We show that zero-dimensional (0D) systems can host nontrivial topology analogous to macroscopic topological materials in higher dimensions. Unlike macroscopic periodic systems with translational symmetry, zero-dimensional materials such as molecules, clusters, and quantum dots can exhibit discrete rotation symmetry. The eigenstates can thus be grouped into discrete bands and Bloch-like wave functions. Since the symmetry is discrete, the Berry phase and the topological indices must be defined by discrete Wilson polygons. Here, we demonstrate nontrivial ${\mathcal{Z}}_2$ orders in two representative 0D molecules, [m]-Cycloparaphenylene and [m]-iso-thianaphthene, where topological transitions occur when modifying the coupling between the repeating units. Similar to macroscopic topological systems in higher dimensions, localized boundary states emerge in composite nanohoops formed by segments that are topologically distinct. This opens up the possibility of nontrivial topological phases in 0D systems.

We introduce a quantum information method for measuring fractional charges in ballistic quantum wires generalizing bipartite fluctuations to the chiral quasiparticles in Luttinger liquids, i.e., analyzing and summing charge and current fluctuations in a region of the wire. Bipartite fluctuations at equilibrium are characterized through a logarithmic scaling with distance encoding the entangled nature of these fractional charges in one-dimensional (1D) fluids. This approach clarifies the physical meaning of the dephasing factor of electronic interferences in a ballistic ring geometry at zero temperature, as a result of charge fractionalization. We formulate an analogy towards ground-state energetics. We show how bipartite current fluctuations represent a useful tool to locate quantum phase transitions associated to Mott physics. We address a spin chain equivalence and verify the fractional charges through an algorithm such as density matrix renormalization group (DMRG). Adding a potential difference between the two sides (parties) of the wire, bipartite fluctuations can detect a bound state localized at the interface through the Jackiw-Rebbi model coexisting with fractional charges.

Understanding and controlling magnon relaxation in two-dimensional (2D) ferromagnets is crucial for applications in spintronics, as it governs magnon-based spin transport and device performance. However, existing studies have been largely confined to systems with low Curie temperatures, leaving near-room-temperature 2D ferromagnets rarely explored. Here, we combine spin-wave theory with first-principles calculations to investigate magnon relaxation in the room-temperature 2D ferromagnet ${\mathrm{MnSe}}_2$, considering both magnon-magnon interactions (MMIs) and magnon-phonon interactions (MPIs) and analyzing their effects under external strain and magnetic fields. We find that a MPI exhibits a strong, nonmonotonic strain dependence due to its coupling to lattice distortions, whereas a MMI is primarily controlled by magnetic fields and is insensitive to strain. At room temperature, a MMI dominates the relaxation process, while the dominant mechanism can be largely reversed by strain and magnetic fields, especially at low temperatures and large wave vectors. These findings highlight the complementary roles of MPIs and MMIs in ${\mathrm{MnSe}}_2$, offering potential strategies for tailoring magnon relaxation in magnon-based spintronic devices.

The generation of pure spin current, that is, spin angular momentum transport without charge flow, is crucial for developing energy-efficient spintronic devices with minimal Joule heating. Here, we introduce the intrinsic nonlinear pure spin Hall effect (NPSHE), where both linear and second-order charge Hall currents vanish. We show intrinsic second-order spin angular momentum transport in metals and insulators through a detailed analysis of the quantum geometric origin of different spin current contributions. Our comprehensive symmetry analysis identifies 39 magnetic point groups that support NPSHE, providing a foundation for material design and experimental realization. Our first principle's calculations demonstrate a significant NPSHE in a prototypical Kramers-Weyl metal RhGe, even at room temperature. This positions Kramers-Weyl metals as potential candidates for NPSHE-based spin-torque devices. Our work lays a practical pathway for realizing charge-free angular momentum transport for the development of next-generation, energy-efficient spintronic devices.

Nonlinear transport in narrow band-gap materials has recently attracted significant attention, being commonly recognized as a useful tool to study a variety of complex physical effects related to Berry phase. Here we demonstrate that a mundane, commonly overlooked mechanism stemming from tunneling between charge puddles, can drive substantial nonlinearities in these narrow band gap materials. As a test bed, we use an epitaxial topological HgTe layer, underscoring that even good crystalline quality materials suffer from this issue. Indeed, we show that signatures of electrical transport through a network of charge puddles are not limited to the linear planar magnetoresistance, but also dominate nonlinear transport. Our findings highlight the need for utmost caution when interpreting nonlinear electrical transport phenomena, as harmonic distortion of the electrical signal, caused by charge puddles, is unavoidable in any realistic narrow-gap system, complicating the identification of other proposed effects related to the Berry phase.

The magnetization field of a quantum Hall ferromagnet (QHFM) can host a variety of spin textures, including skyrmions and magnons. When projected into the lowest Landau level with $\nu =1$ filling, the topological (Pontryagin) charge density of the magnetization field is proportional to the electric charge density, allowing for long-range spin-spin interactions. Inspired by recent experimental developments that enable all-electrical generation and detection of magnons, in this work we theoretically demonstrate two phenomena that can occur due to Coulomb interactions that are unique to QHFMs: Magnons can scatter off of point charges at a distance, and skyrmions can act as transmitters and receivers for magnons to be transduced between separate layers of a bilayer QHFM. The latter Coulomb-mediated spin drag effect occurs at arbitrary distance and could facilitate long-range magnonics, such as detection of spin waves for future experiments in two-dimensional materials.

The two-dimensional (2D) magnetic semiconductor CrSBr shows great potential for spintronic and optoelectronic device applications due to its intrinsic ferromagnetism and a large magnetic anisotropy. In this study, the phononassociated excitonic dynamics in CrSBr is studied using ultrafast reflectance spectroscopy. The signatures of coherent optical phonons (COPs) and coherent acoustic phonons (CAPs) relaxation dynamics were observed in various spectral ranges. The COPs are mainly generated by a spontaneous multi-body excitation, which corresponds to three Raman peaks and exhibits pronounced phononexciton couplings at two resonances corresponding to the exciton magnetic polariton. Remarkably, CAPs predominantly generated by an electronic stress mechanism exhibit wavelength-dependent frequency dispersion ranging from $\sim 24.57\mathrm{GHz}$ at 660 nm to $\sim 9.28\mathrm{GHz}$ at 1400 nm within the same sample, which can be visualized as concentric ripple patterns through spatially resolved femtosecond transient absorption (TA) imaging of surface acoustic waves (SAWs). These observations provide fundamental insights into the exciton magnetic polaron formation in the 2D CrSBr multilayer structure with its anisotropic magnetic profiles and establish a framework for magnonphononmediated quantum information processing devices.

Motivated by recent experiments on rhombohedral stacked multilayer graphene and the observation of the anomalous Hall effect in a spontaneous spin-valley-polarized quarter-metal state, we calculate the anomalous Hall conductivity for this system in the presence of two types of impurities: weak and dense as well as sparse and strong. Our calculation of ${\sigma }_{xy}$ is based on the Kubo-Streda diagrammatic approach. In a model with Gaussian disorder applicable to weak dense impurities, this involves all noncrossing diagrams (intrinsic, side-jump and Gaussian skew-scattering contributions) and additionally, diagrams with two intersecting impurities, X and $\mathrm{\Psi }$, representing diffractive skew-scattering processes. A “Mercedes star” diagram (non-Gaussian skew scattering) is furthermore included to treat in the case of strong, sparse impurities. We supplement our asymptotically exact analytical solutions for an isotropic model without warping effects by seminumerical calculations accounting perturbatively for warping, which plays a crucial role in the low-energy band structure.

Janus MXenes have exhibited many novel properties because of broken inversion symmetry. We systematically investigate the mechanical, electronic, and optical properties and the quantum capacitance ($C_{\mathrm{Q}}$) of Janus $\mathrm{Zr}M\mathrm{C}{T}_2$ ($M=\mathrm{Ti}$, Cr, Mn, Hf; $T=\mathrm{O}$, F, OH) by first-principles calculations. All systems except $\mathrm{ZrCrC}{\left(\mathrm{OH}\right)}_2$ are mechanically stable; ${\mathrm{ZrHfCO}}_2$ is the most resistant to deformation in all directions. ${\mathrm{ZrTiCO}}_2$ and ${\mathrm{ZrHfCO}}_2$ are nonmagnetic semiconductors; the substitution of O with the F group makes ${\mathrm{ZrHfCF}}_2$ and ${\mathrm{ZrTiCF}}_2$ become nonmagnetic conductors, and the substitution of O with the OH group makes $\mathrm{ZrHfC}{\left(\mathrm{OH}\right)}_2$ and $\mathrm{ZrTiC}{\left(\mathrm{OH}\right)}_2$ become magnetic metals. Obvious anisotropy of static dielectric constants $\left[{\varepsilon }_1\left(0\right)\right]$ along the $x$ and $y$ axes is observed for ${\mathrm{ZrTiCF}}_2, {\mathrm{ZrMnCF}}_2, \mathrm{ZrMnC}{\left(\mathrm{OH}\right)}_2$, and ${\mathrm{ZrHfCF}}_2$. $\mathrm{ZrTiC}{\left(\mathrm{OH}\right)}_2$ has the strong absorption of 24.93% along the $x$ axis. For ${\mathrm{ZrTiCT}}_2$, compared with O termination, F and OH terminations greatly improve the maximum $C_{\mathrm{Q}}$ of the system under positive bias in narrow voltage. Compared with O termination, F and OH terminations improve the top $C_{\mathrm{Q}}$ of ${\mathrm{ZrHfCT}}_2$ and ${\mathrm{ZrCrCT}}_2$ under negative bias. The $C_{\mathrm{Q}}$ and surface storage charge ($Q$) of $\mathrm{Zr}M\mathrm{C}{T}_2$ ($M=\mathrm{Ti}$, Mn, Hf) with mixed termination are also explored.

The appearance of angular momentum in the nuclear motion of molecular systems lacking inversion symmetry under imposed thermal gradients presents a novel mechanism with potential implications for spintronics, magnetic response, and energy transport in such systems. Here, we explore this phenomenon, using theoretical analysis and numerical simulations to study angular momentum generation in several driven chiral molecular models. We demonstrate that significant vibrational angular momentum can be induced under both mechanical and thermal driving, with magnitude comparable to those values calculated for optically driven chiral phonons. We find that generation of angular momentum is a common and general phenomenon in driven chiral structures, highlighting the role of symmetry breaking in the (nonequilibrium) internal atomic motion of such systems.

We report on the exchange coupling phenomena emerging at low temperatures in amorphous TbFe-based bilayers with perpendicular magnetic anisotropy. The ferrimagnetic layers have different compositions: one layer is dominated by the magnetic moment of Fe, the other by the magnetic moment of Tb. The antiferromagnetic exchange coupling between the two layers, along with the formation of an intermixing layer at the interface, leads to the emergence of (double) exchange bias [(D)EB] at low temperatures after field cooling. The composition of the Fe-dominated layer and the stacking order of layers are shown to have a significant impact on this behavior, while changes in the Tb-dominated layer and tailoring of the interlayer region influences it to a lesser degree. The most stable DEB was obtained in a ${\mathrm{Tb}}_{36}{\mathrm{Fe}}_{64}/{\mathrm{Tb}}_{19}{\mathrm{Fe}}_{81}$ bilayer structure, with clearly separated, oppositely biased subloops at temperatures up to 120 K. Further, a substantial overcrossing of hysteresis branches between 80 and 140 K was observed for some samples. This is shown to be connected to ultraslow magnetization dynamics similar to magnetic viscosity, which are greatly influenced by the applied magnetic field and temperature. Relaxation times ranging from tens of seconds to several hours are observed. By comparing minor loops with and without relaxation, a simple qualitative model is derived to explain the observed behavior of magnetization relaxation.

Using pump-probe Faraday and Kerr rotation measurements, we investigate the ultrafast magnetization dynamics induced by femtosecond laser pulses in a BiYIG/Co-based heterostructure epitaxially grown on GGG(001) substrate. The Kerr measurements reveal the excitation of high-frequency exchange standing spin-wave modes, whereas the Faraday measurements detect only the ferromagnetic resonance mode. Moreover, the Kerr response exhibits faster remagnetization following the demagnetization dynamics. These differences are explained by considering the average magnetization profile of the magneto-optical (MO) Faraday effect and the unequal depth sensitivity of the MO Kerr effect. They also allow deeper insight into the mechanisms driving the magnetization dynamics. Furthermore, we identify in the Kerr signal a contribution induced by the strain pulse in the BiYIG layer and gadolinium gallium garnet (GGG) substrate, which is absent in the Faraday response and should not be misinterpreted as a magnetic effect. These findings establish important connections with existing literature on laser-induced ultrafast magnetization dynamics in magnetic garnets and advance the fundamental understanding of these dynamics.

Recently, proposals for realizing a nonreciprocal superradiant quantum phase transition (SQPT) have been put forward, based on either nonreciprocal interactions between two spin ensembles or the Sagnac-Fizeau shift in a spinning cavity. However, experimental implementation of such a nonreciprocal SQPT remains challenging. This motivates the search for new mechanisms capable of producing a nonreciprocal SQPT. Here, we propose an alternative approach to realize a nonreciprocal SQPT, induced by the magnon Kerr effect (MKE), in a cavity magnonic system, where magnons in a yttrium iron garnet (YIG) sphere are coupled to cavity photons. The MKE coefficient is positive ($K>0$) when the bias magnetic field is aligned along the crystallographic axis [100], but negative ($K<0$) when aligned along the axis [110]. We show that the steady-state phase diagram for $K>0$ differs markedly from that for $K<0$. This contrast is the origin of the nonreciprocal SQPT. By further studying the steady-state magnon occupation and its fluctuations versus the parametric drive strength, we demonstrate that the SQPT becomes nonreciprocal, characterized by distinct critical thresholds for $K>0$ and $K<0$. Moreover, we introduce a bidirectional contrast ratio to quantify this nonreciprocal behavior. Our work provides a mechanism for realizing the nonreciprocal SQPT, with potential applications in designing nonreciprocal quantum devices.

Spin precession experiments in lateral spin devices are a powerful tool for probing the spin transport properties of materials. These experiments can be quantitatively described using the Bloch diffusion equation, which offers a practical framework for modeling spin-related phenomena. In this work, we present calculations of the spin density across heterogeneous, diffusive spintronic devices. The modeled devices feature spin transport channels that include both isotropic and in-plane anisotropic spin relaxation regions. We analyze how different geometric configurations and spin transport parameters influence the lineshape of spin precession signals under magnetic fields applied in different orientations and compare the results with experimental observations. Our results introduce a theoretical framework for interpreting spin transport measurements in lateral graphene spin devices. The framework is especially relevant when the graphene is partially proximitized by other two-dimensional materials, where proximity-induced spin-orbit coupling leads to anisotropic spin relaxation.