Degenerate Energy Research Articles

First-principles study on structural, electronic, and ferroelectric properties of high-temperature RMn2O5 (R = Sm, Gd, Dy)

As a class of type-II multiferroics, RMn2O5 compounds show intriguing magnetoelectric coupling behaviors. However, the crystallographic characteristics of the high-temperature paramagnetic phase remain controversial. In present work, by employing PBEsol and SCAN exchange-correlation functionals, we have performed first-principles calculations to study the structural, electronic, and ferroelectric properties of the high-temperature RMn2O5 (R = Sm, Gd, Dy) compounds. According to our calculations, it is revealed that the high-temperature phase of RMn2O5 should be viewed as a super-paraelectric state due to the degeneracy of total energy for the centrosymmetric Pbam and polar Pm/P2 structures. Due to its unique structural and electronic properties, under certain external stimuli, the high-temperature RMn2O5 can develops the so-called electronic ferroelectricity through charge transition/transfer between Mn4+ and Mn3+ sites. In addition, the predicted ferroelectric polarization in the monoclinic Pm/P2 structures is no more than 1 μC/cm2. Our results may offer an alternative understanding for the complicated magnetoelectric behaviors occurred in the RMn2O5 family.

Crystal property and spectroscopic investigation on electro-optic and physico-chemical properties of NLO crystal; 3-(3,4-Dihydroxyphenyl)-L-Alanine

In this attempt, in order to obtain high-quality NLO crystal, organic compound; 3-(3,4-Dihydroxyphenyl)-L-Alanine crystal was fabricated. The organic-composite crystal was characterized by crystallographic and spectroscopic tools. The NLO supported parameters like crystal lattice (orthorhombic) and space group (P212121) examined and validated by XRD examination. The SHG test was carried out and SHG efficiency was calculated that1.29 and 1.35 times greater than solid KDP crystal. The laser damage threshold energy density was determined to be 14.51 GW/cm2. By the application of mulliken charge assignment, multiple dielectric cavities were found in crystal material which is able to process the high degree of birefringence gradient. The oscillating chemical potential movement was observed by examining chemical shift, among the core carbons of hexagonal ring and bridge carbons of chain. The chemical softness insists the binding viability of further ligand groups. The π and δ-conjugated interactive complex orbitals recognized on molecular site and participation in optical active mechanism was identified. UV-Visible transmission characteristics of crystal were studied and UV-Visible absorption on degenerate energy states was noted and its band gap energy was estimated. The CT complex of the present case was acknowledged to be COOH group and it causing crystal properties of current organic composite. The hyperactive polarizability was determined as 1775.05 × 10−33 esu and it was found to be five times greater than thiourea. The depletion energy between highly electrophilic zones and protonic zones was estimated to be ±5.241 e 2 causing permanent dielectric characteristics for the title organic composite. The non-superposable on the molecular mirror image was displayed and thereby optical ability was validated.

Poor man’s scaling and Lie algebras

We consider a general model, describing a quantum impurity with degenerate energy levels, interacting with a gas of itinerant electrons, derive general scaling equation for the model, and analyse the connection between its particular forms and the symmetry of interaction. On the basis of this analysis we write down scaling equations for the Hamiltonians which are the direct products of su(3) Lie algebras and have either SU(2) × U(1) or SU(2) symmetry. We also put into a new context anisotropic Coqblin—Schrieffer models proposed by us earlier.

Flux Tube S -Matrix Bootstrap

We bootstrap the S matrix of massless particles in unitary, relativistic two dimensional quantum field theories. We find that the low energy expansion of such S matrices is strongly constrained by the existence of a UV completion. In the context of flux tube (FT) physics, this allows us to constrain several terms in the S matrix low energy expansion or-equivalently-on Wilson coefficients of several irrelevant operators showing up in the FT effective action. These bounds have direct implications for other physical quantities; for instance, they allow us to further bound the ground state energy as well as the level splitting of degenerate energy levels of large FTs. We find that the S matrices living at the boundary of the allowed space exhibit an intricate pattern of resonances with one sharper resonance whose quantum numbers, mass, and width are precisely those of the world-sheet axion proposed by Athenodorou, Bringoltz, and Teper and Dubovsky, Flauger, and Gorbenko. The general method proposed here should be extendable to massless S matrices in higher dimensions and should lead to new quantitative bounds on irrelevant operators in theories of Goldstones and, also, in gauge and gravity theories.

Microwave derived monoclinic Ba1-xSnxNb2O6 materials as an alternative of ITO

Microwave synthesis was optimized for preparing novel monoclinic Tin-doped barium niobate ceramics (Ba1-xSnxNb2O6; x = 0.0, 0.01, 0.05, 0.1, 0.2, 0.3) BSN. The intensity of monoclinic phase formation was observed to decrease on increasing tin as dopant indicating decreased crystallinity. Strained crystalline phase was observed in undoped sample that became severe on doping tin. Monoclinic metal alloy Sn2O3 formation was confirmed on increasing tin doping beyond 5%. Electronic configuration of tin (II) oxide supports the local site-wise monoclinic disorder in crystal structure due to sterically active lone pair. Such a disorder arranges increased number of degenerate energy states and reduce effective energy gap between conduction band minimum and valence band maximum. All BSN compositions were investigated for monoclinic phase stabilization, ultra-violet absorption, dielectric response, raman modes and type of carrier concentration along with hall resistivity. All measurements possessed inflexion corresponding to 5% tin doping indicating successful site substitution and estimated Sn2O3 metal formation beyond this. The values of reducing optical energy band gap, transparency to visible spectrum, hall resistivity and dc conductivity indicated utility of these materials as a target for growing transparent conducting oxide (TCO) films for substituting well-known indium tin oxide (ITO).

Heat flow reversals without reversing the arrow of time: The role of internal quantum coherences and correlations

One of the stunning consequences of quantum correlations in thermodynamics is the reversal of the arrow of time, recently shown experimentally in [K. Micadei, et al., Nat. Commun. 10:2456 (2019)], and manifesting itself by a reversal of the heat flow (from the cold system to the hot one). Here, we show that contrary to what could have been expected, heat flow reversal can happen without reversal of the arrow of time. Moreover, contrasting with previous studies, no initial correlations between system and bath is required. Instead, the heat flow reversal only relies on internal quantum coherences or correlations, which provides practical advantages over previous schemes: one does not need to have access to the bath in order to reverse the heat flow. The underlying mechanism is explained and shown to stem from the collective system-bath coupling and the impact of non-energetic coherences (coherences between degenerate energy levels) on apparent temperatures. The phenomenon is first uncovered in a broad framework valid for diverse quantum systems containing energy degeneracy. By the end of the paper, aiming at experimental realisations, more quantitative results are provided for a pair of two-level systems. Finally, as a curiosity, we mention that our scheme can be adapted as a correlations-to-energy converter, which have the particularity to be able to operate at constant entropy, similarly to ideal work sources.

Bose-Einstein condensate in Bloch bands with an off-diagonal periodic potential

We report the Bose-Einstein condensate (BEC) in the Bloch bands with off-diagonal periodic potential (ODPP), which simultaneously plays the role of spin-orbit coupling (SOC) and Zeeman field. This model can be realized using two independent Raman couplings in the same three level system, in which the time-reversal symmetry ensures the energy degeneracy between the two states with opposite momenta. We find that these two Raman couplings can be used to tune the spin polarization in momentum space, thus greatly modifies the effective scatterings over the Bloch bands. We observe a transition from the Bloch plane wave phase with condensate at one wave vector to the Bloch stripe phase with condensates at the two Bloch states with opposite wave vectors. These two phases will exhibit totally different spin textures and density modulations in real space, which are totally different from that in free space. In momentum space multiple peaks differ by some reciprocal lattice vectors can be observed, reflecting the periodic structure of the ODPP. A three-band effective model is proposed to understand these observations. This system can provide a new platform in investigating of various physics, such as collective excitations, polaron and topological superlfuids, over the Bloch bands.

Dynamic DNA Energy Landscapes and Substrate Complexity in Triplet Repeat Expansion and DNA Repair.

DNA repeat domains implicated in DNA expansion diseases exhibit complex conformational and energy landscapes that impact biological outcomes. These landscapes include ensembles of entropically driven positional interchanges between isoenergetic, isomeric looped states referred to as rollamers. Here, we present evidence for the position-dependent impact on repeat DNA energy landscapes of an oxidative lesion (8oxodG) and of an abasic site analogue (tetrahydrofuran, F), the universal intermediate in base excision repair (BER). We demonstrate that these lesions modulate repeat bulge loop distributions within the wider dynamic rollamer triplet repeat landscapes. We showed that the presence of a lesion disrupts the energy degeneracy of the rollameric positional isomers. This lesion-induced disruption leads to the redistribution of loop isomers within the repeat loop rollamer ensemble, favoring those rollameric isomers where the lesion is positioned to be energetically least disruptive. These dynamic ensembles create a highly complex energy/conformational landscape of potential BER enzyme substrates to select for processing or to inhibit processing. We discuss the implications of such lesion-induced alterations in repeat DNA energy landscapes in the context of potential BER repair outcomes, thereby providing a biophysical basis for the intriguing in vivo observation of a linkage between pathogenic triplet repeat expansion and DNA repair.

Emergent Haldane phase in an alternating-bond Z3 parafermion chain

The Haldane phase represents one of the most important symmetry-protected states in modern physics. This state can be realized using spin-1 and $\mathrm{spin}\ensuremath{-}\frac{1}{2}$ Heisenberg models and bosonic particles. Here we explore the emergent Haldane phase in an alternating bond ${\mathbb{Z}}_{3}$ parafermion chain, which is different from the previous proposals from fundamental statistics and symmetries. We show that this emergent phase can also be characterized by a modified long-range string order, as well as fourfold degeneracy in the ground-state energies and entanglement spectra. This phase is protected by both the charge conjugate and parity symmetry, and the edge modes are shown to satisfy parafermionic statistics in which braiding of the two edge modes yields a $\frac{2\ensuremath{\pi}}{3}$ phase. This model also supports rich phases, including a topological ferromagnetic parafermion (FP) phase, trivial paramagnetic parafermion phase, classical dimer phase, and gapless phase. The boundaries of the FP phase are shown to be gapless and critical with central charge $c=4/5$. Even in the topological FP phase, it is also characterized by long-range string order; thus we observe a drop of string order across the phase boundary between the FP phase and the Haldane phase. This work opens a new way for finding of exotic topological phases with parafermions.

Photoinduced polarization enhancement on biased bilayer graphene in the Landau level regime

We investigate the charge carrier dynamics in bilayer graphene subject to monochromatic laser irradiation within the Landau level quantization regime. Even though the radiation field does not lift the energy degeneracy of the lowest Landau levels (LLs), it nevertheless has a strong effect on the photoinduced pseudospin polarization response for higher LLs (). Our results show that the photoinduced bandgaps lead to a finite response of the averaged pseudospin polarization with nontrivial oscillating behavior. It is shown that the contribution from these higher LL transitions turns out to be crucial to achieve an enhanced photoinduced polarization in radiated bilayer graphene. The experimental feasibility of our findings is also discussed.

Electronic structure and persistent current of hexagonal MoS2 quantum rings: A tight-binding calculations

The electronic structure and the persistent current of the hexagonal MoS2 quantum rings are investigated within the multi-orbital tight-binding formalism. It is shown that the degenerate energy levels split owing to spin-orbit coupling of Mo and S atoms, especially for the highest occupied state. The energy levels show the periodic oscillation with the increase of perpendicular magnetic flux. The spin-up electrons induced by magnetic flux flow clockwise around the rings resulting in paramagnetism. However, the spin-down electrons flow anticlockwise around the rings resulting in diamagnetism. Thus the hexagonal MoS2 quantum rings exhibit paramagnetic or diamagnetic effect, which is the result of competition of spin-up and spin down states.

Effect of Lateral Electric Field on the Transition Energies of Heavy Hole State and Light Hole State in a Semiconductor Quantum Dot

Using effective mass approximation, this study theoretically investigated how the ground state of the valence band in an inverse parabolic quantum dot (IPQD) reaches the light hole state by strengthening the confinement in the QD plane through the application of a lateral electric field on IPQDs. We calculated the exact analytical solution in the form of the Heun function associated with a complex eigenvalue for the degenerate and excited energy states of the electron, light hole (LH), and heavy hole (HH), in the IPQD context. Our results showed that, for a large QD size, the inverse parabolic quantum confinement (IPQC) potential removed the electron and hole degeneracy, while in the strong confinement region (small QD size), crossing occurred. In addition, the results offered another way of switching the ground state energy of the valence band inside QDs from the HH to LH state when applying a lateral electric field on a QD with an IPQC potential. Consequently, different optical transitions were established, and from the lateral electric field, we obtained a larger quantum Stark shift in IPQD compared with the parabolic QD (PQD), and the excited state energies showed a linear Stark shift in both the QDs.

Valley-dependent exciton fine structure and Autler-Townes doublets from Berry phases in monolayer MoSe2.

The Berry phase of Bloch states can have profound effects on electron dynamics1-3 and lead to novel transport phenomena, such as the anomalous Hall effect and the valley Hall effect4-6. Recently, it was predicted that the Berry phase effect can also modify the exciton states in transition metal dichalcogenide monolayers, and lift the energy degeneracy of exciton states with opposite angular momentum through an effective valley-orbital coupling1,7-11. Here, we report the observation and control of the Berry phase-induced splitting of the 2p exciton states in monolayer molybdenum diselenide (MoSe2) using the intraexciton optical Stark spectroscopy. We observe the time-reversal-symmetric analogue of the orbital Zeeman effect resulting from the valley-dependent Berry phase, which leads to energy difference of +14 (-14) meV between the 2p+ and 2p- exciton states in the K (K') valley, consistent with the ordering from our ab initio GW-Bethe-Salpeter equation results. In addition, we show that the light-matter coupling between intraexciton states is remarkably strong, leading to a prominent valley-dependent Autler-Townes doublet under resonant driving. Our study opens up pathways to coherently manipulate the quantum states and excitonic excitation with infrared radiation in two-dimensional semiconductors.

Energy levels in a self-similar fractal cluster

The energy spectrum of atomic clusters with a fractal structure corresponding to a Sierpiński triangle on a hexagonal lattice are studied theoretically using a simple tight-binding Hamiltonian. The evolution of the energy levels and degeneracy with the growing generation of the fractal cluster is investigated. The energy states are classified into two groups: growing states and temporary states. States belonging to the first group continue to grow after appearing at a certain generation, while those of the second group do not grow. The self-similar structure of the cluster model is reflected in the growing states, which consist of three distinct types. The energy levels of the growing states, whose degeneracy obeys a recurrence relation, can be expressed by an iterated or multi-nested function including the infinitely nested square root function.

Degenerate Landau–Zener model in the presence of quantum noise

The degenerate Landau–Zener–Majorana–Stückelberg model consists of two degenerate energy levels whose energies vary with time and in the presence of an interaction which couples the states of the two levels. In the adiabatic limit, it allows for the populations transfer from states of one level to the states of the other level. The presence of an interaction with the environment influences the efficiency of the process. Nevertheless, identification of possible decoherence-free subspaces permits to engineer coupling schemes for which the effects of quantum noise can be made negligible.

Landau quantization of nearly degenerate bands and full symmetry classification of Landau level crossings

Semiclassical quantization rules compactly describe the energy dispersion of Landau levels, and are predictive of quantum oscillations in transport and thermodynamic quantities. Such rules -- as formulated by Onsager, Lifshitz and Roth -- apply when the spin-orbit interaction dominates over the Zeeman interaction (or vice versa), but does not generally apply when the two interactions are comparable in strength. In this work, we present a generalized quantization rule which treats the spin-orbit and Zeeman interactions on equal footing, and therefore has wider applicability to spin-orbit-coupled materials lacking a spatial inversion center, or having magnetic order. More generally, our rule describes the Landau quantization of any number of nearly degenerate energy bands -- in any symmetry class. The resultant Landau-level spectrum is generically non-equidistant but may contain spin (or pseudospin) degeneracies. To tune to such degeneracies in the absence of crystalline point-group symmetries, three real parameters are needed. We have exhaustively identified all symmetry classes of cyclotron orbits for which this number is reduced from three, thus establishing symmetry-enforced 'non-crossing rules' for Landau levels. In particular, only one parameter is needed in the presence of spatial rotation or inversion; this single parameter may be the magnitude or orientation of the field. Signatures of single-parameter tunability include (i) a smooth crossover between period-doubled and -undoubled quantum oscillations in the low-temperature Shubnikov-de Haas effect, as well as (ii) 'magic-angle' magnetoresistance oscillations. We demonstrate the utility of our quantization rule, as well as the tunability of Landau-level degeneracies, for the Rashba-Dresselhaus two-dimensional electron gas -- subject to an arbitrarily oriented magnetic field.

Enhanced E1 transition between weakly-bound excited states in the nucleus Ne27

Inspired by the recently-reported strong electric-dipole (E1) transition between the weakly-bound first and second excited states, 3/2- at 765 keV and 1/2+ at 885 keV, in the nucleus 27Ne, the E1 transition is estimated in a model by properly taking into account the effect of both deformation and weakly-bound neutrons. In addition to both the spin-parities, 1/2+ and 3/2-, and observed nearly degenerate energies of the two excited states, the observed order of magnitude of the E1 transition strength between the two states is very naturally explained in the case that these two excited states are prolately deformed, in terms of the transitions between the halo components of the wave functions of the weakly-bound odd-neutrons, s1/2 -> p3/2 and s1/2 -> p1/2, in addition to the large probability of the p3/2 component in the weakly-bound neutron [330 1/2] orbit. The large probability is the result of the shell-structure unique in weakly-bound or resonant neutrons.

Effects of noise-induced coherence on the fluctuations of current in quantum absorption refrigerators.

We investigate the effects of noise-induced coherence on average current and current fluctuations in a simple model of a quantum absorption refrigerator with degenerate energy levels. We describe and explain the differences and similarities between the system behavior when it operates in the classical regime, where the populations and coherences in the corresponding quantum optical master equation decouple in a suitably chosen basis, and in the quantum regime, where such a transformation does not exist. The differences between the quantum and the classical cases are observable only close to the maximum current regime, where the system steady-state becomes nonunique. This allows us to approximate the system dynamics by an analytical model based on a dichotomous process that explains the behavior of the average current both in the classical and in the quantum cases. Due to the nonuniqueness, the scaled cumulant generating function for the current at the vicinity of the critical point exhibits behavior reminiscent of the dynamical first-order phase transition. Unless the system parameters are fine-tuned to a single point in the parameter space, the corresponding current fluctuations are moderate in the quantum case and large in the classical case.

Interacting chains of orbital polarons in the colossal magnetoresistance material La1−xSrxMnO3 revealed by spin and lattice dynamics

The origin of the effect of colossal magneto-resistance (CMR) remains still unexplained. In this work we revisit the spin dynamics of the pseudo-cubic La1-xSrxMnO3 along the Mn-O-Mn bond direction at four x doping values (x < 0.5) and various temperatures and report a new lattice dynamics study at x0=0.2, representative of the optimal doping for CMR. We propose an interpretation of the spin dynamics in terms of orbital polarons. This picture is supported by the observation of a discrete magnetic energy spectrum Enmag (q) characteristic of the internal excitations of "orbital polarons" defined by Mn3+ neighbors surrounding a Mn4+ center with a hole. Because of its hopping, the hole mixes up dynamically all the possible orbital configurations of its surrounding Mn3+ sites with degenerate energies. The Enmag values indicate a lift of orbital degeneracy by phonon excitations. The number n varies with the spatial dimension D of the polaron and the q-range determines its size. At x=0.125 and x=0.3 the spectrum reveals 2D polarons coupled by exchange and 3D "free" polarons respectively, with sizes l=1.67a < 2a in all bond directions. At x0=0.2, the spin and the lattice dynamics provide evidence for chains of orbital polarons of size l=2a with a periodic distribution over ~ 3a and an interaction energy ~ 3 meV. At T < Tc the charges propagate together with the longitudinal acoustic phonons along the chains enhancing their ferromagnetic character. The phase separation between metallic and ferromagnetic chains in a non-metallic matrix may be crucial for CMR.

Photonic quantum memory using an intra-atomic frequency comb

Photonic quantum memory, such as an atomic frequency comb (AFC), is essential to make photonic quantum computation and long distance quantum communication scalable and feasible. In standard AFC the frequency of different atoms must be stable relative to each other which presents difficulties in realizing the quantum memory. Here we propose a quantum memory using an intra-atomic frequency comb which does not require frequency stabilization. We show that the transitions between two degenerate energy levels of a single atom can be used to construct the frequency comb. The spacing between the teeth of the comb is controlled by applying an external magnetic field. Since the frequency comb is constructed from individual atoms, these atoms can be used alone or in ensembles to realize the quantum memory. Furthermore, the ensemble based quantum memory with intra-AFC is robust against Doppler broadening which makes it useful for high-temperature quantum memory. As an example, we numerically show the intra-AFC in cesium atoms and demonstrate a photon echo which is essential for quantum memory.