Found 34 papers in cond-mat


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A first-principles study and mesoscopic modeling of two-dimensional spin and orbital fluctuations in FeSe
Abyay Ghosh, Piotr Chudzinski, Myrta Gr\"uning
arXiv:2403.04802v1 Announce Type: new Abstract: We calculated the structural, electronic and magnetic properties of FeSe within density-functional theory at the generalized gradient approximation level. First, we studied how the bandwidth of the d-bands at the Fermi energy are renormalized by adding simple corrections: Hubbard U, Hunds J and by introducing long-range magnetic orders. We found that introducing either a striped or a staggered dimer antiferromagnetic order brings the bandwidths -- which are starkly overestimated at the generalized gradient approximation level -- closer to those experimentally observed. Second, for the ferromagnetic, the striped, checkerboard and the staggered dimer antiferromagnetic order, we investigate the change in magnetic formation energy with local magnetic moment of Fe at a pressure up to 6 GPa. The bilinear and biquadratic exchange energies are derived from the Heisenberg model and noncollinear first-principles calculations, respectively. We found a non-trivial behavior of the spin-exchange parameters on the magnetization, and we put forward a field-theory model that rationalizes these results in terms of two-dimensional spin and orbital fluctuations. The character of these fluctuations can be either that of a standard density wave or a topological vortex. Topological vortexes can result in mesoscopic magnetization structures.

Extreme anti-ohmic conductance enhancement in neutral diradical acene-like molecular junctions
Brent Lawson, Efrain Vidal, Michael M. Haley, Maria Kamenetska
arXiv:2403.04906v1 Announce Type: new Abstract: We achieve, at room temperature, conductance enhancements over two orders of magnitude in single molecule circuits formed with polycyclic benzoquinoidal (BQn) diradicals upon increasing molecular length by ~0.5 nm. We find that this extreme and atypical anti-ohmic conductance enhancement at longer molecular lengths is due to the diradical character of the molecules, which can be described as a topologically non-trivial electronic state. We adapt the 1D-SSH model originally developed to examine electronic topological order in linear carbon chains to the polycyclic systems studied here and find that it captures the anti-ohmic trends in this molecular series. The mechanism of conductance enhancement with length is revealed to be constructive quantum interference (CQI) between the frontier orbitals with non-trivial topology, which is present in acene-like, but not in linear, molecular systems. Importantly, we predict computationally and measure experimentally that anti-ohmic trends can be engineered through synthetic adjustments of the diradical character of the acene-like molecules. Overall, we achieve an experimentally unprecedented anti-ohmic enhancement and mechanistic insight into electronic transport in a class of materials that we identify here as promising candidates for creating highly conductive and tunable nanoscale wires.

Enhanced polarization switching characteristics of HfO2 ultrathin films via acceptor-donor co-doping
Chao Zhou, Liyang Ma, Yanpeng Feng, Chang-Yang Kuo, Yu-Chieh Ku, Cheng-En Liu, Xianlong Cheng, Jingxuan Li, Yangyang Si, Haoliang Huang, Yan Huang, Hongjian Zhao, Chun-Fu Chang, Sujit Das, Shi Liu, Zuhuang Chen
arXiv:2403.04994v1 Announce Type: new Abstract: In the realm of ferroelectric memories, HfO2-based ferroelectrics stand out because of their exceptional CMOS compatibility and scalability. Nevertheless, their switchable polarization and switching speed are not on par with those of perovskite ferroelectrics. It is widely acknowledged that defects play a crucial role in stabilizing the metastable polar phase of HfO2. Simultaneously, defects also pin the domain walls and impede the switching process, ultimately rendering the sluggish switching of HfO2. Herein, we present an effective strategy involving acceptor-donor co-doping to effectively tackle this dilemma. Remarkably enhanced ferroelectricity and the fastest switching process ever reported among HfO2 polar devices are observed in La3+-Ta5+ co-doped HfO2 ultrathin films. Moreover, robust macro-electrical characteristics of co-doped films persist even at a thickness as low as 3 nm, expanding potential applications of HfO2 in ultrathin devices. Our systematic investigations further demonstrate that synergistic effects of uniform microstructure and smaller switching barrier introduced by co-doping ensure the enhanced ferroelectricity and shortened switching time. The co-doping strategy offers an effective avenue to control the defect state and improve the ferroelectric properties of HfO2 films.

Crystal structure, properties and pressure-induced insulator-metal transition in layered kagome chalcogenides
Hong Du, Yu Zheng, Cuiying Pei, Chi-Ming Yim, Yanpeng Qi, Ruidan Zhong
arXiv:2403.05001v1 Announce Type: new Abstract: Layered materials with kagome lattice have attracted a lot of attention due to the presence of nontrivial topological bands and correlated electronic states with tunability. In this work, we investigate a unique van der Waals (vdW) material system, $A_{2}M_{3}X_{4}$ ($A$ = K, Rb, Cs; $M$ = Ni, Pd; $X$ = S, Se), where transition metal kagome lattices, chalcogen honeycomb lattices and alkali metal triangular lattices coexist simultaneously. A notable feature of this material is that each Ni/Pd atom is positioned in the center of four chalcogen atoms, forming a local square-planar environment. This crystal field environment results in a low spin state $S$ = 0 of Ni$^{2+}$/Pd$^{2+}$. A systematic study of the crystal growth, crystal structure, magnetic and transport properties of two representative compounds, Rb$_{2}$Ni$_{3}$S$_{4}$ and Cs$_{2}$Ni$_{3}$Se$_{4}$, has been carried out on powder and single crystal samples. Both compounds exhibit nonmagnetic $p$-type semiconducting behavior, closely related to the particular chemical environment of Ni$^{2+}$ ions and the alkali metal intercalated vdW structure. Additionally, Cs$_{2}$Ni$_{3}$Se$_{4}$ undergoes an insulator-metal transition (IMT) in transport measurements under pressure up to 87.10 GPa without any structural phase transition, while Rb$_{2}$Ni$_{3}$S$_{4}$ persists in its semiconducting behavior.

Superconductivity in Two-Dimentional Systems with Unconventional Rashba Bands
Ran Wang, Jiayang Li, Xinliang Huang, Rui Song, Ning Hao
arXiv:2403.05051v1 Announce Type: new Abstract: In two-dimensional system with Rashba spin-orbit coupling, it is well-known that superconductivity can have mixed spin-singlet and -triplet parity, and the $\boldsymbol{d}$-vector of spin-tripet pairing is parallel to $\boldsymbol{g}$-vector of Rashba spin-orbit coupling. Here, we propose a model to describe a two-dimensional system with unconventional Rashba bands and study its superconductivity. We show that the $\boldsymbol{d}$-vector of spin-tripet pairing can be either parallel or perpendicular to $\boldsymbol{g}$-vector of Rashba spin-orbit coupling depending on the different pairing interaction. We also propose a junction to generate tunneling spin current depending on the direction of $\boldsymbol{d}$-vector. It provides a detecable evidence to distinguish these two different but very similar pairing channels. Furthermore, we find this model can give arise to a subleading spin-singlet chiral $p$-wave topological superconducting state. More significantly, we find that such unconventional Rashba bands and unconventional superconudcting pairings can be realized on surface of some superconducting topological materials, such as trigonal layered PtBi$_{2}$.

Trapping Hard-Core Bosons in Flatband Lattices
Sanghoon Lee, Alexei Andreanov, Tigran Sedrakyan, Sergej Flach
arXiv:2403.05084v1 Announce Type: new Abstract: We investigate 1D and 2D cross-stitch lattices with hard-core bosons and analytically construct exact groundstates that feature macroscopic degeneracy. The construction relies on the presence of a flatband in the single particle spectrum and the orthogonality of the associated compact localized states (CLS). Up to filling fraction $\nu=1/2$, the groundstate is constructed by occupying the CLS. Exactly at $\nu=1/2$, the groundstate becomes a Wigner crystal. For higher filling fractions, the groundstate is constructed by filling the CLS sites completely one by one. Macroscopic degeneracy arises from the multiple choices available when occupying or filling the CLS sites. An occupied CLS acts as an impenetrable barrier for bosons both in 1D and 2D, leading to Hilbert space fragmentation. A similar phenomenology also holds for hard-core bosons on the diamond chain and its higher dimensional generalizations. We also discuss the mapping of these hard-core models onto spin models with quantum many-body scars.

The Uhlmann Phase Winding in Bose-Einstein Condensates
Chang-Yan Wang, Yan He
arXiv:2403.05127v1 Announce Type: new Abstract: The Uhlmann phase is a generalization of the celebrated Berry phase, and can characterize topological properties at finite temperature and for mixed states. In this paper, we investigate the Uhlmann phase of Bose-Einstein Condensates at finite temperature, which are quantum systems with rich and diverse phenomena. Using the $SU(1,1)$ symmetry of Bogoliubov Hamiltonian, we derive a general formula of Uhlmann phase for BEC and numerically show that it can differ from the Berry phase in zero-temperature limit, in contrast to previous studies on Uhlmann phase. We also find that the Uhlmann phase exhibits a winding behavior as the temperature increases, and relate the total winding degree to the Berry phase. This winding behavior indicates that the Uhlmann phase takes values in a Riemann surface. We further propose a scheme to measure the Uhlmann phase of BEC experimentally based on purification of the density matrix using an atomic interferometer.

Imprinting of Antiferromagnetic Vortex States in NiO-Fe Nanostructures
M. \'Sl\k{e}zak, T. Wagner, V. K. Bharadwaj, O. Gomonay, A. Kozio{\l}-Rachwa{\l}, T. O. Mente\c{s}, A. Locatelli, M. Zaj\k{a}c, D. Wilgocka-\'Sl\k{e}zak, P. Dr\'o\.zd\.z, T. \'Sl\k{e}zak
arXiv:2403.05151v1 Announce Type: new Abstract: Magnetic vortices are topological spin structures frequently found in ferromagnets, yet novel to antiferromagnets. By combining experiment and theory, we demonstrate that in a nanostructured antiferromagnetic-ferromagnetic NiO(111)-Fe(110) bilayer, a magnetic vortex is naturally stabilized by magnetostatic interactions in the ferromagnet and is imprinted onto the adjacent antiferromagnet via interface exchange coupling. We use micromagnetic simulations to construct a corresponding phase diagram of the stability of the imprinted antiferromagnetic vortex state. Our in depth analysis reveals that the interplay between interface exchange coupling and the antiferromagnet magnetic anisotropy plays a crucial role in locally reorienting the N\'eel vector out-of-plane in the prototypical in-plane antiferromagnet NiO and thereby stabilizing the vortices in the antiferromagnet.

Bending-Rotation coupling in the viscoelasticity of semi-flexible polymers -- Rigorous perturbation analysis from the rod limit
Zhongqiang Xiong, Ryohei Seto, Masao Doi
arXiv:2403.05173v1 Announce Type: new Abstract: Brownian motion and viscoelasticity of semi-flexible polymers is a subject that has been studied for many years. Still, rigorous analysis has been hindered due to the difficulty in handling the constraint that polymer chains cannot be stretched along the contour. Here, we show a straightforward method to solve the problem. We consider a stiff polymer that has a persistent length $L_p$ much larger than the contour length $L$. We express the polymer configuration using three types of variables: the position vector of the center of mass $R_c$, the unit vector $n$ along the main axis, and the normal coordinates $u_p$ for bending. Solving the Smoluchowski equation for the distribution function of these variables, we calculate the equilibrium time correlation function $ \langle P(t)\cdot P(0) \rangle$ of the end-to-end vector $P$ and the complex modulus $G^*(\omega)$ of dilute solution. They include the bending effect to the first order in $\theta \equiv L/L_p$ and reduce to the exact results for the rigid rod in the limit of $\theta \to 0$. The rotational diffusion coefficient increases slightly by the semi-flexibility because the equilibrium length of the semi-flexible polymer is smaller than that of the rigid rod with the same contour length. The storage modulus shows the same asymptotic dependence $G'(\omega) \sim \omega^{3/4}$ predicted by Shankar, Pasquali, and Morse [J. Rheol. 2002, 46, 1111--1154]. The high-frequency viscosity is predicted to be dependent on the thickness of the semi-flexible polymers.

Superconductivity in kagome metal ThRu3Si2
Yi Liu, Jing Li, Wu-Zhang Yang, Jia-Yi Lu, Bo-Ya Cao, Hua-Xun Li, Wan-Li Chai, Si-Qi Wu, Bai-Zhuo Li, Yun-Lei Sun, Wen-He Jiao, Wang Cao, Xiao-Feng Xu, Ren Zhi, Guang-Han Cao
arXiv:2403.05227v1 Announce Type: new Abstract: We report the physical properties of ThRu$_3$Si$_2$ featured with distorted Ru kagome lattice. The combined experiments of resistivity, magnetization and specific heat reveal bulk superconductivity with $T_{\rm{c}}$ = 3.8 K. The specific heat jump and calculated electron-phonon coupling indicate a moderate coupled BCS superconductor. In comparison with LaRu$_3$Si$_2$, the calculated electronic structure in ThRu$_3$Si$_2$ shows an electron-doping effect with electron filling lifted from 100 meV below flat bands to 300 meV above it. This explains the lower superconducting transition temperature and weaker electron correlations observed in ThRu$_3$Si$_2$. Our work suggests the $T_{\rm{c}}$ and electronic correlations in kagome superconductor could have intimate connection with the flat bands.

Disorder-induced instability of a Weyl nodal loop semimetal towards a diffusive topological metal with protected multifractal surface states
Jo\~ao S. Silva, Miguel Gon\c{c}alves, Eduardo V. Castro, Pedro Ribeiro, Miguel A. N. Ara\'ujo
arXiv:2403.05298v1 Announce Type: new Abstract: Weyl nodal loop semimetals are gapless topological phases that, unlike their insulator counterparts, may be unstable to small perturbations that respect their topology-protecting symmetries. Here, we analyze a clean system perturbed by chiral off-diagonal disorder using numerically exact methods. We establish that the ballistic semimetallic phase is unstable towards the formation of an unconventional topological diffusive metal hosting topological multifractal surface states. Although, as in the clean case, surface states are exponentially localized along the direction perpendicular to the nodal loop, disorder induces a multifractal structure in the remaining directions. Surprisingly, the number of these states also increases with a small amount of disorder. Eventually, as disorder is further increased, the number of surface states starts decreasing. In the strong disordered regime we predict that some types of disorder induce an Anderson transition into an electrically-polarized insulator whose signature may be detected experimentally.

Onset of Spin Entanglement in Doped Carbon Nanotubes Studied by EPR
Andreas Sperlich, Klaus H. Eckstein, Florian Oberndorfer, Bernd K. Sturzda, Michael Auth, Vladimir Dyakonov, Roland Mitric, Tobias Hertel
arXiv:2403.05361v1 Announce Type: new Abstract: Nanoscale semiconductors with isolated spin impurities have been touted as promising materials for their potential use at the intersection of quantum, spin, and information technologies. Electron paramagnetic resonance (EPR) studies of spins in semiconducting carbon nanotubes have overwhelmingly focused on spins more strongly localized by $\rm sp^3$-type lattice defects. However, the creation of such impurities is irreversible and requires specific reactions to generate them. Shallow charge impurities, on the other hand, are more readily and widely produced by simple redox chemistry, but have not yet been investigated for their spin properties. Here we use EPR to study p-doped (6,5) semiconducting single-wall carbon nanotubes (s-SWNTs) and elucidate the role of impurity-impurity interactions in conjunction with exchange and correlation effects for the spin behavior of this material. A quantitative comparison of the EPR signals with phenomenological modeling combined with configuration interaction electronic structure calculations of impurity pairs shows that orbital overlap, combined with exchange and correlation effects, causes the EPR signal to disappear due to spin entanglement for doping levels corresponding to impurity spacings of $14\,\rm nm$ (at 30 K). This transition is predicted to shift to higher doping levels with increasing temperature and to lower levels with increasing screening, providing an opportunity for improved spin control in doped s-SWNTs.

Anisotropic effects in two-dimensional materials
Alexander N. Rudenko, Mikhail I. Katsnelson
arXiv:2403.05374v1 Announce Type: new Abstract: Among a huge variety of known two-dimensional materials, some of them have anisotropic crystal structures; examples include so different systems as a few-layer black phoshphorus (phosphorene), beryllium nitride BeN$_4$, van der Waals magnet CrSBr, rhenium dichalgogenides ReX$_2$. As a consequence, their optical and electronic properties turn out to be highly anisotropic as well. In some cases, the anisotropy results not just in a smooth renormalization of observable properties in comparison with the isotropic case but in the appearance of dramatically new physics. The examples are hyperbolic plasmons and excitons, strongly anisotropic ordering of adatoms at the surface of two-dimensional or van der Waals materials, essential change of transport and superconducting properties. Here, we present a systematic review of electronic structure, transport and optical properties of several representative groups of anisotropic two-dimensional materials including semiconductors, anisotropic Dirac and semi-Dirac materials, as well as superconductors.

Experimental set-up for thermal measurements at the nanoscale using an SThM probe with niobium nitride thermometer
R. Swami, G. Julie, S. Le-Denmat, G. Pernot, D. Singhal, J. Paterson, J. Maire, J. F. Motte, N. Paillet, H. Guillou, S. Gomes, O. Bourgeois
arXiv:2403.05405v1 Announce Type: new Abstract: Scanning Thermal Microscopy (SThM) has become an important measurement tool for characterizing the thermal properties of materials at the nanometer scale. This technique requires a SThM probe that combines an Atomic Force Microscopy (AFM) probe and a very sensitive resistive thermometry; the thermometer being located at the apex of the probe tip allows the mapping of temperature or thermal properties of nanostructured materials with very high spatial resolution. The high interest of the SThM technique in the field of thermal nanoscience currently suffers from a low temperature sensitivity despite its high spatial resolution. To address this challenge, we developed a high vacuum-based AFM system hosting a highly sensitive niobium nitride (NbN) SThM probe to demonstrate its unique performance. As a proof of concept, we utilized this custom-built system to carry out thermal measurements using the 3$\omega$ method. By measuring the $V_{3\omega}$ voltage on the NbN resistive thermometer in vacuum conditions we were able to determine the SThM probe's thermal conductance and thermal time constant. The performance of the probe is demonstrated by doing thermal measurements in-contact with a sapphire sample.

Observation of Topological Hall Effect and Skyrmions in Pt/Co/Ir/Co/Pt System
Shaktiranjan Mohanty, Brindaban Ojha, Minaxi Sharma, Subhankar Bedanta
arXiv:2403.05469v1 Announce Type: new Abstract: The interlayer exchange coupling (IEC) between two ferromagnetic (FM) layers separated by a non-magnetic (NM) spacer layer gives rise to different types of coupling with the variation of spacer layer thickness. When the NM is metallic, the IEC is attributed to the well known Ruderman Kittel Kasuya Yosida (RKKY) interaction which shows an oscillatory decaying nature with increasing thickness. Due to this, it is possible to tune the coupling between the two FM to be either ferromagnetic or antiferromagnetic. In this work we have studied a Pt/Co/Ir/Co/Pt system where the Co thickness has been taken in the strong perpendicular magnetic anisotropy regime which is much less than the spin reorientation transition thickness. By tuning the Ir thickness to 2.0 nm, a canted state of magnetization reversal in the system is observed which gives rise to a possibility of nucleating topologically non trivial spin textures like skyrmions. Further, with the combination of transport and magnetic force microscopy (MFM) measurements, we have confirmed the presence of skyrmions in our system. These findings may be useful for potential applications in emerging spintronic and data storage technologies using skyrmions.

Orthorhombic metal carbide-borides MeC$_2$B$_{12}$ (Me=Mg, Ca, Sr) from first principles: structure, stability and mechanical properties
Oleksiy Bystrenko, Jingxian Zhang, Tianxing Sun, Hu Ruan, Yusen Duan, Kaiqing Zhang, Xiaoguang Li
arXiv:2403.05517v1 Announce Type: new Abstract: First principle DFT simulations are employed to study structural and mechanical properties of orthorhombic B12-based metal carbide-borides. The simulations predict the existence of Ca- and Sr- based phases with the structure similar to that of experimentally observed earlier compound MeC$_2$B$_{12}$. Dynamical stability of both phases is demonstrated, and the phase MeC$_2$B$_{12}$ is found to be thermodynamically stable. According to simulations, Ca- and Sr- based phases have significantly enhanced mechanical characteristics, which suggest their potential application as superhard materials. Calculated shear and Young moduli of these phases are nearly 250 and 540 GPa, respectively, and estimated Vickers hardness is 45-55 GPa.

Anomalous Hall Crystals in Rhombohedral Multilayer Graphene II: General Mechanism and a Minimal Model
Tomohiro Soejima, Junkai Dong, Taige Wang, Tianle Wang, Michael P. Zaletel, Ashvin Vishwanath, Daniel E. Parker
arXiv:2403.05522v1 Announce Type: new Abstract: We propose a minimal "three-patch model" for the anomalous Hall crystal (AHC), a topological electronic state that spontaneously breaks both time-reversal symmetry and continuous translation symmetry. The proposal for this state is inspired by the recently observed integer and fractional quantum Hall states in rhombohedral multilayer graphene at zero magnetic field. There, interaction effects appear to amplify the effects of a weak moir\'e potential, leading to the formation of stable, isolated Chern bands. It has been further shown that Chern bands are stabilized in mean field calculations even without a moir\'e potential, enabling a realization of the AHC state. Our model is built upon the dissection of the Brillouin zone into patches centered around high symmetry points. Within this model, the wavefunctions at high symmetry points fully determine the topology and energetics of the state. We extract two quantum geometrical phases of the non-interacting wavefunctions that control the stability of the topologically nontrivial AHC state. The model predicts that the AHC state wins over the topological trivial Wigner crystal in a wide range of parameters, and agrees very well with the results of full self-consistent Hartree-Fock calculations of the rhombohedral multilayer graphene Hamiltonian.

Photon mediated energy, linear and angular momentum transport in fullerene and graphene systems beyond local equilibrium
Jian-Sheng Wang, Mauro Antezza
arXiv:2307.11361v2 Announce Type: cross Abstract: Based on a tight-binding model for the electron system, we investigate the transfer of energy, momentum, and angular momentum mediated by electromagnetic fields among buckminsterfullerene (C$_{60}$) and graphene nano-strips. Our nonequilibrium Green's function approach enables calculations away from local thermal equilibrium where the fluctuation-dissipation theorem breaks down. For example, the forces between C$_{60}$ and current-carrying nano-strips are predicted. It is found that the presence of current usually enhances the van der Waals attractive forces. For two current-carrying graphene strips rotated at some angle, the fluctuational force and torque are much stronger at the nanoscale compared to that of the static Biot-Savart law.

Playing nonlocal games across a topological phase transition on a quantum computer
Oliver Hart, David T. Stephen, Dominic J. Williamson, Michael Foss-Feig, Rahul Nandkishore
arXiv:2403.04829v1 Announce Type: cross Abstract: Many-body quantum games provide a natural perspective on phases of matter in quantum hardware, crisply relating the quantum correlations inherent in phases of matter to the securing of quantum advantage at a device-oriented task. In this paper we introduce a family of multiplayer quantum games for which topologically ordered phases of matter are a resource yielding quantum advantage. Unlike previous examples, quantum advantage persists away from the exactly solvable point and is robust to arbitrary local perturbations, irrespective of system size. We demonstrate this robustness experimentally on Quantinuum's H1-1 quantum computer by playing the game with a continuous family of randomly deformed toric code states that can be created with constant-depth circuits leveraging mid-circuit measurements and unitary feedback. We are thus able to tune through a topological phase transition - witnessed by the loss of robust quantum advantage - on currently available quantum hardware. This behavior is contrasted with an analogous family of deformed GHZ states, for which arbitrarily weak local perturbations destroy quantum advantage in the thermodynamic limit. Finally, we discuss a topological interpretation of the game, which leads to a natural generalization involving an arbitrary number of players.

Phase Transitions in Ising models: the Semi-infinite with decaying field and the Random Field Long-range
Jo\~ao Maia
arXiv:2403.04921v1 Announce Type: cross Abstract: In this thesis, we present results on phase transition for two models: the semi-infinite Ising model with a decaying field, and the long-range Ising model with a random field. We study the semi-infinite Ising model with an external field $h_i = \lambda |i_d|^{-\delta}$, $\lambda$ is the wall influence, and $\delta>0$. This external field decays as it gets further away from the wall. We are able to show that when $\delta>1$ and $\beta > \beta_c(d)$, there exists a critical value $0< \lambda_c:=\lambda_c(\delta,\beta)$ such that, for $\lambda<\lambda_c$ there is phase transition and for $\lambda>\lambda_c$ we have uniqueness of the Gibbs state. In addition, when $\delta<1$ we have only one Gibbs state for any positive $\beta$ and $\lambda$. For the model with a random field, we extend the recent argument by Ding and Zhuang from nearest-neighbor to long-range interactions and prove the phase transition in the class of ferromagnetic random field Ising models. Our proof combines a generalization of Fr\"ohlich-Spencer contours to the multidimensional setting proposed by Affonso, Bissacot, Endo and Handa, with the coarse-graining procedure introduced by Fisher, Fr\"ohlich, and Spencer. Our result shows that the Ding-Zhuang strategy is also useful for interactions $J_{xy}=|x-y|^{- \alpha}$ when $\alpha > d$ in dimension $d\geq 3$ if we have a suitable system of contours, yielding an alternative proof that does not use the Renormalization Group Method (RGM), since Bricmont and Kupiainen claimed that the RGM should also work on this generality. We can consider i.i.d. random fields with Gaussian or Bernoulli distributions.

Quantum Many-body Scar Models in One Dimensional Spin Chains
Jia-Wei Wang, Xiang-Fa Zhou, Guang-Can Guo, Zheng-Wei Zhou
arXiv:2403.05015v1 Announce Type: cross Abstract: The phenomenon of quantum many-body scars has received widespread attention both in theoretical and experimental physics in recent years due to its unique physical properties. In this paper, based on the $su(2)$ algebraic relations, we propose a general method for constructing scar models by combining simple modules.This allows us to investigate many-body scar phenomena in high-spin systems. We numerically verify the thermalization and non-integrability of this model and demonstrate the dynamical properties of the scar states. We also provide a theoretical analysis of the properties of these scar states. For spin-$1$ case, we find that our 1D chain model reduces to the famous PXP model[C. J. Turner et al. Phys. Rev. B 98, 155134(2018)] under special parameter condition. In addition, due to the continuous tunability of the parameters, our model also enables us to investigate the transitions of QMBS from non-integrable to integrable system.

Experimental Evidence of Direct Exchange Interaction Mediating Intramolecular Singlet Fission in Weakly-Coupled Dimers
Oskar Kefer, Pavel V. Kolesnichenko, Lukas Ahrens, Jan Freudenberg, Uwe H. F. Bunz, Tiago Buckup
arXiv:2403.05163v1 Announce Type: cross Abstract: The electronic interaction between an optically active singlet state ($S_1S_0$) and a dark state of singlet multiplicity, known as correlated triplet pair ($^1[TT]$), plays a crucial role in the effective transformation from $S_1S_0$ to $^1[TT]$ during intramolecular singlet fission (iSF). This process is understood through mechanisms such as direct exchange coupling and incoherent processes that involve super-exchange coupling through charge-transfer states. However, most insights into these mechanisms are derived from theoretical studies due to the difficulties in obtaining experimental evidence. In this study, we investigate the excited-state interactions between $S_1S_0$ and $^1[TT]$ in spiro-conjugated iSF sensitizers by employing transient two-dimensional electronic spectroscopy. This approach allows us to focus on the early stages of the conversion from $S_1S_0$ to $^1[TT]$. Upon optical excitation, a superposition of $S_1S_0$ and $^1[TT]$ is created, which gradually transitions to favor $^1[TT]$ within the characteristic time frames of iSF. The observed high-order signals indicate circular repopulation dynamic that effectively reinitiates the iSF process from higher energy electronic states. Our findings, supported by semi-quantum-mechanical simulations of the experimental data, suggest the presence of a direct iSF mechanism in the dimers, facilitated by weak non-adiabatic coupling between $S_1S_0$ and $^1[TT]$. This experiment provides new insights into the equilibrium between the two electronic states, a phenomenon previously understood primarily through theoretical models.

Cornertronics in Two-Dimensional Second-Order Topological Insulators
Yilin Han, Chaoxi Cui, Xiao-Ping Li, Ting-Ting Zhang, Zeying Zhang, Zhi-Ming Yu, Yugui Yao
arXiv:2306.08384v2 Announce Type: replace Abstract: Traditional electronic devices rely on electron's intrinsic degrees of freedom (d.o.f.) to process information. However, additional d.o.f. like the valley, can emerge in the low-energy states of certain systems. Here, we show that the quantum dots (QDs) constructed from two-dimensional (2D) second-order topological insulator (SOTI) posses a new kind of d.o.f., namely corner freedom, related to the topological corner states that reside at different corners of the systems. Since the corner states are well separated in real space, they can be intuitively addressed and manipulated individually, giving rise to the concept of cornertronics. Via symmetry analysis and material search, we identify the TiSiCO-family monolayers as the first prototype of cornertronics materials, where the corner states can be controlled by both electric and optical fields, due to the novel corner-layer coupling (CLC) effect and the corner-contrasted optical selection rules. Furthermore, we find that the band gap of the TiSiCO nanodisk lies in the terahertz region and is robust to size reduction. These results indicate that the TiSiCO nanodisks can be used to design terahertz devices with ultrasmall size, electric-field tunable band gap, and the ability to simultaneously detect the strength and polarization of terahertz waves, which is not possible for previously reported QDs. Our findings not only pave the way for the cornertronics, but also open a new direction for the research in 2D SOTI, QD and terahertz electronics.

Light-induced switching between singlet and triplet superconducting states
Steven Gassner, Clara S. Weber, Martin Claassen
arXiv:2306.13632v2 Announce Type: replace Abstract: While the search for topological triplet-pairing superconductivity has remained a challenge, recent developments in optically stabilizing metastable superconducting states suggest a new route to realizing this elusive phase. Here, we devise a testable theory of competing superconducting orders that permits ultrafast switching to an opposite-parity superconducting phase in centrosymmetric crystals with strong spin-orbit coupling. Using both microscopic and phenomenological models, we show that dynamical inversion symmetry breaking with a tailored light pulse can induce odd-parity (spin triplet) order parameter oscillations in a conventional even-parity (spin singlet) superconductor, which when driven strongly can send the system to a competing minimum in its free energy landscape. Our results provide new guiding principles for engineering unconventional electronic phases using light, suggesting a fundamentally non-equilibrium route toward realizing topological superconductivity.

Symmetry breaking and ascending in the magnetic kagome metal FeGe
Shangfei Wu, Mason Klemm, Jay Shah, Ethan T. Ritz, Chunruo Duan, Xiaokun Teng, Bin Gao, Feng Ye, Masaaki Matsuda, Fankang Li, Xianghan Xu, Ming Yi, Turan Birol, Pengcheng Dai, Girsh Blumberg
arXiv:2309.14314v2 Announce Type: replace Abstract: Spontaneous symmetry breaking-the phenomenon where an infinitesimal perturbation can cause the system to break the underlying symmetry-is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. In the paramagnetic state at 460K, we confirm that the crystal structure is indeed hexagonal kagome lattice. On cooling to TN, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10^(-4) and the associated splitting of the double degenerate phonon mode of the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice shows a small negative thermal expansion, and the crystal structure becomes more symmetric gradually upon further cooling. Increasing the crystalline symmetry upon cooling is unusual, it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter, hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system and sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.

Engineering the in-plane anomalous Hall effect in Cd$_3$As$_2$ thin films
Wangqian Miao, Binghao Guo, Susanne Stemmer, Xi Dai
arXiv:2309.15457v2 Announce Type: replace Abstract: We predict two topological phase transitions for cadmium arsenide (\ce{Cd3As2}) thin films under in-plane magnetic field, taking advantage of a four-band $k\cdot p$ model and effective $g$ factors calculated from first principles. Film thickness, growth direction and in-plane Zeeman coupling strength can all serve as control parameters to drive these phase transitions. For (001) oriented \ce{Cd3As2} thin films, a two dimensional Weyl semimetal phase protected by $C_{2z}\mathcal{T}$ symmetry can be realized using an in-plane magnetic field, which has recently been reported in our companion paper. We then put forth two pathways to achieve in-plane anomalous Hall effects (IPAHE). By either introducing a trigonal warping term or altering the growth orientation, the emergent $C_{2z} \mathcal{T}$ symmetry can be broken. Consequently, in the clean limit and at low temperatures, quantized Hall plateaus induced by in-plane Zeeman fields become observable.

Inheritance of the exciton geometric structure from Bloch electrons in two-dimensional layered semiconductors
Jianju Tang, Songlei Wang, Hongyi Yu
arXiv:2310.14856v2 Announce Type: replace Abstract: We theoretically studied the exciton geometric structure in layered semiconducting transition metal dichalcogenides. Based on a three-orbital tight-binding model for Bloch electrons which incorporates their geometric structures, an effective exciton Hamiltonian is constructed and solved perturbatively to reveal the relation between the exciton and its electron/hole constituent. We show that the electron-hole Coulomb interaction gives rise to a non-trivial inheritance of the exciton geometric structure from Bloch electrons, which manifests as a valley-dependent center-of-mass anomalous Hall velocity of the exciton when two external fields are applied on the electron and hole constituents, respectively. The obtained center-of-mass anomalous velocity is found to exhibit a non-trivial dependence on the fields, as well as the wave function and valley index of the exciton. These findings can serve as a general guide for the field-control of the valley-dependent exciton transport, enabling the design of novel quantum optoelectronic and valleytronic devices.

Circuit realisation of a two-orbital non-Hermitian tight-binding chain
Dipendu Halder, Ronny Thomale, Saurabh Basu
arXiv:2311.15014v2 Announce Type: replace Abstract: We examine a non-Hermitian (NH) tight-binding system comprising of two orbitals per unit cell and their electrical circuit analogues. We distinguish the PT-symmetric and non-PT symmetric cases characterised by non-reciprocal nearest neighbour couplings and onsite gain/loss terms, respectively. The localisation of the edge modes or the emergence of the topological properties are determined via the maximum inverse participation ratio, which has distinct dependencies on the parameters that define the Hamiltonian. None of the above scenarios exhibits the non-Hermitian skin effect. We investigate the boundary modes corresponding to the topological phases in a suitably designed electrical circuit by analyzing the two-port impedance and retrieve the admittance band structure of the circuit via imposing periodic boundary conditions. The obtained results are benchmarked against the Hermitian version of the two-orbital model to compare and discriminate against those obtained for the NH variants.

Four-dimensional Floquet topological insulator with an emergent second Chern number
Zheng-Rong Liu, Rui Chen, Bin Zhou
arXiv:2312.16013v2 Announce Type: replace Abstract: Floquet topological insulators have been widely investigated in lower-dimensional systems. However, Floquet topological insulators induced by time-periodic driving in higher-dimensional systems remain unexplored. In this work, we study the effects of time-periodic driving in a four-dimensional (4D) normal insulator, focusing on topological phase transitions at the resonant quasienergy gap. We consider two types of time-periodic driving, including a time-periodic onsite potential and a time-periodic vector potential. We reveal that both types of time-periodic driving can transform the 4D normal insulator into a 4D Floquet topological insulator characterized by an emergent second Chern number. Moreover, it is found that the topological phase of the 4D system can be modulated by tuning the strength and frequency of the time-periodic driving. Our work will be helpful for the future investigation of Floquet topological insulators in higher dimensions.

3D ferroelectric phase field simulations of polycrystalline multi-phase hafnia and zirconia based ultra-thin films
Prabhat Kumar, Michael Hoffmann, Andrew Nonaka, Sayeef Salahuddin, Zhi Yao
arXiv:2402.05331v2 Announce Type: replace Abstract: HfO$_2$- and ZrO$_2$-based ferroelectric thin films have emerged as promising candidates for the gate oxides of next generation electronic devices. Recent work has experimentally demonstrated that a tetragonal/orthorhombic (t/o-) phase mixture with partially in-plane polarization can lead to negative capacitance (NC) stabilization. However, there is a discrepancy between experiments and the theoretical understanding of domain formation and domain wall motion in these multi-phase, polycrystalline materials. Furthermore, the effect of anisotropic domain wall coupling on NC has not been studied so far. Here we apply 3D phase field simulations of HfO$_2$- and ZrO$_2$-based mixed-phase ultra-thin films on silicon to understand the necessary and beneficial conditions for NC stabilization. We find that smaller ferroelectric grains and a larger angle of the polar axis with respect to the out-of-plane direction enhances the NC effect. Furthermore, we show that theoretically predicted negative domain wall coupling even along only one axis prevents NC stabilization. Therefore, we conclude that topological domain walls play a critical role in experimentally observed NC phenomena in HfO$_2$- and ZrO$_2$-based ferroelectrics.

Triplet fermions in MXenes: The Applications for spintronic-based devices
Phusit Nualpijit, Bumned Soodchomshom
arXiv:2402.19039v2 Announce Type: replace Abstract: We investigate the electronic properties of MXenes by three bands tight-binding model of \d_{z^2} , \d_{xy} , and \d_{x^2-y^2} orbitals. The three corresponding bands touch each other at high symmetry K point in the case of absence of spin-orbit interaction. The proper parameters can be obtained by Slater-Koster parameters related to chemical bonding, \pi, \sigma, and \delta bonds. The model calculated for these band structures make an agreement with the same trend as discussed in DFT calculation which the hopping parameters may be identified roughly by fermi velocity. Furthermore, the triplet fermion occurs around K point hosting by flat band, leading to super-Klein tunnelling and anti-super-Klein tunnelling for gapped and gapless pseudospin-1 fermion, respectively. These may apply for nanodevices operated by spin polarization which is more stable than that of the conventional two-dimensional materials.

Time reversal invariant topological 1D and 2D superconductors: doubling the Sau-Luchtin-Tewari-Sarma and Oreg-Refael-von Oppen proposals
Garry Goldstein
arXiv:2403.01696v2 Announce Type: replace Abstract: In this work we present a doubled version of the Sau-Luchtin-Tewari-Sarma and Oreg-Refael-von Open proposals thereby obtaining time reversal invariant p-wave superconductivity in both 1D and 2D. This construction is much like the Kane-Mele spin Hall model which is a time reversal invariant doubling of the Haldane model. We show that the low energy effective action for these doubled versions of the Sau-Luchtin-Tewari-Sarma and Oreg-Refael-von Open models corresponds to a single band p-wave time reversal invariant superconductors with pseudo spin degree of freedom instead of spin degree of freedom. There are Majorana fermions at the ends of wires or in vortex cores of these superconductors. Furthermore these are shown to be stable to small perturbations. In the supplement we present physical realization of the system with cold atoms and show a related "no-go" theorem given in Haim et. al. (2019) has too restrictive assumptions to apply to this proposal.

Features of a spin glass in the random field Ising model
Sourav Chatterjee
arXiv:2307.07634v3 Announce Type: replace-cross Abstract: A longstanding open question in the theory of disordered systems is whether short-range models, such as the random field Ising model or the Edwards-Anderson model, can indeed have the famous properties that characterize mean-field spin glasses at nonzero temperature. This article shows that this is at least partially possible in the case of the random field Ising model. Consider the Ising model on a discrete $d$-dimensional cube under free boundary condition, subjected to a very weak i.i.d. random external field, where the field strength is inversely proportional to the square-root of the number of sites. It turns out that in $d\ge 2$ and at subcritical temperatures, this model has some of the key features of a mean-field spin glass. Namely, (a) the site overlap exhibits one step of replica symmetry breaking, (b) the quenched distribution of the overlap is non-self-averaging, and (c) the overlap has the Parisi ultrametric property. Furthermore, it is shown that for Gaussian disorder, replica symmetry does not break if the field strength is taken to be stronger than the one prescribed above, and non-self-averaging fails if it is weaker, showing that the above order of field strength is the only one that allows all three properties to hold. However, the model does not have two other features of mean-field models. Namely, (a) it does not satisfy the Ghirlanda-Guerra identities, and (b) it has only two pure states instead of many.

Non-equilibrium Green's function approach to low-energy fission dynamics
K. Uzawa, K. Hagino, G. F. Bertsch
arXiv:2403.04255v2 Announce Type: replace-cross Abstract: The concept of a compound nucleus was proposed by Bohr in 1936 to explain narrow resonances in neutron scattering off a nucleus. While a compound nucleus has been understood in terms of statistical mechanics, its description based on a many-body Hamiltonian has yet to be developed. Here we present a microscopic modeling of a compound nucleus starting from a nucleonic degree of freedom. We focus in particular on a decay of a heavy compound nucleus, that is, fission and radiative capture. To this end, we develop an approach based on a non-equilibrium Green's function, which is combined with a configuration interaction (CI) approach based on a constrained density-functional theory (DFT). We apply this approach to a barrier-top fission of $^{236}$U, restricting the model space to seniority zero configurations of neutrons and protons. Our calculation with a Skyrme energy functional yields the fission-to-capture branching ratio of around 0.07. While this value is still reasonable, the calculation underestimates the branching ratio by about a factor of 40 as compared to the empirical value, indicating a necessity of seniority non-zero configurations in the model space. We also find that the distribution of the fission probability approximately follows the chi-squared distribution with the number of degrees of freedom of the order of 1, which is consistent with the experimental finding.