Nonmagnetic Disorder Research Articles

On the nonmagnetic atom disorder influence in the magnetic and magnetocaloric properties of the Er2Cu0.92Si2.76 off-stoichiometric single-phase compound

This paper presents and discusses material aspects, as well as the magnetic and magnetocaloric properties of nominal Er2Cu0.92Si2.76 alloy synthesized by arc melting. The stoichiometric alloy, prepared with a precise 2(Er):1(Cu):3(Si) ratio, adopts dual symmetries manifesting AlB2-type hexagonal and ThSi2-type tetragonal structures. It was observed that stoichiometric deviations by reducing both Cu/Si concentrations imply a decrease in the tetragonal phase fraction. An 8 % reduction in Cu and Si contents leads to the formation of an off-stoichiometric single-phase: the Er2Cu0.92Si2.76 phase - the focus of this investigation. Notably, Er2Cu0.92Si2.76 showed a uniform grain microstructure without porous and with a homogeneous elemental composition. On the other hand, a long-range antiferromagnetic ordering around 5.6 K was noticed, with a weak influence of magnetic frustration likely arising from nonmagnetic atomic disorder and vacancy at the Cu/Si site. An external magnetic field (greater than 10 kOe) induces ferromagnetic-like ordering, with a saturation tendency at ≈ 50 kOe, corresponding to half the expected magnetic moment for fully ordered Er3+ ions. Er2Cu0.92Si2.76 exhibits large magnetocaloric properties, with isothermal entropy change and relative cooling power values reaching 14.6 J/kgK and 312 J/kg, respectively, for a magnetic field change of 50 kOe. The absence of magnetic and thermal hysteresis is also an attractive prospect for this material aimed at magnetic refrigeration applications at low temperatures.

Effects of forward disorder on quasi-one-dimensional superconductors

We study the competition between disorder and singlet superconductivity in a quasi-one-dimensional (1D) system. We investigate the applicability of the Anderson theorem, namely that time-reversal conserving (nonmagnetic) disorder does not impact the critical temperature, by opposition to time-reversal breaking disorder (magnetic). To do so, we examine a quasi-1D system of spin 1/2 fermions with attractive interactions and forward scattering disorder using field theory (bosonization). By computing the superconducting critical temperature (Tc), we find that, for nonmagnetic disorder, the Anderson theorem also holds in the quasi-1D geometry. In contrast, magnetic disorder has an impact on the critical temperature, which we investigate by deriving renormalization group equations describing the competition between disorder and interactions. Computing the critical temperature as a function of disorder strength, we observe different regimes depending on the strength of interactions. We discuss possible platforms where this can be observed in cold atoms and condensed matter. Published by the American Physical Society 2024

Disorder-Induced Singularity of the Quantum Metric

The quantum weight is a new concept to describe gap electronic states of matter. This quantity is obtained by integrating the quantum metric (the real part of the quantum metric tensor) in the same way as the Berry phase is obtained by integrating the Berry curvature (the imaginary part of the quantum metric tensor). The quantum weight determines a number of kinetic quantities such as the nonlinear anomalous Hall effect, optical conductivity, and photovoltaic effect. In this work, it is shown that nonmagnetic disorder in topological insulators can induce a singularity in the quantum metric and quantum weight.

Insensitivity of Tc to the residual resistivity in high-Tc cuprates and the tale of two domes

One of the few undisputed facts about hole-doped high-Tc cuprates is that their superconducting gap Δ has d-wave symmetry. According to ‘dirty’ d-wave BCS theory, even structural (non-magnetic) disorder can suppress Δ, the transition temperature Tc and the superfluid density ρs. The degree to which the latter is affected by disorder depends on the nature of the scattering. By contrast, Tc is only sensitive to the total elastic scattering rate (as estimated from the residual resistivity ρ0) and should follow the Abrikosov-Gor’kov pair-breaking formula. Here, we report a remarkable robustness of Tc in a set of Bi2201 single crystals to large variations in ρ0. We also survey an extended body of data, both recent and historical, on the LSCO family which challenge key predictions from dirty d-wave theory. We discuss the possible causes of these discrepancies, and argue that either we do not understand the nature of disorder in cuprates, or that the dirty d-wave scenario is not an appropriate framework. Finally, we present an alternative (non-BCS) scenario that may account for the fact that the superconducting dome in Tl2201 extends beyond that seen in Bi2201 and LSCO and suggest ways to test the validity of such a scenario.

Proximity-induced gapless superconductivity in two-dimensional Rashba semiconductor in magnetic field

Two-dimensional semiconductor-superconductor heterostructures form the foundation of numerous nanoscale physical systems. However, measuring the properties of such heterostructures, and characterizing the semiconductor in-situ is challenging. A recent experimental study by [Phys. Rev. Lett. 128, 107701 (2022)] was able to probe the semiconductor within the heterostructure using microwave measurements of the superfluid density. This work revealed a rapid depletion of superfluid density in semiconductor, caused by the in-plane magnetic field which in presence of spin-orbit coupling creates so-called Bogoliubov Fermi surfaces. The experimental work used a simplified theoretical model that neglected the presence of non-magnetic disorder in the semiconductor, hence describing the data only qualitatively. Motivated by experiments, we introduce a theoretical model describing a disordered semiconductor with strong spin-orbit coupling that is proximitized by a superconductor. Our model provides specific predictions for the density of states and superfluid density. Presence of disorder leads to the emergence of a gapless superconducting phase, that may be viewed as a manifestation of Bogoliubov Fermi surface. When applied to real experimental data, our model showcases excellent quantitative agreement, enabling the extraction of material parameters such as mean free path and mobility, and estimating gg-tensor after taking into account the orbital contribution of magnetic field. Our model can be used to probe in-situ parameters of other superconductor-semiconductor heterostructures and can be further extended to give access to transport properties.

Nodal superconductivity in miassite Rh17S15

Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Here, we report compelling evidence of unconventional nodal superconductivity in synthetic samples of Rh17S15 (Tc = 5.4 K), which is also found in nature as the mineral miassite. We investigated the temperature-dependent variation of the London penetration depth Δλ(T) and the disorder evolution of the critical superconducting temperature Tc and the upper critical field Hc2(T) in single crystalline Rh17S15. We found a T − linear temperature variation of Δλ(T) below 0.3Tc, which is consistent with the presence of nodal lines in the superconducting gap of Rh17S15. The nodal character of the superconducting state is supported by the observed suppression of Tc and Hc2(T) in samples with a controlled level of non-magnetic disorder introduced by 2.5 MeV electron irradiation. We propose a nodal sign-changing superconducting gap in the A1g irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the derived superfluid density. To the best of our knowledge, this establishes miassite as the only mineral known so far that reveals unconventional superconductivity in its clean synthetic form, though it is unlikely that it is present in natural crystals because of unavoidable impurities that quickly destroy nodal superconductivity.

Anisotropic multiband superconductivity in 2M−WS2 probed by controlled disorder

The intrinsically superconducting Dirac semimetal 2M−WS2 is a promising candidate for realizing proximity-induced topological superconductivity in its protected surface states. A precise characterization of the bulk superconducting state is essential to understand the nature of surface superconductivity in the system. Here, we report a detailed experimental study of the temperature-dependent London penetration depth, λ(T), the upper critical field, Hc2(T), and the effects of nonmagnetic disorder on these quantities, as well as on the superconducting transition temperature Tc in single crystals of 2M−WS2. We observe a power-law variation of λ(T)∝T3 at temperatures below 0.35Tc. Nonmagnetic pointlike disorder induced by 2.5 MeV electron irradiation at various doses results in a significant suppression of Tc. These observations are markedly different from expectations for a fully gapped isotropic s−wave superconductor. Together with the substantial increase of slope, dHc2/dT|T=Tc, with increasing disorder, our results suggest a strongly anisotropic s++ multiband superconducting state. These results have direct consequences for the expected proximity-induced superconductivity of the topological surface states. Published by the American Physical Society 2024

Non-homogeneous pairing in disordered two-orbital s-wave superconductors

We investigate the effects of non-magnetic disorder in a hybridized two-dimensional two-orbital s-wave superconductor (SC) model. The situation in which electronic orbitals overlap such that the hybridization among them is antisymmetric, under inversion symmetry, was taken into account. The on-site disorder is given by a random impurity potential W. We find that while the random disorder acts to the detriment of superconductivity, hybridization proceeds to favor it. Accordingly, hybridization plays an important role in two-orbital models of superconductivity, in order to hold the long-range order against the increase of disorder. This makes the present model eligible to describe real materials, since the hybridization may be induced by pressure or doping. In addition, the regime from moderate to strong disorder reveals that the system is broken into SC islands with correlated local order parameters. These correlations persist to distances of several order lattice spacing which corresponds to the size of the SC-Islands.

Evidence of Nodal Superconductivity in Monolayer 1H-TaS2 with Hidden Order Fluctuations.

Unconventional superconductors represent one of the fundamental directions in modern quantum materials research. In particular, nodal superconductors are known to appear naturally in strongly correlated systems, including cuprate superconductors and heavy-fermion systems. Van der Waals materials hosting superconducting states are well known, yet nodal monolayer van der Waals superconductors have remained elusive. Here, using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments, it is shown that pristine monolayer 1H-TaS2 realizes a nodal superconducting state. Non-magnetic disorder drives the nodal superconducting state to a conventional gapped s-wave state. Furthermore, many-body excitations emerge close to the gap edge, signalling a potential unconventional pairing mechanism. The results demonstrate the emergence of nodal superconductivity in a van der Waals monolayer, providing a building block for van der Waals heterostructures exploiting unconventional superconductingstates.

Ion-Selective Scattering Studied Using the Variable-Energy Electron Irradiation in the Ba0.2K0.8Fe2As2 Superconductor.

Low-temperature variable-energy electron irradiation was used to induce non-magnetic disorder in a single crystal of a hole-doped iron-based superconductor, Ba1-xKxFe2As2, x = 0.80. To avoid systematic errors, the beam energy was adjusted non-consequently for five values between 1.0 and 2.5 MeV when sample resistance was measured in situ at 22 K. For all energies, the resistivity raises linearly with the irradiation fluence suggesting the creation of uncorrelated dilute point-like disorder (confirmed by simulations). The rate of the resistivity increase peaks at energies below 1.5 MeV. Comparison with calculated partial cross-sections points to the predominant creation of defects in the iron sublattice. Simultaneously, superconducting Tc, measured separately between the irradiation runs, is monotonically suppressed as expected, since it depends on the total scattering rate, hence on the total cross-section, which is a monotonically increasing function of the energy. Our work experimentally confirms an often-made assumption of the dominant role of the iron sub-lattice in iron-based superconductors.

Exchange Interaction‐Driven Surface State Hybridization in Bi2Se3Topological Insulator

AbstractTopological insulator has gapless surface state, which results from bulk band inversion due to strong spin–orbit coupling. This nontrivial topology robust to nonmagnetic disorders and defects is considered one of the biggest obstacles for modulations of the surface state. In this work, the suppression of surface properties in Bi2Se3of various thicknesses grown on antiferromagnetic NiO, which has a strong exchange interaction is investigated. Under perpendicular magnetic fields, a drastic decrease in mobilityµand in the number of phase coherent channelsαin 5 QL Bi2Se3on NiO are observed. In addition, the THz transmission study shows that an increase in the surface penetration depthξcan accelerate the hybridization of surface states, which is also verified using the optical pump THz probe. This rapid collapse of surface states indicates the unique role of antiferromagnetic materials in band overlap, suggesting that the topological surface nature can be modulated by forming an antiferromagnet‐topological insulator heterostructure.

Emergent Halperin–Saslow mode, gauge glass and quenched disorders in quantum Ising magnet TmMgGaO4

Quenched disorders could bring novel quantum excitations and models to certain quantum magnets. Motivated by recent experiments on the quantum Ising magnet TmMgGaO4, we explore the effects of the quenched disorder and the interlayer coupling in this triangular lattice Ising antiferromagnet. It is pointed out that the weak quenched (nonmagnetic) disorder would convert the emergent 2D Berezinskii–Kosterlitz–Thouless (BKT) phase and the critical region into a U(1) gauge glass. There will be an emergent Halperin–Saslow mode associated with this gauge glass. Using the Imry-Ma’s renormalization group result, we explain the fate of the finite-field [Formula: see text] symmetry breaking transition at the low temperatures. The ferromagnetic interlayer coupling would suppress the BKT phase and generate a tiny ferromagnetism. With quenched disorders, this interlayer coupling changes the 2D gauge glass into a 3D gauge glass, and the Halperin–Saslow mode persists. This work merely focuses on addressing a phase regime in terms of emergent U(1) gauge glass behaviors and hope to inspire future works and thoughts in weakly disordered frustrated magnets in general.

Absence of magnetic order and emergence of unconventional fluctuations in the Jeff=12 triangular-lattice antiferromagnet YbBO3

We present the ground state properties of a new quantum antiferromagnet, ${\mathrm{YbBO}}_{3}$, in which the isotropic ${\mathrm{Yb}}^{3+}$ triangular layers are separated by a nonmagnetic layer of partially occupied B and O(2) sites. The magnetization and heat capacity data establish a spin-orbit entangled effective spin ${J}_{\mathrm{eff}}=\frac{1}{2}$ state of ${\mathrm{Yb}}^{3+}$ ions at low temperatures, interacting antiferromagnetically with an intralayer coupling $J/{k}_{\mathrm{B}}\ensuremath{\simeq}0.53$ K. The absence of oscillations and a $1/3$ tail in the zero-field muon asymmetries rules out the onset of magnetic long-range order as well as spin freezing down to 20 mK. An anomalous broad maximum in the temperature-dependent heat capacity with an unusually reduced value and a broad anomaly in the zero-field muon depolarization rate centered at ${T}^{*}\ensuremath{\simeq}0.7J/{k}_{\mathrm{B}}$ provide compelling evidence for a wide fluctuating regime $(0.2\ensuremath{\lesssim}T/J\ensuremath{\le}1.5)$ with slow relaxation. We infer that the fluctuating regime is a universal feature of highly frustrated triangular-lattice antiferromagnets while the absence of magnetic long-range order is due to perfect two-dimensionality of the spin lattice protected by nonmagnetic site disorder.

Влияние немагнитного беспорядка на фазовые переходы первого рода

Using the Monte Carlo method, weakly dilute magnetic systems described by two-dimensional five-component Potts model, which in the pure state undergoes a phase transition first order. Numerical studies have revealed that an insignificant quenched disorder, realized in the form of non-magnetic impurities, can change the order of phase transition in magnetic systems described by the two-dimensional five-component Potts model on a square lattice, for which a phase transition of the first order is observed in a homogeneous state. It is shown that such a phase change the transition is due to the fact that non-magnetic disorder prevents the coexistence of local phases characteristic of a phase transition of the first order at T = Tl .

The effect of nonmagnetic disorder on phase transitions of the first-order

The effect of nonmagnetic disorder on phase transitions of the first-order

Anisotropic superconductivity of niobium based on its response to nonmagnetic disorder

Niobium is one of the most studied superconductors, both theoretically and experimentally. It is tremendously important for applications, and it has the highest superconducting transition temperature, ${T}_{c}=9.32$ K, of all pure metals. In addition to power applications in alloys, pure niobium is used for sensitive magnetosensing, radio-frequency cavities, and, more recently, as circuit metallization layers in superconducting qubits. A detailed understanding of its electronic and superconducting structure, especially its normal and superconducting state anisotropies, is crucial for mitigating the loss of quantum coherence in such devices. Recently, a microscopic theory of the anisotropic properties of niobium with the disorder was put forward. To verify theoretical predictions, we studied the effect of disorder produced by 3.5 MeV proton irradiation of thin Nb films grown by the same team and using the same protocols as those used in transmon qubits. By measuring the superconducting transition temperature and upper critical fields, we show a clear suppression of ${T}_{c}$ by potential (nonmagnetic) scattering, which is directly related to the anisotropic order parameter. We obtain a very close quantitative agreement between the theory and the experiment.

Breakdown of topological protection due to nonmagnetic edge disorder in two-dimensional materials in the quantum spin Hall phase

We study the suppression of the conductance quantization in quantum spin Hall systems by a combined effect of electronic interactions and edge disorder, that is ubiquitous in exfoliated and chemical vapor deposition grown two-dimensional (2D) materials. We show that the interplay between the electronic localized states due to edge defects and electron-electron interactions gives rise to local magnetic moments, that break time-reversal symmetry and the topological protection of the edge states in 2D topological systems. Our results suggest that edge disorder leads to small deviations of a perfect quantized conductance in short samples and to a strong conductance suppression in long ones. Our analysis is based on the Kane-Mele model, an unrestricted Hubbard mean-field Hamiltonian, and on a self-consistent recursive Green's function technique to calculate the transport quantities.

Band Structure and Quantum Transport of Bent Bilayer Graphene.

We investigate the band structures and transport properties of a zigzag-edged bent bilayer graphene nanoribbon under a uniform perpendicular magnetic field. Due to its unique geometry, the edge and interface states can be controlled by an electric field or local potential, and the conductance exhibits interesting quantized behavior. When Zeeman splitting is considered, the edge states are spin-filtered, and a weak quantum spin Hall (WQSH) phase appears. In the presence of an electric field or local potential, a WQSH-QH junction or WQSH-spin-unbalanced QSH junction can be achieved, respectively, while fully spin-polarized currents appear in the interface region. Zeeman splitting lifts the spin degeneracy, leading to a WQSH around zero energy with a quantized two-terminal conductance of 4e2/h, which is robust against weak nonmagnetic disorder. These results provide a way to manipulate the band structures and transport properties of the system using an electric field, local potential, and Zeeman splitting.

Disorder-induced phase transitions in double Weyl semimetals

The double Weyl semimetal (DWSM) is a newly proposed topological material that hosts Weyl points with chiral charge n=2. The disorder effect in DWSM is investigated by adopting the tight-binding Hamiltonian. Using the transfer matrix method and the noncommutative Kubo formula, we numerically calculate the localization length and the Hall conductivity in the presence of the on-site nonmagnetic disorder or orbital (spin-flip) disorders, and give the corresponding global phase diagrams. For the on-site nonmagnetic disorder, the system undergoes the DWSM-3D quantum anomalous hall (3D QAH) and normal insulator (NI)-DWSM phase transitions, and evolves into the diffusive metal (DM) phase before being localized by strong disorders, which is consistent with the Weyl semimetal. For \sigma_x orbital disorder, we find that increasing disorder can generate a pair of Weyl nodes at the boundary of the Brillouin zone and induce a 3D QAH-DWSM phase transition. Then we investigate the combined effect of orbital disorders for both disordered 3D QAH phase and DWSM phase. The disorder-induced transitions can be well understood in terms of an effective medium theory based on self-consistent Born approximation.

Universal suppression of superfluid weight by non-magnetic disorder in $s$-wave superconductors independent of quantum geometry and band dispersion

Motivated by the experimental progress in controlling the properties of the energy bands in superconductors, significant theoretical efforts have been devoted to study the effect of the quantum geometry and the flatness of the dispersion on the superfluid weight. In conventional superconductors, where the energy bands are wide and the Fermi energy is large, the contribution due to the quantum geometry is negligible, but in the opposite limit of flat-band superconductors the superfluid weight originates purely from the quantum geometry of Bloch wave functions. Here, we study how the energy band dispersion and the quantum geometry affect the disorder-induced suppression of the superfluid weight. In particular, we consider non-magnetic disorder and s-wave superconductivity. Surprisingly, we find that the disorder-dependence of the superfluid weight is universal across a variety of models, and independent of the quantum geometry and the flatness of the dispersion. Our results suggest that a flat-band superconductor is as resilient to disorder as a conventional superconductor.