Condensation Of Defects Research Articles

Electrophysical parameters of p-i-n photodiodes, irradiated with 60Co γ-quanta

The results of studies of changes in the electrical parameters of p-i-n photodiodes manufactured on monocrystalline silicon wafers of p-type conductivity orientation (100) with ρ = 1000 Ohm cm, when irradiated with γ-quanta from a 60Co source, are presented. It has been established that as a result of irradiation of p-i-n photodiodes with doses up to 2·1015 quanta/cm2, the reverse dark current increases by more than an order of magnitude. However, the shape of the curve of the dependence of the current on the applied reverse voltage of irradiated p-i-n photodiodes does not change qualitatively, as for the original devices there are three regions with different dependences of current on voltage: sublinear, superlinear and linear, due to different mechanisms of generation-recombination processes in the depletion region of the p-n junction. The main reason for the increase in the reverse current of p-i-n photodiodes as a result of irradiation with γ-quanta is the formation of generation-recombination centers of radiation origin due to the condensation of primary radiation defects (vacancies and/or self-interstitial atoms) on technological residual structural defects formed during the growth of silicon single crystals, and during subsequent high-temperature treatments during the formation of devices.

Lorentz violation emergence in a superconductivity phase transition scenario

In this letter, we use a condensation of topological defects mechanism to show how a Lorentz-violating (LV) term can emerge. The approach used here was originally developed to describe phase transitions due to a vortice proliferation in systems such as superconductors. First, as a consequence of our approach, we obtain a Podolsky-like LV term as a result of this work. Second, a condensation of topological defects in the theory restores the original Lorentz symmetric phase of the theory. The approach presented here can be seen as an early description of a mechanism to describe a phase transition between a Lorentz symmetric phase and a non-symmetric one. Besides the usage of this mechanism to show how LV terms can emerge, we also show how to extend the condensation mechanism to scalar theories. The condensation mechanism was originally designed for gauge theories. As a result, our scalar extension recovers the Ginzburg-Landau (GL) description of both regular and non-local superconductors. The GL description, in our approach, arises as a consequence of the condensation of topological defects.

Intragranular thermal fatigue of Cu thin films: Near-grain boundary hardening, strain localization and voiding

In order to obtain a fundamental understanding of the phenomena accompanying thermomechanical fatigue of Cu metallization used in power electronics, as well as the resulting deterioration of electric properties, there is a need to assess intragranular microstructure and strain evolution within individual Cu grains and near grain boundaries. Here, synchrotron dark field X-ray microscopy (DFXM) is used to characterize as-deposited and 5 × 104 times thermally-cycled 20 µm thick Cu films. The cycling was performed using a dedicated test chip in the range of 100–400 °C applying a heating rate of 106 K/s. The thermomechanical treatment results in severe shear deformation of Cu grains, film surface roughening, gradual grain microstructural refinement, the emergence of microscopic voids preferably at high angle grain boundaries (HAGBs) and finally in the voids' percolation, as revealed by in-situ and ex-situ scanning electron microscopy. DFXM provides experimental evidence that mosaicity of Cu grains, residual tensile and compressive elastic strain concentrations and full width at half maximum of Cu 111 reflections increase simultaneously in the vicinity of the HAGBs. The latter is interpreted as resulting from vacancy condensation in front of the HAGBs, after dislocations have moved across cycled grains and partly annihilated. Moreover, the observed HAGB decohesion and gradual void formation are correlated with the supposed hardening of regions near HAGBs, which thus lose their ductility during cyclic elasto-plastic deformation. The results are complemented with the ex-situ X-ray nanotomography data, which document the voids' percolation across the film's thickness with a “crack” width of up to ∼2 µm. In general, the study identifies the inhomogeneous intragranular microstructural refinement and a gradual condensation of structural defects near HAGBs as driving forces for void formation in thick Cu metallizations during fast thermo-mechanical fatigue.

Massive Carroll-Field-Jackiw model from a condensation of topological defects

In this letter we demonstrated precisely that the mass of the massive Carroll-Field-Jackiw model can emerge from a condensation of the topological defects. To accomplish the task we have used an approach named Julia-Toulouse mechanism and we described how a condensation of topological defects can drive the system to a phase with a massive theory.

Searching for fracton orders via symmetry defect condensation

We propose a set of constraints on the ground-state wavefunctions of fracton phases, which provide a possible generalization of the string-net equations used to characterize topological orders in two spatial dimensions. Our constraint equations arise by exploiting a duality between certain fracton orders and quantum phases with "subsystem" symmetries, which are defined as global symmetries on lower-dimensional manifolds, and then studying the distinct ways in which the defects of a subsystem symmetry group can be consistently condensed to produce a gapped, symmetric state. We numerically solve these constraint equations in certain tractable cases to obtain the following results: in $d=3$ spatial dimensions, the solutions to these equations yield gapped fracton phases that are distinct as conventional quantum phases, along with their dual subsystem symmetry-protected topological (SSPT) states. For an appropriate choice of subsystem symmetry group, we recover known fracton phases such as Haah's code, along with new, symmetry-enriched versions of these phases, such as non-stabilizer fracton models which are distinct from both the X-cube model and the checkerboard model in the presence of global time-reversal symmetry, as well as a variety of fracton phases enriched by spatial symmetries. In $d=2$ dimensions, we find solutions that describe new weak and strong SSPT states, such as ones with both line-like subsystem symmetries and global time-reversal symmetry. In $d=1$ dimension, we show that any group cohomology solution for a symmetry-protected topological state protected by a global symmetry, along with lattice translational symmetry necessarily satisfies our consistency conditions.

Podolsky electrodynamics from a condensation of topological defects

In this paper we demonstrate the arising of higher-derivative contributions to the effective action of electrodynamics on the basis of the generalized Julia-Toulouse mechanism and explicitly show that the complete effective action arising within this methodology is nonlocal.

Superconductivity from the condensation of topological defects in a quantum spin-Hall insulator

The discovery of quantum spin-Hall (QSH) insulators has brought topology to the forefront of condensed matter physics. While a QSH state from spin-orbit coupling can be fully understood in terms of band theory, fascinating many-body effects are expected if it instead results from spontaneous symmetry breaking. Here, we introduce a model of interacting Dirac fermions where a QSH state is dynamically generated. Our tuning parameter further allows us to destabilize the QSH state in favour of a superconducting state through proliferation of charge-2e topological defects. This route to superconductivity put forward by Grover and Senthil is an instance of a deconfined quantum critical point (DQCP). Our model offers the possibility to study DQCPs without a second length scale associated with the reduced symmetry between field theory and lattice realization and, by construction, is amenable to large-scale fermion quantum Monte Carlo simulations.

Remarks on a B ∧ F model with topological mass from gauging spin

Aspects of screening and confinement are reassessed for a model with topological mass with the gauging of spin. Our discussion is carried out using the gauge-invariant, but path-dependent, variables formalism. We explicitly show that the static potential profile is the sum of a Yukawa and a linear potential, leading to the confinement of static external charges. Interestingly enough, similar results are obtained in a theory of antisymmetric tensor fields that results from the condensation of topological defects as a consequence of the Julia-Toulouse mechanism.

Emergent Higgsless Superconductivity

We present a new Higgsless model of superconductivity, inspired from anyon superconductivity but P- and T-invariant and generalizable to any dimension. While the original anyon superconductivity mechanism was based on incompressible quantum Hall fluids as average field states, our mechanism involves topological insulators as average field states. In D space dimensions it involves a (D-1)-form fictitious pseudovector gauge field which originates from the condensation of topological defects in compact low-energy effective BF theories. There is no massive Higgs scalar as there is no local order parameter. When electromagnetism is switched on, the photon acquires mass by the topological BF mechanism. Although the charge of the gapless mode (2) and the topological order (4) are the same as those of the standard Higgs model, the two models of superconductivity are clearly different since the origins of the gap, reflected in the high-energy sectors are totally different. In 2D this type of superconductivity is explicitly realized as global superconductivity in Josephson junction arrays. In 3D this model predicts a possible phase transition from topological insulators to Higgsless superconductors.

Condensation of lattice defects and melting transitions in quantum Hall phases

Motivated by recent progress in understanding the interplay between lattice and electronic topological phases, we consider quantum-melting transitions of {\it weak} quantum liquid crystals, a crystal and a nematic phase, in which electrons form a quantum Hall state. In certain classes of Chern band insulators and quantum Hall phases, it has been previously demonstrated that there are topological Chern-Simons terms such as a Hall viscosity term and a gravitational Chern-Simons term for local lattice deformations. The Chern-Simons terms can induce anyonic statistics for the topological lattice defects and furthermore dress the defects with certain symmetry quantum numbers. On the other hand, the melting transitions of such liquid-crystalline orders are driven by the condensation of lattice defects. Based on these observations, we show how the topological terms can change the nature of the proximate disordered phases of the quantum liquid crystalline phases. We derive and study the effective dual field theories for the liquid crystalline phases with the geometric Chern-Simons terms, and carefully examine the symmetry quantum numbers and statistics of defects. We show that a crystal may go through a continuous phase transition into another crystal with the new discrete translational symmetries because the dislocation, the topological defect in the crystal, carries non-zero crystal momentum due to the Hall viscosity term. For the nematic phase, the disclination will condense at the phase transition to the isotropic phase, and we show that the isotropic phase may support a deconfined fractionally charged excitation due to the Wen-Zee term, and thus the isotropic phase and the nematic phase have different electromagnetic Hall responses.

Electric-field-induced point defect redistribution in single-crystal TiO2–x and effects on electrical transport

The spatial redistribution of non-stoichiometric point defects in rutile TiO2 is studied as a function of voltage and time. Single crystals are equilibrated initially to a well-defined stoichiometry with n-type conductivity and a carrier concentration on the order of 1018cm−3. The crystals are subsequently electroded with Pt contacts that exhibit Schottky behavior. When subjected to an applied voltage of 15V, a time-dependent increase and saturation in the leakage current is observed, which is associated with an accumulation of point defects and an attendant decrease in stoichiometry at the cathode electrode. This local change in stoichiometry degrades the Schottky barrier, leading to asymmetric electrodes and thus macroscopic rectifying behavior. Cathodoluminescence spectroscopy shows that Ti interstitials dominate the point defect redistribution process. Under larger applied voltages, of around 30V, qualitatively different behavior is observed in which the resistivity increases as a function of time. This behavior is associated with condensation of point defects into a region of extended defects and Magnéli phases near the cathode, sufficient to increase the bulk stoichiometry and resistivity. These experiments demonstrate that a one-dimensional drift-diffusion process, as opposed to filamentary growth, dominates in these experimental conditions and that the Pt–TiO2–Pt system remains closed, with no significant oxygen transport across the Pt–TiO2 interfaces. We believe this is the first observation of a second higher-voltage regime in which the bulk stoichiometry and thus resistivity is increased as large concentrations of defects condense into metallic Magnéli phases in the near-electrode regions.

Higgsless superconductivity from topological defects in compact BF terms

We present a new Higgsless model of superconductivity, inspired from anyon superconductivity but P- and T-invariant and generalisable to any dimension. While the original anyon superconductivity mechanism was based on incompressible quantum Hall fluids as average field states, our mechanism involves topological insulators as average field states. In D space dimensions it involves a (D−1)-form fictitious pseudovector gauge field which originates from the condensation of topological defects in compact low-energy effective BF theories. In the average field approximation, the corresponding uniform emergent charge creates a gap for the (D−2)-dimensional branes via the Magnus force, the dual of the Lorentz force. One particular combination of intrinsic and emergent charge fluctuations that leaves the total charge distribution invariant constitutes an isolated gapless mode leading to superfluidity. The remaining massive modes organise themselves into a D-dimensional charged, massive vector. There is no massive Higgs scalar as there is no local order parameter. When electromagnetism is switched on, the photon acquires mass by the topological BF mechanism. Although the charge of the gapless mode (2) and the topological order (4) are the same as those of the standard Higgs model, the two models of superconductivity are clearly different since the origins of the gap, reflected in the high-energy sectors are totally different. In 2D this type of superconductivity is explicitly realised as global superconductivity in Josephson junction arrays. In 3D this model predicts a possible phase transition from topological insulators to Higgsless superconductors.

Effects of tritium content on lattice parameter, 3He retention, and structural evolution during aging of titanium tritide

Effects of tritium content on crystal lattice, 3He retention and structure evolution during aging of Ti tritides were investigated using X-ray diffraction, together with analysis of 3He release. Phase transformation and lattice parameters of the Ti tritides varied apparently depending on tritium stoichiometry. Initial tritium content in the Ti tritides showed significant effects on 3He retention due to the existence of α + δ phase boundaries and lattice symmetry in the Ti tritide. The critical contents of 3He release in the α + δ phase region and ε phase region were found to be smaller than that in the δ phase region. Variation of crystal lattice structures of α + δ, δ and ε phases in the Ti tritides has been investigated, and evolution of 3He during aging is mainly governed by the finite defects of self-interstitial atoms, dislocation loops, 3He bubbles, and dislocations created by formation and growth of 3He bubbles. In the α + δ two phase region, the phase boundaries played an important role to accumulate 3He bubbles and cause inter-bubble fracture. In the ε phase region, a preferred condensation of finite defects in basal plane of Ti tritide lattice and formation of a dislocation network were identified.

Defect mastering - one of the topic challenges for crystal growth

The quality of single crystals, epitaxial layers and devices made there from are very sensitively influenced by structural and atomistic deficiencies generated during the crystal growth. Crystalline imperfections comprise point defects, dislocations, grain boundaries, second-phase particles. Over more than a half-century of the development of crystal growth, most of the important defect-forming mechanisms have become well understood [1-2]. As a result, the present state of technology makes it possible to produce crystals of remarkably high quality. However, that is not to say that all problems are already solved. For instance, in comparison with silicon the point defect dynamics in semiconductor and oxide compounds is not nearly as well understood. The density of equivalent defect types and antisites in each sub-lattice is determined by deviation from stoichiometry. Their charge state depends on the Fermi level position leading via interaction with dopants to certain compensation level and complex formation. One measure proves to be the in situ control of stoichiometry. Due to high-temperature dislocation dynamics heterogeneous dislocation substructures are formed. Both, acting thermo-mechanical stress and given point defect situation force the dislocation to glide and climb. In the course of enthalpy minimization the long-range character of dislocation interaction produces agglomerates and patterns with polygonized cell walls, i.e. small angle grain boundaries [3]. Thanks to the rules of correspondence of Taylor and Kuhlmann-Wilsdorf one is able to estimate the interaction between shear stress, dislocation density and cell diameter (Fig.). In epitaxy the Nye tensor, describing dislocation distribution inhomogeneity, affects the layer stress considerably. The growth under minimum stress, solution hardening and in situ stoichiometry control are effective counteracting methods. One of the most serious consequences during cooling down of as-grown crystals is the point defect condensation in precipitates and micro-voids decorating dislocation patterns or inducing high mechanical misfit stress that generates dislocation loops. It proves to be favourable to anneal the crystal a few degrees below the melting point in order to dissolve the particles and re-diffuse their into the crystal matrix.

Three-dimensional Lorentz-violating action

We demonstrate the generation of the three-dimensional Chern-Simons-like Lorentz-breaking ``mixed" quadratic action via an appropriate Lorentz-breaking coupling of vector and scalar fields to the spinor field and study some features of the scalar QED with such a term. We show that the same term emerges through a nonpertubative method, namely the Julia-Toulouse approach of condensation of charges and defects.

Landau theory for stress-induced order-disorder transitions in phase change materials

We propose a Landau theory for phase change materials (PCMs), which describes stress-induced amorphization in vacancy-free, ordered PCMs as a condensation of defects in analogy to equilibrium gas-liquid transitions. Three dimensionless parameters suffice to deduce a highly accurate equation of state for both phases from it. The reference data for our model alloy Ge${}_{x}$Sb${}_{1\ensuremath{-}x}$ are produced from molecular dynamics simulations and synchrotron x-ray diffraction. Raman spectroscopy is used to estimate the density of tetrahedrally coordinated germanium atoms, which we relate to the order parameter. All methods provide consistent support for the reversibility of the transition.

Modifications of the Meissner screening profile in YBa2Cu3O7−δthin films by gold nanoparticles

Adding Au nanoparticles to YBa$_2$Cu$_3$O$_{7-\delta}$ thin films leads to an increase of the superconducting transition temperature $T_{\rm c}$ and the critical current density $j_{\rm c}$. While the higher $j_{\rm c}$ can be understood in terms of a stronger pinning of the flux vortices at the Au nanoparticles, the enhanced $T_{\rm c}$ is still puzzling. In the present study, we determined the microscopic magnetic penetration profiles and the corresponding London penetration depths $\lambda_{\rm L}$ in the Meissner state of optimally doped YBa$_2$Cu$_3$O$_{7-\delta}$ thin films with and without Au nanoparticles by low-energy muon spin rotation. By Rutherford backscattering spectrometry, we show that the Au nanoparitcles are distributed over the whole thickness of the thin-film samples. The superfluid density $n_{\rm s} \propto 1/\lambda_{\rm L}^2$ was found to increase in the films containing Au nanoparticles. We attribute this increase of $n_{\rm s}$ to a reduction of the defect density possibly due to defect condensation at the Au nanoparticles.

Phase stability in the two-dimensional anisotropic boson Hubbard Hamiltonian

The two-dimensional square lattice hard-core boson Hubbard model with near-neighbor interactions has a ``checkerboard'' charge density wave insulating phase at half filling and sufficiently large intersite repulsion. When doped, rather than forming a supersolid phase in which long-range charge density wave correlations coexist with a condensation of superfluid defects, the system instead phase separates. However, it is known that there are other lattice geometries and interaction patterns for which such coexistence takes place. In this paper we explore the possibility that anisotropic hopping or anisotropic near-neighbor repulsion might similarly stabilize the square lattice supersolid. By considering the charge density wave structure factor and superfluid density for different ratios of interaction strength and hybridization in the $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{x}$ and $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{y}$ directions, we conclude that phase separation still occurs.

Julia-Toulouse approach to (d+1)-dimensional bosonized Schwinger model with an application to largeNQCD

The Julia-Toulouse approach for condensation of charges and defects is used to show that the bosonized Schwinger model can be obtained through a condensation of electric charges in 1+1 dimensions. The massive model is derived by taking into account the presence of vortices over the electric condensate, while the massless model is obtained when these vortices are absent. This construction is then straightforwardly generalized for arbitrary d+1 spacetime dimensions. The d=3 case corresponds to the large N chiral dynamics of SU(N) QCD in the limit $N\rightarrow\infty$.

From topological insulators to superconductors and confinement

Topological matter in three dimensions (3D) is characterized by the presence of a topological BF term in its long-distance effective action. We show that, in 3D, there is another marginal term that must be added to the action in order to fully determine the physical content of the model. The quantum phase structure is governed by three parameters that drive the condensation of topological defects: the BF coupling, the electric permittivity and the magnetic permeability of the material. For intermediate levels of electric permittivity and magnetic permeability, the material is a topological insulator. We predict, however, new states of matter when these parameters cross critical values: a topological superconductor when the electric permittivity is increased and the magnetic permeability is lowered and a charge confinement phase in the opposite case of low electric permittivity and high magnetic permeability. Synthetic topological matter may be fabricated as 3D arrays of Josephson junctions.