Degenerate Valence Research Articles

PtS2/WSe2 heterostructure for thermoelectric and Li-ion battery Applications: A First-Principles study

Van der Waals’ two-dimensional heterostructure engineering offers great potential in applications of electronic devices, battery applications, thermoelectric. Encouraged by the successful synthesis of PtS2/WSe2 heterostructure, we explore its potential in thermoelectric and Li-ion battery applications by first-principles calculations. Due to these multi-valley degenerate valence and conduction bands, a large power factor of 0.007 W/mK2 is attained. Due to the significant interface effect, PtS2/WSe2 heterostructure can strongly adsorb Li atoms with −2.69 eV adsorption strength on the middle of PtS2/WSe2 heterostructure layers. Also, an extremely fast Li diffusion rate was found in this heterostructure (as low as 20 meV), which is close to the recently reported MXene. Large theoretical volumetric storage capacity of 1283.98 mA h/cm3, in the meanwhile a rather low average Open-circuit voltage (0.07 V) also purports that PtS2/WSe2 heterostructure is a quite promising anode material in Li-ion batteries with high power densities and fast charge/discharge rates.

First principles study of stability and thermoelectric response of GeH monolayer

Thermoelectric characteristics of two-dimensional GeH structure have been studied using first-principles density functional technique and Boltzmann transport theory. Positive phonon modes and negative formation energy both attest to the stability of structure. A high value of elastic constant suggests strong chemical bonding and hence high mechanical strength. A direct band gap semiconducting character of GeH monolayer with 1.64 eV band gap is revealed by examination of electronic band structure. Flat conduction band in one direction and degenerate valence band near Fermi level improves density of states. As a consequence, both Seebeck coefficient and electrical conductivity shows excellent response and results in high power factor. At room temperature, the ZeT value of 5.51 (5.45) is obtained for p-(n) type doping, which shows its high practical capability of thermoelectric conversion. These findings suggest that GeH monolayer is an exciting possibility for 2D thermoelectric applications.

Multi-zone resonance spectrometry of inelastic electron scattering of light and the manifestation of strong spin-orbit interaction in nanostructures with quantum dots

Highly sensitive methods of inelastic light scattering spectrometry have been developed to detect individual photons in the visible and near-infrared spectral ranges (0.4-1.35) μm. Precision optical measurements of vibrational and electronic states of nanostructures with quantum objects are performed, using the example of quantum-dimensional nanoheterostructures (311)In si-GaAs/InAs with InAs quantum dots. A distinctive attractive feature of such structures with a degenerate valence band of symmetry 8 is the presence of a strong spin-orbit interaction. In this case, the intrinsic moment of the holes can effectively interact with the electric field of the incident light wave. It was found that such interaction causes intense luminescence and inelastic scattering of the light wave in the IR region of the 0.9-1.35 μm spectrum. Effective generation of nonequilibrium electron-hole (e-h) plasma with concentrations n=p=1.0·1017 cm-3 and T_e=T_h=25 K,was revealed under resonant excitation conditions, while the lattice temperature T_L=5.1 K. New mechanisms have been discovered for the formation of an abnormally intense integration spectrum of multiplex inelastic light scattering of a rather complex shape formed by various separable resonant contributions of the processes of quasi-elastic scattering by charge carriers, light scattering by acoustic plasmons of electron-hole plasma, as well as inelastic intra- and inter-band light scattering by heavy holes. It has been established that the anomalous intensity gain of such multi-zone selective resonant light scattering is more than 105 times higher than the intensity of Thomson light scattering on individual charge carriers. There is agreement between the estimated calculated and experimental spectra, and for the most part for the difficult-to-interpret significant width of the observed light scattering line. It is shown that new mechanisms of various contributions to the formation of scattered photon radiation are important diagnostic elements that are clearly manifested in the spectra of the resulting enhanced inelastic light scattering. Keywords: nanostructures, quantum objects, resonant scattering of light by charge carriers, exchange interaction, plasma diagnostics.

Full-momentum exciton landscape and macroscopic quantum phase diagram in two-dimensional gallium oxides

High-temperature exciton macroscopic quantum phases have a wide application prospect in many fields such as ultrahigh power photonics and quantum information, and are extremely difficult to realize due to the strong thermal dissociation effect. In this study, the exciton dispersions and macroscopic quantum phase diagrams of two-dimensional (2D) ultrawide bandgap semiconductors I–Ga2O3 and II-Ga2O3 are investigated for the first time. Both I–Ga2O3 and II-Ga2O3 possess the complex excitonic composition and morphology, resulting from their highly degenerate and flat valence bands. For the exciton B1 (B1') with an ultra-large binding energy of 1.14 (1.03) eV in I–Ga2O3, the phase transition temperature from free exciton to Bose-Einstein condensation (TBEC) is as high as 356 (679) K, respectively. The critical carrier densities for the formation of electron-hole liquid at room temperature (NEHL) are 1.3 × 1013 cm−2 and 1.8 × 1013 cm−2, respectively. For the free exciton B1 with an ultra-large binding energy of 1.49 eV in II-Ga2O3, TBEC reaches 365 K and NEHL is 5.9 × 1013 cm−2. Our findings extend the material basis for the research of exciton macroscopic quantum phases and will boost their applications in novel optoelectronic devices.

Band engineering and improved thermoelectric performance in p-type SmMg2Sb2: A first-principles study

The thermoelectric properties of p-type SmMg2Sb2 are investigated by using the density functional theory and the Boltzmann transport equation method. It is found that SmMg2Sb2 shows a narrow band gap characteristic, and the crystal field splitting and spin-orbital coupling effect result in a singly degenerate valence band maximum. Meanwhile, SmMg2Sb2 exhibits an intrinsic low lattice thermal conductivity owing to the obvious phonon softening in the low-frequency acoustic branches. Based on a careful analysis of the band structure, we propose a scheme of metallic dopant at Sm site to promote the valence band convergence and improve the Seebeck coefficient. The improved thermoelectric properties are numerically confirmed in the Ba-doped p-type SmMg2Sb2, where a significantly improved ZT of 1.6 at 750 K is predicted with the synergistical effects of band engineering and enhanced phonon scattering.

Topological nodal surface semimetal states in Sr5X3 compounds (X=As,Sb,Bi)

Topological nodal surface semimetals are a class of materials characterized by degenerate conduction and valence bands on closed nodal surfaces in the Brillouin zone that are protected by crystalline symmetries. These materials attract considerable attention for exhibiting intriguing, topologically driven quantum phenomena that are fundamentally interesting and practically useful. Here we identify by systematic ab initio calculations and topological quantum chemistry analysis prominent topological nodal surface semimetal states in ${\mathrm{Sr}}_{5}{X}_{3}(X$ = As, Sb, Bi) compounds in hexagonal $P{6}_{3}/mcm$ (${D}_{6h}^{3}$) symmetry. These nodal surface states comprise degenerate the highest occupied band and the lowest unoccupied band that overlap in a single band at the ${k}_{z}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ plane and are protected by time-reversal and nonsymmorphic twofold screw rotation symmetry. Topological quantum chemistry theory analysis reveals a twofold band degeneracy without spin-orbit coupling at the high-symmetry and generic $\mathbit{k}$ points located at the ${k}_{z}=\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}$ plane, which is formed by the bands of (${\mathrm{A}}_{1}\ensuremath{\bigoplus}{\mathrm{B}}_{2})@6g$ band representations. Since the compounds studied here have been already experimentally synthesized, the present results have an immediate impact on the experimental detection and characterization of the theoretically predicted topological physics.

Solvation Effects on Quantum Tunneling Reactions.

A decisive factor for obtaining high yields and selectivities in organic synthesis is the choice of the proper solvent. Solvent selection is often guided by the intuitive understanding of transition state-solvent interactions. However, quantum-mechanical tunneling can significantly contribute to chemical reactions, circumventing the transition state and thus depriving chemists of their intuitive handle on the reaction kinetics. In this Account, we aim to provide rationales for the effects of solvation on tunneling reactions derived from experiments performed in cryogenic matrices.The tunneling reactions analyzed here cover a broad range of prototypical organic transformations that are subject to strong solvation effects. Examples are the hydrogen tunneling probability for the cis-trans isomerization of formic acid which is strongly reduced upon formation of hydrogen-bonded complexes and the [1,2]H-shift in methylhydroxycarbene where a change in product selectivity is predicted upon interaction with hydrogen bond acceptors.Not only hydrogen but also heavy atom tunneling can exhibit strong solvent effects. The direction of the nearly degenerate valence tautomerization between benzene oxide and oxepin was found to reverse upon formation of a halogen or hydrogen bond with ICF3 or H2O. But even in the absence of strong noncovalent interactions such as hydrogen or halogen bonding, solvation can have a decisive effect on tunneling as evidenced by the Cope rearrangement of semibullvalenes via heavy-atom tunneling. Can quantum tunneling be catalyzed? The acceleration of the ring expansion of 1H-bicyclo[3.1.0.]-hexa-3,5-dien-2-one by complexation with Lewis acids provides a proof-of-concept for tunneling catalysis.Two concepts are central for the explanation and prediction of solvation effects on tunneling phenomena: a simple approach expands the Born-Oppenheimer approximation by separating nuclear degrees of freedom into intra- and intermolecular degrees. Intermolecular movements represent the slowest motions within molecular aggregates, thus effectively freezing the position of the solvent in relation to the reactant during the tunneling process. Another useful approach is to treat reactants and products by separate single-well potentials, where the intersection represents the transition state. Thus, stabilization of the reactants via solvation should result in an increase in barrier heights and widths which in turn lowers tunneling probabilities. These simple models can predict trends in tunneling kinetics and provide a rational basis for controlling tunneling reactions via solvation.

Band-edge absorption characteristics of semi-insulating indium phosphide under unified Franz-Keldysh and Einstein models

The foundational Franz-Keldysh effect and Einstein model are applied in this work to characterize semiconductor band-edge absorption---and its departures from ideality. We unify the Franz-Keldysh and Einstein models to fully characterize the field-induced tunneling of photoexcited electrons from degenerate valence bands into the conduction band, with encroachment into the band gap arising as an Urbach tail. Our unified model is implemented for semi-insulating indium phosphide (SI-InP) with strong agreement seen between the theoretical and experimental results for varied photon energies and electric fields.

On the role of Zn doping on tuning the electronic and optical properties of MnCr2O4 spinel via Mn0.5Zn0.5Cr2O4 doping scheme: A first-principles quantum computational analysis

Herein, we report on the impact of Zn doping on electronic and optical properties of manganese di-chromium oxide spinel MnCr2O4 via Mn0.5Zn0.5Cr2O4 doping scheme. For the quantum computational analysis on the targeted properties, we employed first-principles density functional theory approach within full-potential linearized augmented plane wave method by Modified Becke–Johnson potential. The calculated partial and total density of states revealed a strong hybridization among electronic orbitals of the constituent species while the asymmetric spin-polarized profiles of density of states showed magnetic nature of the spinels. The major interspecies spin interaction in the degenerate valence band is via (Mn + Cr + O ) from −7 eV to Fermi level for spin up polarization. Interestingly, we predict a reduction in band gap of MnCr2O4 to order 1eV subjected to the Zn doping, which considerably modified the electronic and optical properties of the doped spinels Mn0.5Zn0.5Cr2O4 in contrast to its undoped counterpart. In particular, we predict an enhancement in absorption coefficient, energy loss function, reflectivity, dielectric constant and refractive index for Mn0.5Zn0.5Cr2O4 spinels at some regions of ultraviolet energy spectrum. The calculated static refractive index of MnCr2O4 is 1.59 and for Mn0.5Zn0.5Cr2O4 it is 1.4. The optoelectronic properties indicate the promising prospects of the Zn-doped spinels for potential application in optoelectronic devices.

Finding the order in complexity: The electronic structure of 14-1-11 zintl compounds

Yb14MnSb11 and Yb14MgSb11 have rapidly risen to prominence as high-performing p-type thermoelectric materials. However, the fairly complex crystal structure of A14MX11 Zintl compounds renders the interpretation of the electronic band structure obscure, making it difficult to chemically guide band engineering and optimization efforts. In this work, we delineate the valence-balanced Zintl chemistry of A14MX11 compounds using the molecular orbital theory. By analyzing the electronic band structures of Yb14MgSb11 and Yb14AlSb11, we show that the conduction band minimum is composed of either an antibonding molecular orbital originating from the (Sb3)7– trimer or a mix of atomic orbitals of A, M, and X. The singly degenerate valence band is comprised of non-bonding Sb pz orbitals primarily from the Sb atoms in the (MSb4)m– tetrahedra and of isolated Sb atoms distributed throughout the unit cell. Such a chemical understanding of the electronic structure enables strategies to engineer electronic properties (e.g., the bandgap) of A14MX11 compounds.

Promising Thermoelectric Performance in Two-Dimensional Semiconducting Boron Monolayer.

A heavy element is a special character for high thermoelectric performance since it generally guarantees a low lattice thermal conductivity. Here, we unexpectedly found a promising thermoelectric performance in a two-dimensional semiconducting monolayer consisting of a light boron element. Using first-principles combined with the Boltzmann transport theory, we have shown that in contrast to graphene or black phosphorus, the boron monolayer has a low lattice thermal conductivity arising from its complex crystal of hexagonal vacancies. The conduction band with an intrinsic camelback shape leads to the high DOS and a high n-type Seebeck coefficient, while the highly degenerate valence band along with the small hole effective mass contributes to the high p-type power factor. As a result, we obtained the p-type thermoelectric figure of merit up to 0.96 at 300 K, indicating that the boron monolayer is a promising p-type thermoelectric material.

Interacting spin- 32 fermions in a Luttinger semimetal: Competing phases and their selection in the global phase diagram

We compute the effects of electronic interactions on gapless spin-3/2 excitations that in a noninteracting system emerge at a bi-quadratic touching of Kramers degenerate valence and conduction bands, known as Luttinger semimetal. This model can describe the low-energy physics of HgTe, gray-Sn, 227 pyrochlore iridates and half-Heuslers. For the sake of concreteness we only consider the short-range components of the Coulomb interaction. By combining mean-field analysis with a renormalization group (RG) calculation, we construct multiple cuts of the global phase diagram of interacting spin-3/2 fermions at zero and finite temperature and chemical doping. Such phase diagrams display a rich confluence of competing orders, among which rotational symmetry breaking nematic insulators and time reversal symmetry breaking magnetic orders are the prominent excitonic phases. We also show that even repulsive interactions can be conducive for both s-wave and d-wave pairings. The reconstructed band structure inside the ordered phases allows us to organize them according to the energy (entropy) gain in the following (reverse) order: s-wave pairing, nematic phases, magnetic orders and d-wave pairings, at zero chemical doping. But, the paired states are always energetically superior over the excitonic ones for finite doping. The phase diagrams obtained from the RG analysis show that an ordered phase with higher energy (entropy) gain is realized at low (high) temperature. In addition, we establish a "selection rule" between the interaction channels and the resulting ordered phases, suggesting that repulsive interactions in the magnetic (nematic) channels are conducive for the nucleation of d-wave (s-wave) pairing. The proposed methodology can shed light on the global phase diagram of other strongly interacting multi-band systems, such as doped Dirac semimetal, topological insulators and the like.

Zinc gallate spinel dielectric function, band-to-band transitions, and Γ-point effective mass parameters

We determine the dielectric function of the emerging ultrawide bandgap semiconductor ZnGa2O4 from the near-infrared (0.75 eV) into the vacuum ultraviolet (8.5 eV) spectral regions using spectroscopic ellipsometry on high quality single crystal substrates. We perform density functional theory calculations and discuss the band structure and the Brillouin zone Γ-point band-to-band transition energies, their transition matrix elements, and effective band mass parameters. We find an isotropic effective mass parameter (0.24 me) at the bottom of the Γ-point conduction band, which equals the lowest valence band effective mass parameter at the top of the highly anisotropic and degenerate valence band (0.24 me). Our calculated band structure indicates the spinel ZnGa2O4 is indirect, with the lowest direct transition at the Γ-point. We analyze the measured dielectric function using critical-point line shape functions for a three-dimensional, M0-type van Hove singularity, and we determine the direct bandgap with an energy of 5.27(3) eV. In our model, we also consider contributions from Wannier–Mott type excitons with an effective Rydberg energy of 14.8 meV. We determine the near-infrared index of refraction from extrapolation (1.91) in very good agreement with results from recent infrared ellipsometry measurements (ε∞=1.94) [M. Stokey, Appl. Phys. Lett. 117, 052104 (2020)].

The promising thermoelectric performance of newly synthesized bulk SrCu2GeSe4 and BaCu2SnSe4 associated with superior band degeneracy

The thermoelectric transport properties with the variations of carrier concentration, temperature and strain of bulk SrCu2GeSe4 and BaCu2SnSe4 are studied by using the first-principles calculations with the Boltzmann transport equation. A multi-valley degenerate valence band is gained for BaCu2SnSe4, which is responsible for its comparatively high power factor. The power factor of p-type doped SrCu2GeSe4 is enhanced to 35 μW cm−1 K−2 when applying a 2% tensile strain at 300 K. The ZT values of 1.01 (0.94) and 1.60 (1.36) are acquired at 700 K for n- and p-type BaCu2SnSe4 (SrCu2GeSe4), respectively, predicting that both the two compounds are hopeful thermoelectric materials.

Effect of Bi-substitution on structural stability and improved thermoelectric performance of p-type half-Heusler TaSbRu: A first-principles study

Recently, Fang et al. have predicted a high thermoelectric (TE) figure of merit (ZT) of 1.54 in TaSbRu alloys at 1200 K from first-principles calculations without considering spin-orbit interaction, accurate electronic structure, details of phonon scattering, and energy-dependent holes relaxation time. Here, we report the details of structural stability and thermoelectric performance of Bi-substituted p-type TaSbRu from first-principles calculations considering these important parameters. This indirect bandgap semiconductor (Eg = 0.8 eV by Tran-Blaha modified Becke Jonshon potential (TB-mBJ) including spin-orbit coupling (SOC) effect) has highly dispersive and degenerate valence bands, which lead to a maximum power factor, 3.8 mW m−1 K−2 at 300 K. As Sb-5p has a small contribution to the bandgap formation, the substitution of Bi on the Sb site does not cause significant change to the electronic structure. Although the Seebeck coefficient increases by Bi due to slight changes in the bandgap, electrical conductivity decreases, and hence, the power factor reduces to ~3 mW m−1 K−2 at 300 K (50% Bi). On the other side, lattice thermal conductivity drops effectively to 5 from 20 W m−1 K−1 as Bi introduces a significant contribution to the acoustic phonons and intensifies phonon scattering. Thus, ZT value is improved through Bi-substitution, reaching 1.1 (50% Bi) at 1200 K from 0.45 (pure TaSbRu) only. Therefore, the present study suggests how to improve the TE performance of Sb-based half-Heusler compounds and TaSbRu (with 50% Bi) is a promising material for high-temperature TE applications.

High Thermoelectric Performance of AgSb1-xPbxSe2 Prepared by Fast Nonequilibrium Synthesis.

AgSbSe2 is a typical member of cubic I-V-VI2 semiconductors, which are known for their extremely low lattice thermal conductivity (κl). However, the low electrical conductivity of AgSbSe2, below ∼10 S cm-1 at room temperature, has hindered its thermoelectric performance. In this work, single-phase AgSbSe2 bulk samples with much higher electrical conductivity were synthesized via self-propagating high-temperature synthesis (SHS) combined with spark plasma sintering (SPS) for the first time. Pb doping through the nonequilibrium process further increases the electrical conductivity to >100 S cm-1. Furthermore, continuously increased effective mass md* can be achieved upon Pb doping because of the multiple degenerate valence bands of AgSbSe2 and the energy-filtering effect induced by in situ-formed nanodots. The simultaneous enhancement of both the electrical conductivity and Seebeck coefficient contributes to an unprecedentedly high average power factor of 6.75 μW cm-1 K-2. Meanwhile, the introduced dense grain boundaries and point defects enhance the phonon scattering and consequently suppress κl, yielding a high ZT value of 1.2 at 723 K in AgSb0.94Pb0.06Se2. This study opens a new avenue for rapid, low-cost, large-scale production of AgSbSe2-based materials and demonstrates that Pb-doped AgSbSe2 prepared via the SHS-SPS method is a promising candidate for thermoelectric applications.

Ultrafast photoinduced band splitting and carrier dynamics in chiral tellurium nanosheets

Trigonal tellurium (Te) is a chiral semiconductor that lacks both mirror and inversion symmetries, resulting in complex band structures with Weyl crossings and unique spin textures. Detailed time-resolved polarized reflectance spectroscopy is used to investigate its band structure and carrier dynamics. The polarized transient spectra reveal optical transitions between the uppermost spin-split H4 and H5 and the degenerate H6 valence bands (VB) and the lowest degenerate H6 conduction band (CB) as well as a higher energy transition at the L-point. Surprisingly, the degeneracy of the H6 CB (a proposed Weyl node) is lifted and the spin-split VB gap is reduced upon photoexcitation before relaxing to equilibrium as the carriers decay. Using ab initio density functional theory (DFT) calculations, we conclude that the dynamic band structure is caused by a photoinduced shear strain in the Te film that breaks the screw symmetry of the crystal. The band-edge anisotropy is also reflected in the hot carrier decay rate, which is a factor of two slower along the c-axis than perpendicular to it. The majority of photoexcited carriers near the band-edge are seen to recombine within 30 ps while higher lying transitions observed near 1.2 eV appear to have substantially longer lifetimes, potentially due to contributions of intervalley processes in the recombination rate. These new findings shed light on the strong correlation between photoinduced carriers and electronic structure in anisotropic crystals, which opens a potential pathway for designing novel Te-based devices that take advantage of the topological structures as well as strong spin-related properties.

Strain-engineered higher-order topological phases for spin- 32 Luttinger fermions

Recently, the notion of topological phases of matter has been extended to higher-order incarnations, supporting gapless modes on even lower dimensional boundaries, such as corners and hinges. We here identify a collection of cubic spin-3/2 fermions with biquadratic touching of Kramers degenerate valence and conduction bands as a platform to strain-engineer higher-order topological (HOT) phases: external uniaxial strain gives birth to a HOT Dirac semimetal or an insulator, depending on its sign, featuring topological \emph{hinge} modes in the strain direction. The insulator in fact exhibits \emph{mixed} topology, and in addition supports edge modes on orthogonal planes. These outcomes are germane when the external strain is applied along one of the $C_{4v}$ or coordinate axes, as well as $C_{3v}$ or body-diagonal, directions. Our findings place HgTe, gray-Sn, 227 pyrochlore iridates and half-Heusler compounds at the frontier of strain-engineered electronic HOT phases.

Superconductors, orbital magnets and correlated states in magic-angle bilayer graphene.

Superconductivity can occur under conditions approaching broken-symmetry parent states1. In bilayer graphene, the twisting of one layer with respect to the other at 'magic' twist angles of around 1 degree leads to the emergence of ultra-flatmoiré superlattice minibands. Such bands are a rich and highly tunable source of strong-correlation physics2-5, notably superconductivity, which emerges close to interaction-induced insulating states6,7. Here we report the fabrication of magic-angle twisted bilayer graphene devices with highly uniform twist angles. The reduction in twist-angle disorder reveals the presence of insulating states at all integer occupancies of the fourfold spin-valley degenerate flat conduction and valence bands-that is, at moiré band filling factors ν=0,±1,±2,±3. At ν≈-2, superconductivity is observed below critical temperatures of up to 3 kelvin. We also observe three new superconducting domes at much lower temperatures, close to the ν=0 and ν=±1 insulating states. Notably, at ν=±1 we find states with non-zero Chern numbers. For ν=-1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field greater than 3.6 tesla is applied, which is consistent with a field-driven phase transition. Our study shows that broken-symmetry states, interaction-driven insulators, orbital magnets, states with non-zero Chern numbers and superconducting domes occur frequently across a wide range of moiré flat band fillings, including close to charge neutrality. This study provides a more detailed view of the phenomenology of magic-angle twisted bilayer graphene, adding to our evolving understanding of its emergent properties.

Emergence and spontaneous breaking of approximate O(4) symmetry at a weakly first-order deconfined phase transition

We investigate approximate emergent nonabelian symmetry in a class of weakly first order `deconfined' phase transitions using Monte Carlo simulations and a renormalization group analysis. We study a transition in a 3D classical loop model that is analogous to a deconfined 2+1D quantum phase transition in a magnet with reduced lattice symmetry. The transition is between the N\'eel phase and a twofold degenerate valence bond solid (lattice-symmetry-breaking) phase. The combined order parameter at the transition is effectively a four-component superspin. It has been argued that in some weakly first order `pseudocritical' deconfined phase transitions, the renormalization group flow can take the system very close to the ordered fixed point of the symmetric $O(N)$ sigma model, where $N$ is the total number of `soft' order parameter components, despite the fact that $O(N)$ is not a microscopic symmetry. This yields a first order transition with unconventional phenomenology. We argue that this occurs in the present model, with $N=4$. This means that there is a regime of lengthscales in which the transition resembles a `spin-flop' transition in the ordered $O(4)$ sigma model. We give numerical evidence for (i) the first order nature of the transition, (ii) the emergence of $O(4)$ symmetry to an accurate approximation, and (iii) the existence of a regime in which the emergent $O(4)$ is `spontaneously broken', with distinctive features in the order parameter probability distribution. These results may be relevant for other models studied in the literature, including 2+1D QED with two flavours, the `easy-plane' deconfined critical point, and the N\'eel--VBS transition on the rectangular lattice.