by 16 | Viewed by 3911

Abstract

Strange metal behavior refers to a linear temperature dependence of the electrical resistivity that is not due to electron–phonon scattering. It is seen in numerous strongly correlated electron systems, from the heavy fermion compounds, via transition metal oxides and iron pnictides, to magic [...] Read more.

Strange metal behavior refers to a linear temperature dependence of the electrical resistivity that is not due to electron–phonon scattering. It is seen in numerous strongly correlated electron systems, from the heavy fermion compounds, via transition metal oxides and iron pnictides, to magic angle twisted bi-layer graphene, frequently in connection with unconventional or “high temperature” superconductivity. To achieve a unified understanding of these phenomena across the different materials classes is a central open problem in condensed matter physics. Tests whether the linear-in-temperature law might be dictated by Planckian dissipation—scattering with the rate

\sim k_{B} T / ℏ

—are receiving considerable attention. Here we assess the situation for strange metal heavy fermion compounds. They allow to probe the regime of extreme correlation strength, with effective mass or Fermi velocity renormalizations in excess of three orders of magnitude. Adopting the same procedure as done in previous studies, i.e., assuming a simple Drude conductivity with the above scattering rate, we find that for these strongly renormalized quasiparticles, scattering is much weaker than Planckian, implying that the linear temperature dependence should be due to other effects. We discuss implications of this finding and point to directions for further work. Full article

(This article belongs to the Special Issue New Spin on Metal-Insulator Transitions)

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