Max Planck Institute for Chemical Physics of Solids

Bistable switching typically arises from ferroic orders, such as ferroelectricity and ferromagnetism, in which the bistable states are encoded in charge or spin degrees offreedom1,2. Here we report the observation of bistable superlattice switching in monolayer TaIrTe4, a dual quantum spin Hall insulator3-5. Switching occurs between two lattice configurations with sharply contrasting periodicities. In particular, in a pristine monolayer, we observe the spontaneous emergence of a long-period superlattice that can be programmed on and off in a non-volatile manner by electrostatic tuning of low-energy electronic states. This switching toggles the system between two structural configurations with unit cell areas differing by two orders of magnitude. Mechanistically, our results reveal two independent and distinct instabilities, one in the lattice and the other in the quantum spin Hall electrons. These instabilities are coupled, leading to electrostatic control of lattice configurations with non-volatile memory. This finding is enabled by combining linear and nonlinear transport measurements6-13, Raman spectroscopy and scanning tunnelling microscopy, which probe complementary aspects of the underlying orders. Notably, this non-volatile memory stabilizes a spontaneous superlattice with a periodicity on the few-nanometre scale that remains robust across a wide doping range, persists over days and survives above 70 K. Our preliminary data also show the emergence of new insulating states at fractional superlattice fillings, which can be switched on and off together with the superlattice.

Intermetallic compounds play a key role as catalysts in numerous important chemical reactions. Iron‐ and platinum‐based intermetallic compounds have gained a lot of attention because of their advantageous properties in industrial processes and their widespread application. To understand the atomic and electronic structure as well as the catalytic properties of the compounds, it is crucial to analyze the phases and determine the parameter range (i.e., composition and temperature) over which these compounds remain stable as even minor deviations in the composition can substantially alter a compound's chemical and physical properties. This makes the exact determination of homogeneity ranges particularly critical. The range of existence of FeSi and PtSi is revisited, and for FeSi, no measurable homogeneity range is observed at 800°C, while PtSi demonstrates a narrow phase width at 825°C. Phase boundaries of the neighboring phases (Fe 3 Si, FeSi 2 , Pt 6 Si 5 , and (Si, Pt)) and their corresponding lattice parameters are reported in addition.

ABSTRACTTwisted bilayer MoTe2 (tMoTe2) has recently emerged as an exceptional platform for realizing strongly correlated and topological quantum phases. Yet, its microscopic electronic structure remains largely unexplored. Here, we use scanning tunneling microscopy/spectroscopy (STM/STS) to directly image the moiré flat bands in dual-gated tMoTe2 devices with twist angles of 2.3°–3.8°. A dual-gate design allows independent tuning of band filling and displacement field, enabling detailed spectroscopic mapping. We find that the low-energy flat bands are localized at MX and XM sites and form a topological honeycomb lattice at zero electric field. An applied electric field lifts the degeneracy of the layer, driving a transition to two decoupled triangular lattices with trivial topology. Our results match first-principles calculations, revealing K-valley hybridization as the microscopic origin. At large moiré potential, we observe Wigner molecular crystals forming a Kagome lattice at filling νMX = 3, demonstrating electric-field control of topology and correlation in tMoTe2.

Medium-entropy semiconductors represent a unique category of entropy-engineered materials. They possess a considerable level of randomness in atomic mixing, although this is not sufficient to conclusively achieve single-phase structure stabilization, in contrast to high-entropy materials. This introduces strong competition between the formation of different phases, which can potentially lead to structural heterogeneity. In this work, we uncover endotaxial nanoprecipitates in the microscopically identified homogeneous medium-entropy semiconductor AgMnSbPbTe4. These nanoprecipitates initially crystallize in a cubic phase (Fm3̅m) within kinetically stabilized AgMnSbPbTe4, subsequently evolving into a thermodynamically stable monoclinic phase (P21/c) during thermal annealing while maintaining an endotaxial relationship with the matrix lattice. This nanophase segregation and the resultant lattice mismatch at interfaces introduce strain fluctuations up to 5% at intervals of 20 nm across the entire microstructure. Within the matrix phase, atomic displacement of up to 23 pm was observed. This structural heterogeneity results in glass-like thermal transport behavior, achieving an ultralow lattice thermal conductivity κL = 0.312 Wm-1 K-1 at 800 K, which is in accordance with the amorphous limit predicted by the Cahill model. The synergy of band convergence effect and well-maintained carrier mobility leads to a maximum ZT of 1.72 at 800 K and an average ZTavg of 1.02 over the temperature range of 300-825 K. This study highlights that the underexplored structural heterogeneity in medium-entropy semiconductors can potentially yield beneficial phenomena, such as the phonon-glass electron-crystal transport behavior in this case, which holds promise for advancing thermoelectric applications.

The interplay between magnetism and charge transport is central to understanding colossal magnetoresistance (CMR), a phenomenon well studied in ferromagnets. Recently, antiferromagnetic (AFM) EuCd2P2 has attracted considerable interest due to its remarkable CMR, for which magnetic fluctuations and the formation of ferromagnetic clusters have been proposed as key mechanisms. Here we provide direct evidence that these effects originate from the formation and percolation of magnetic polarons. We employ a complementary set of sensitive probes that allows for a direct comparison of electronic and magnetic properties on multiple time scales revealing pronounced electronic and magnetic phase separation below T* ≈ 2TN. These measurements indicate an inhomogeneous, percolating electronic system below T* and well above the magnetic ordering temperature TN = 11 K. In applied magnetic fields, the onset of the pronounced negative MR in the paramagnetic regime emerges at a universal critical magnetization. The characteristic size of the magnetic polarons near the percolation threshold is estimated to be ~6−10 nm. Our results establish dynamic polaron percolation within an AFM matrix as the microscopic origin of CMR in EuCd2P2, providing a unified framework for magnetotransport in Eu-based correlated semiconductors.

The recent discovery of altermagnetism was in part motivated by the research of compensated magnets towards highly scalable spintronic technologies. Simultaneously, altermagnetism shares the anisotropic higher-partial-wave nature of ordering with unconventional superfluid phases, which have been at the forefront of research for the past several decades. These examples illustrate the interest in altermagnetism from a broad range of science and technology perspectives. Here we review the symmetry, microscopy and spectroscopy signatures of altermagnetism. We describe the spontaneously broken and retained symmetries that delineate altermagnetism as a distinct phase of matter with d-, g- or i-wave compensated collinear spin ordering. In materials ranging from weakly interacting metals to strongly correlated insulators, the microscopic crystal-structure realizations of the altermagnetic symmetries feature a characteristic ferroic order of anisotropic higher-partial-wave components of atomic-scale spin densities. These symmetry and microscopy signatures of altermagnetism are directly reflected in spin-dependent electronic spectra and responses. We review salient band-structure features originating from the altermagnetic ordering, and from its interplay with spin-orbit coupling and topological phenomena. Throughout, we compare altermagnetism with traditional ferromagnetism and Néel antiferromagnetism, and with magnetic phases with symmetry-protected compensated non-collinear spin orders. We accompany the theoretical discussions with references to relevant experiments.

Disordered two-dimensional (2D) lattices, including hexatic and various glassy states, are observed in a wide range of 2D systems including colloidal nanoparticle assemblies and fluxon lattices. Their disordered nature determines the stability and mobility of these systems, as well as their response to the external stimuli. Here we report on the controlled creation and characterization of a disordered 2D lattice of nontopological magnetic bubbles in the noncentrosymmetric ferrimagnetic alloy <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Mn</a:mi> <a:mrow> <a:mn>1.4</a:mn> </a:mrow> </a:msub> <a:mi>PtSn</a:mi> </a:mrow> </a:math> . By analyzing the type and frequency of fundamental lattice defects, such as dislocations, the orientational correlation, as well as the induced motion of the lattice in an external field, a nonergodic glassy state, stabilized by directional application of an external field, is revealed.

Recent experiments have revealed that many kagome metals exhibit sublinear temperature dependence in resistivity up to room temperature. Here, we develop a minimal semiclassical two-pocket model—comprising a Dirac cone and a Van Hove singularity—and show that, within an extended Fermi liquid scattering framework, internode electron-electron interactions naturally lead to sublinear scaling in both electrical and thermal transport at low temperatures. At higher temperatures, distinct scattering channels for charge and heat currents lead to a violation of the Wiedemann-Franz law. Our work provides a simple and broadly applicable framework for understanding anomalous transport in kagome metals, capturing non-Fermi-liquid behavior without requiring fine-tuning or exotic interactions.