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Condens. Matter, Volume 6, Issue 2 (June 2021) – 10 articles

Cover Story (view full-size image): Two-dimensional systems with topologically protected edge states and corner modes are particularly relevant for application in quantum electronics, as well as quantum technologies, such as directional antennas, programmable coding insulators, on-chip integrated mechanical insulators. Here, we propose a high-order topological superconductor based on a Majorana Benalcazar–Bernevig–Hughes (BBH) model which obeys an extended topological bulk-boundary correspondence, providing a new avenue to confine electron wavefunctions. We gather some clues on the finite size scaling behavior of the topological properties, such as lowest energy gap and correlation functions. Our results can serve to design artificial 2D materials with a non-zero topological invariant and robust edge states. View this paper.
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9 pages, 2647 KiB  
Article
Electrochemical Potential of the Metal Organic Framework MIL-101(Fe) as Cathode Material in Li-Ion Batteries
by Fatemeh Keshavarz, Marius Kadek, Bernardo Barbiellini and Arun Bansil
Condens. Matter 2021, 6(2), 22; https://doi.org/10.3390/condmat6020022 - 16 Jun 2021
Cited by 10 | Viewed by 3779
Abstract
We discuss the characteristic factors that determine the electrochemical potentials in a metal-organic framework used as cathode for Li-ion batteries via density functional theory-based simulations. Our focus is on MIL-101(Fe) cathode material. Our study gives insight into the role of local atomic environment [...] Read more.
We discuss the characteristic factors that determine the electrochemical potentials in a metal-organic framework used as cathode for Li-ion batteries via density functional theory-based simulations. Our focus is on MIL-101(Fe) cathode material. Our study gives insight into the role of local atomic environment and structural deformations in generating electrochemical potential. Full article
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9 pages, 2592 KiB  
Article
Tuning the Electrical Parameters of p-NiOx-Based Thin Film Transistors (TFTs) by Pulsed Laser Irradiation
by Poreddy Manojreddy, Srikanth Itapu, Jammalamadaka Krishna Ravali and Selvendran Sakkarai
Condens. Matter 2021, 6(2), 21; https://doi.org/10.3390/condmat6020021 - 11 Jun 2021
Cited by 6 | Viewed by 3365
Abstract
We utilized laser irradiation as a potential technique in tuning the electrical performance of NiOx/SiO2 thin film transistors (TFTs). By optimizing the laser fluence and the number of laser pulses, the TFT performance was evaluated in terms of mobility, threshold [...] Read more.
We utilized laser irradiation as a potential technique in tuning the electrical performance of NiOx/SiO2 thin film transistors (TFTs). By optimizing the laser fluence and the number of laser pulses, the TFT performance was evaluated in terms of mobility, threshold voltage, on/off current ratio and subthreshold swing, all of which were derived from the transfer and output characteristics. The 500 laser pulses-irradiated NiOx/SiO2 TFT exhibited an enhanced mobility of 3 cm2/V-s from a value of 1.25 cm2/V-s for as-deposited NiOx/SiO2 TFT, subthreshold swing of 0.65 V/decade, on/off current ratio of 6.5 × 104 and threshold voltage of −12.2 V. The concentration of defect gap states as a result of light absorption processes explains the enhanced performance of laser-irradiated NiOx. Additionally, laser irradiation results in complex thermal and photo thermal changes, thus resulting in an enhanced electrical performance of the p-type NiOx/SiO2 TFT structure. Full article
(This article belongs to the Section Surface and Interfaces)
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9 pages, 270 KiB  
Article
Translation-Invariant Excitons in a Phonon Field
by Victor D. Lakhno
Condens. Matter 2021, 6(2), 20; https://doi.org/10.3390/condmat6020020 - 06 Jun 2021
Cited by 1 | Viewed by 2049
Abstract
Large-radius excitons in polar crystals are considered. It is shown that translation invariant description of excitons interacting with a phonon field leads to a nonzero contribution of phonons into the exciton ground state energy only in the case of weak or intermediate electron-phonon [...] Read more.
Large-radius excitons in polar crystals are considered. It is shown that translation invariant description of excitons interacting with a phonon field leads to a nonzero contribution of phonons into the exciton ground state energy only in the case of weak or intermediate electron-phonon coupling. A conclusion is made that self-trapped excitons cannot exist in the limit of strong coupling. Peculiarities of the absorption and emission spectra of translation invariant excitons in a phonon field are discussed. Conditions when the hydrogen-like exciton model remains valid in the case of electron-phonon interaction are found. Full article
(This article belongs to the Section Condensed Matter Theory)
12 pages, 5778 KiB  
Article
Micromagnetic Simulation of Round Ferromagnetic Nanodots with Varying Roughness and Symmetry
by Pia Steinmetz and Andrea Ehrmann
Condens. Matter 2021, 6(2), 19; https://doi.org/10.3390/condmat6020019 - 26 May 2021
Cited by 2 | Viewed by 3295
Abstract
Magnetic nanodots are of high interest for basic research due to their broad spectrum of possible magnetic states and magnetization reversal processes. Besides, they are of technological interest since they can be applied in magnetic data storage, especially if vortex states occur in [...] Read more.
Magnetic nanodots are of high interest for basic research due to their broad spectrum of possible magnetic states and magnetization reversal processes. Besides, they are of technological interest since they can be applied in magnetic data storage, especially if vortex states occur in closed dots or open rings. While producing such nanorings and nanodots from diverse magnetic materials by lithographic techniques is quite common nowadays, these production technologies are naturally prone to small deviations of the borders of these nanoparticles. Here we investigate the influence of well-defined angular-dependent roughness of the edges, created by building the nanoparticles from small cubes, on the resulting hysteresis loops and magnetization reversal processes in five different round nanodots with varying open areas, from a thin ring to a closed nanodot. By varying the orientation of the external magnetic field, the impact of the angle-dependent roughness can be estimated. Especially for the thinnest ring, significant dependence of the transverse magnetization component on the field orientation can be found. Full article
(This article belongs to the Section Magnetism)
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8 pages, 1716 KiB  
Article
Topologically Protected Wormholes in Type-III Weyl Semimetal Co3In2X2 (X = S, Se)
by Christopher Sims
Condens. Matter 2021, 6(2), 18; https://doi.org/10.3390/condmat6020018 - 16 May 2021
Cited by 11 | Viewed by 3574
Abstract
The observation of wormholes has proven to be difficult in the field of astrophysics. However, with the discovery of novel topological quantum materials, it is possible to observe astrophysical and particle physics effects in condensed matter physics. It is proposed in this work [...] Read more.
The observation of wormholes has proven to be difficult in the field of astrophysics. However, with the discovery of novel topological quantum materials, it is possible to observe astrophysical and particle physics effects in condensed matter physics. It is proposed in this work that wormholes can exist in a type-III Weyl phase. In addition, these wormholes are topologically protected, making them feasible to create and measure in condensed matter systems. Finally, Co3In2X2 (X = S, Se) are identified as ideal type-III Weyl semimetals and experiments are put forward to confirm the existence of a type-III Weyl phase. Full article
(This article belongs to the Section Condensed Matter Theory)
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11 pages, 3096 KiB  
Article
Magnetization Reversal in Concave Iron Nano-Superellipses
by Emre Öncü and Andrea Ehrmann
Condens. Matter 2021, 6(2), 17; https://doi.org/10.3390/condmat6020017 - 12 May 2021
Cited by 2 | Viewed by 2409
Abstract
Square magnetic nanodots can show intentional or undesired shape modifications, resulting in superellipses with concave or convex edges. Some research groups also concentrated on experimentally investigating or simulating concave nano-superellipses, sometimes called magnetic astroids due to their similarity to the mathematical shape of [...] Read more.
Square magnetic nanodots can show intentional or undesired shape modifications, resulting in superellipses with concave or convex edges. Some research groups also concentrated on experimentally investigating or simulating concave nano-superellipses, sometimes called magnetic astroids due to their similarity to the mathematical shape of an astroid. Due to the strong impact of shape anisotropy in nanostructures, the magnetization-reversal process including coercive and reversibility fields can be expected to be different in concave or convex superellipses than that in common squares. Here, we present angle-dependent micromagnetic simulations on magnetic nanodots with the shape of concave superellipses. While magnetization reversal occurs via meander states, horseshoe states or the 180° rotation of magnetization for the perfect square, depending on the angle of the external magnetic field, more complicated states occur for superellipses with strong concaveness. Even apparently asymmetric hysteresis loops can be found along the hard magnetization directions, which can be attributed to measuring minor loops since the reversibility fields become much larger than the coercive fields. Full article
(This article belongs to the Section Magnetism)
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59 pages, 608 KiB  
Article
Critical Temperature in the BCS-BEC Crossover with Spin-Orbit Coupling
by Luca Dell’Anna and Stefano Grava
Condens. Matter 2021, 6(2), 16; https://doi.org/10.3390/condmat6020016 - 30 Apr 2021
Cited by 3 | Viewed by 3299
Abstract
We review the study of the superfluid phase transition in a system of fermions whose interaction can be tuned continuously along the crossover from Bardeen–Cooper–Schrieffer (BCS) superconducting phase to a Bose–Einstein condensate (BEC), also in the presence of a spin–orbit coupling. Below a [...] Read more.
We review the study of the superfluid phase transition in a system of fermions whose interaction can be tuned continuously along the crossover from Bardeen–Cooper–Schrieffer (BCS) superconducting phase to a Bose–Einstein condensate (BEC), also in the presence of a spin–orbit coupling. Below a critical temperature the system is characterized by an order parameter. Generally a mean field approximation cannot reproduce the correct behavior of the critical temperature Tc over the whole crossover. We analyze the crucial role of quantum fluctuations beyond the mean-field approach useful to find Tc along the crossover in the presence of a spin–orbit coupling, within a path integral approach. A formal and detailed derivation for the set of equations useful to derive Tc is performed in the presence of Rashba, Dresselhaus and Zeeman couplings. In particular in the case of only Rashba coupling, for which the spin–orbit effects are more relevant, the two-body bound state exists for any value of the interaction, namely in the full crossover. As a result the effective masses of the emerging bosonic excitations are finite also in the BCS regime. Full article
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12 pages, 2990 KiB  
Article
Topological Edge States of a Majorana BBH Model
by Alfonso Maiellaro and Roberta Citro
Condens. Matter 2021, 6(2), 15; https://doi.org/10.3390/condmat6020015 - 09 Apr 2021
Cited by 9 | Viewed by 3353
Abstract
We investigate a Majorana Benalcazar–Bernevig–Hughes (BBH) model showing the emergence of topological corner states. The model, consisting of a two-dimensional Su–Schrieffer–Heeger (SSH) system of Majorana fermions with π flux, exhibits a non-trivial topological phase in the absence of Berry curvature, while the Berry [...] Read more.
We investigate a Majorana Benalcazar–Bernevig–Hughes (BBH) model showing the emergence of topological corner states. The model, consisting of a two-dimensional Su–Schrieffer–Heeger (SSH) system of Majorana fermions with π flux, exhibits a non-trivial topological phase in the absence of Berry curvature, while the Berry connection leads to a non-trivial topology. Indeed, the system belongs to the class of second-order topological superconductors (HOTSC2), exhibiting corner Majorana states protected by C4 symmetry and reflection symmetries. By calculating the 2D Zak phase, we derive the topological phase diagram of the system and demonstrate the bulk-edge correspondence. Finally, we analyze the finite size scaling behavior of the topological properties. Our results can serve to design new 2D materials with non-zero Zak phase and robust edge states. Full article
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13 pages, 392 KiB  
Article
Effect of Mismatched Electron-Hole Effective Masses on Superfluidity in Double Layer Solid-State Systems
by Sara Conti, Andrea Perali, François M. Peeters and David Neilson
Condens. Matter 2021, 6(2), 14; https://doi.org/10.3390/condmat6020014 - 07 Apr 2021
Cited by 1 | Viewed by 2416
Abstract
Superfluidity has been predicted and now observed in a number of different electron-hole double-layer semiconductor heterostructures. In some of the heterostructures, such as GaAs and Ge-Si electron-hole double quantum wells, there is a strong mismatch between the electron and hole effective masses. We [...] Read more.
Superfluidity has been predicted and now observed in a number of different electron-hole double-layer semiconductor heterostructures. In some of the heterostructures, such as GaAs and Ge-Si electron-hole double quantum wells, there is a strong mismatch between the electron and hole effective masses. We systematically investigate the sensitivity to unequal masses of the superfluid properties and the self-consistent screening of the electron-hole pairing interaction. We find that the superfluid properties are insensitive to mass imbalance in the low density BEC regime of strongly-coupled boson-like electron-hole pairs. At higher densities, in the BEC-BCS crossover regime of fermionic pairs, we find that mass imbalance between electrons and holes weakens the superfluidity and expands the density range for the BEC-BCS crossover regime. This permits screening to kill the superfluid at a lower density than for equal masses. Full article
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6 pages, 309 KiB  
Article
Standard Behaviour of Bi2Sr2CaCu2O8+δ Overdoped
by Giovanni Alberto Ummarino
Condens. Matter 2021, 6(2), 13; https://doi.org/10.3390/condmat6020013 - 25 Mar 2021
Cited by 2 | Viewed by 1954
Abstract
I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg theory with only one free parameter in order to reproduce the experimental data relative to [...] Read more.
I calculated the critical temperature and superconducting gap in the framework of one band d wave Eliashberg theory with only one free parameter in order to reproduce the experimental data relative to Bi2Sr2CaCu2O8+δ(BSCCO) in the overdoped regime. The theoretical calculations are in excellent agreement with the experimental data and indicate that cuprates in the overdoped regime are well described by standard d-wave Eliashberg theory with coupling provided by antiferromagnetic spin fluctuations. Full article
(This article belongs to the Section Superconductivity)
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