Nematicity, where rotational symmetry of the crystal lattice is spontaneously broken, is a ubiquitous phenomenon in correlated quantum matter, often intertwining with other orders to produce a richer spectrum of phases. Here we report a new phase transition in high-quality ScV6Sn6 bilayer kagome metal at a temperature T^*, occurring seven Kelvins below the charge density wave (CDW) transition at T_CDW, as indicated by thermodynamic, transport, and optical measurements. This emerging intermediate phase does not exhibit spontaneous time-reversal-symmetry breaking, as evidenced by zero-field Sagnac interferometer experiments. However, it displays a strong, spontaneous (strain- and field-free) anisotropy in the kagome plane between T^* and T_CDW, as revealed by transport and optical polarization rotation measurements. Additionally, a pronounced depolarization effect detected by the Sagnac interferometer further confirms its nematic nature. This intermediate nematic phase, alongside the recently discovered intra-unit cell nematic order at much lower temperatures, presents a diverse landscape of nematicities at multiple length and temperature scales, distinguishing it from those observed in kagome metals AV3Sb5. Our findings highlight ScV6Sn6 and the broader RM6X6 intermetallic family as fertile platforms for realizing symmetry-breaking phases driven by a unique interplay of competing CDW instabilities, kagome physics, and Van Hove singularities.

The kagome lattice ferromagnet, Co 3 Sn 2 S 2 , serves as template for a host of materials that exhibit exotic topological states, as well as a giant anomalous Hall state—a momentous realization in condensed matter physics. The anisotropic exchange interactions are probed by applying external magnetic fields in various directions along major crystal axes and the magneto‐optical response is recorded. When magnetic fields are applied along the a–b plane, long‐range magnetism forms and grows with increasing temperature. This counterintuitive behavior does not take place when the magnetic field is applied at a slight angle off the a–b plane. In fact, the opposite effect is observed for this exact configuration, as well as for other fields arrangements. Ab initio theoretical calculations reveal that anisotropic exchange interactions are the underlying mechanism leading to this peculiar behavior. Furthermore, the long‐range ferromagnetic order along the c ‐axis is thought to coexist with an antiferromagnetic, or spin glass, state in the a–b plane, before becoming paramagnetic above the Curie temperature ( T c ). These two coexisting magnetic states are thought to compete as the temperature approaches T c , with the antiferromagnetic state gaining strength compared to the ferromagnetic order.

The kagome motif is a versatile platform for condensed matter physics, hosting rich interactions between magnetic, electronic, and structural degrees of freedom. In recent years, the discovery of a charge density wave (CDW) in the AV(3)Sb(5) superconductors and structurally-derived bond density waves (BDW) in FeGe and ScV6Sn6 have stoked the search for new kagome platforms broadly exhibiting density wave (DW) transitions. In this work, we evaluate the known AM(6)X(6) chemistries and construct a stability diagram that summarizes the structural relationships among the >125 member family. Subsequently, we introduce our discovery of the broader LnNb(6)Sn(6) (Ln:Ce-Nd,Sm,Gd-Tm,Lu,Y) family of kagome metals and an analogous DW transition in LuNb6Sn6. Our X-ray scattering measurements clearly indicate a (1/3, 1/3, 1/3) ordering wave vector (3x3x3 superlattice) and diffuse scattering on half-integer L-planes. Our analysis of the structural data supports the "rattling mode" DW model proposed for ScV6Sn6 and paints a detailed picture of the steric interactions between the rare-earth filler element and the host Nb-Sn kagome scaffolding. We also provide a broad survey of the magnetic properties within the HfFe6Ge6-type LnNb(6)Sn(6) members, revealing a number of complex antiferromagnetic and metamagnetic transitions throughout the family. This work integrates our new LnNb(6)Sn(6) series of compounds into the broader AM(6)X(6) family, providing new material platforms and forging a new route forward at the frontier of kagome metal research.

The study of magnetoresistance (MR) phenomena has been pivotal in advancing magnetic sensors and spintronic devices. Helimagnets present an intriguing avenue for spintronics research. Theoretical predictions suggest that MR magnitude in the helimagnetic (HM) regime surpasses that in the ferromagnetic (FM) regime by over an order of magnitude. However, in metallic helimagnets like MnP, MR in the HM phase remains modest (<10%), limiting its application in MR devices. Here, a groundbreaking approach is presented to achieve a giant low-field MR effect in nanostructured MnP by leveraging confinement and strain effects along with spin helicity. Unlike the modest MR observed in bulk MnP single crystals and large-grain polycrystalline films, which exhibit a small negative MR in the FM region (∼2%) increasing to ∼8% in the HM region across 10-300 K, a grain size-dependent giant positive MR (∼90%) is discovered near FM to HM transition temperature (TN ∼ 110 K), followed by a rapid decline to a negative MR below ∼55 K in MnP nanocrystalline films. These findings illuminate a strain-mediated spin helicity phenomenon in nanostructured helimagnets, presenting a promising pathway for the development of high-performance MR sensors and spintronic devices through the strategic utilization of confinement and strain effects.

The density waves that develop in kagome metals ScV6Sn6 and LuNb6Sn6 at low temperature appear to arise from underfilled atomic columns within a V-Sn or Nb-Sn scaffolding. Compressing this network with applied pressure in ScV6Sn6 suppressed the structural transition temperature by constraining atomic rattling and inhibiting the shifts that define the structural modulation. We predicted that the density wave transition in LuNb6Sn6 at 68 K would be suppressed by pressure as well. In this Letter, we examine the pressure dependence of the density wave transition by measuring resistance vs temperature up to 2.26 GPa. We found the transition temperature is smoothly depressed and disappears around 1.9 GPa. This result not only addresses our prediction, but strengthens the rattling chains origin of structural instabilities in the HfFe6Ge6-type kagome metals.

The discovery of iron-based superconductors proves that high-temperature superconductivity is not limited to cuprates. Here, we use transport measurements to determine the in-plane anisotropy of the upper critical field (Hc2) in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence of the Hc2 anisotropy for fields along inter-planar and in-plane directions was found to increase in the over-doped regime. For underdoped samples with a spin-nematic phase below the tetragonal-to-orthorhombic structural transition temperature, we find that Hc2 along the a axis is considerably lower than that along the b axis. This Hc2 anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with previous works, we conclude that superconductivity in underdoped iron pnictides is orbital selective—with a dominant contribution from electrons with the dyz orbital character and from being intimately associated with spin excitations.

Metals with kagome lattice provide bulk materials to host both the flat-band and Dirac electronic dispersions. A new family of kagome metals is recently discovered inAV6Sn6. The Dirac electronic structures of this material needs more experimental evidence to confirm. In the manuscript, we investigate this problem by resolving the quantum oscillations in both electrical transport and magnetization in ScV6Sn6. The revealed orbits are consistent with the electronic band structure models. Furthermore, the Berry phase of a dominating orbit is revealed to be aroundπ, providing direct evidence for the topological band structure, which is consistent with calculations. Our results demonstrate a rich physics and shed light on the correlated topological ground state of this kagome metal.

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including BaFe_2As_2, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic (AF) ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned BaFe_2As_2 and compare the results to the non-magnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, BaFe_2As_2 exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in BaFe_2As_2 that is not present in the Ni-doped material.

The recent discovery of a charge density (CDW) state in ScV$_6$Sn$_6$ at $T_{\textrm{CDW}}$ = 91 K offers new opportunities to understand the origins of electronic instabilities in topological kagome systems. By comparing to the isostructural non-CDW compound LuV$_6$Sn$_6$, we unravel interesting electrical transport properties in ScV$_6$Sn$_6$, above and below the charge ordering temperature. We observed that by applying a magnetic field along the $a$ axis, the temperature behavior of the longitudinal resistivity in ScV$_6$Sn$_6$ changes from metal-like to insulator-like above the CDW transition. We show that in the charge ordered state ScV$_6$Sn$_6$ follows the Fermi liquid behavior while above that, it transforms into a non-Fermi liquid phase in which the resistivity varies sublinearly over a broad temperature range. The sublinear resistivity, which scales by $T^{3/5}$ is a common feature among other vanadium-containing kagome compounds exhibiting CDW states such as KV$_3$Sb$_5$, RbV$_3$Sb$_5$, and CsV$_3$Sb$_5$. By contrast, the non-Fermi liquid behavior does not occur in LuV$_6$Sn$_6$. We explain the $T^{3/5}$ universal scaling behavior from the Coulomb scattering between Dirac electrons and Van Hove singularities; common features in the electronic structure of kagome materials. Finally, we show anomalous Hall-like behavior in ScV$_6$Sn$_6$ below $T_{\textrm{CDW}}$, which is absent in the Lu compound. Comparing the transport properties of ScV$_6$Sn$_6$ and LuV$_6$Sn$_6$ is valuable to highlight the impacts of the unusual CDW in the Sc compound.

The kagome motif is a versatile platform for condensed matter physics, hosting rich interactions between magnetic, electronic, and structural degrees of freedom. In recent years, the discovery of a charge density wave (CDW) in the AV_3Sb_5 superconductors and structurally-derived bond density waves in FeGe and ScV_6Sn_6 have stoked the search for new kagome platforms broadly exhibiting density wave (DW) transitions. In this work, we evaluate the known AM_6X_6 chemistries and construct a stability diagram that summarizes the structural relationships between the ≈125 member family. Subsequently we introduce our discovery of the broader LnNb_6Sn_6 (Ln:Ce-Nd,Sm,Gd-Tm,Lu,Y) family of kagome metals and an analogous DW transition in LuNb_6Sn_6. Our X-ray scattering measurements clearly indicate a (1/3, 1/3, 1/3) ordering wave vector (√(3)×√(3)×3 superlattice) and diffuse scattering on half-integer L-planes. Our analysis of the structural data supports the “rattling mode” DW model proposed for ScV_6Sn_6 and paints a detailed picture of the steric interactions between the rare-earth filler element and the host Nb-Sn kagome scaffolding. We also provide a broad survey of the magnetic properties within the HfFe_6Ge_6-type LnNb_6Sn_6 members, revealing a number of complex antiferromagnetic and metamagnetic transitions throughout the family. This work integrates our new LnNb_6Sn_6 series of compounds into the broader AM_6X_6 family, providing new material platforms and forging a new route forward at the frontier of kagome metal research.

Unconventional plasmonic materials beyond traditional noble metals extend applications of nanotechnology to novel optical, electrical, and magnetic devices. For example, the low photoluminescence (PL) efficiency of two-dimensional (2D) magnetic materials hinders their effective utilization in magnetooptical studies and practical applications, despite their significant role in information storage and spintronic devices. Plasmon-enhanced PL is a promising route toward efficient magneto-optical applications. Here, we report the first observations of enhanced PL and Raman signals in a multilayered 2D antiferromagnet MnPS3, which are attributed to the near-field edge plasmon antenna enhancement in few hundred nm thick flakes. We observed two in-gap near-infrared emission signals and studied their thickness dependence. For the first time, we performed tip-enhanced photoluminescence (TEPL) imaging of MnPS3 in classical (tapping mode) and quantum plasmonic (contact mode) regimes. Classical TEPL showed signal enhancement via plasmonic gap-mode and surface guided waves. Quantum plasmonic TEPL showed evidence for edge plasmons in MnPS3 via tunneling-induced PL suppression, revealing a 300 nm wide edge plasmon size. Our work opens new possibilities for plasmonic applications of MnPS3, while quantum plasmonic imaging may be used to discover novel plasmonic materials.

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge -spin interactions in the system. Although the parent state of iron -based high -temperature superconductors, including BaFe2As2, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in -plane MR effect due to twin domains in the orthorhombic antiferromagnetic ordered state. Here, we study the in -plane magnetoresistance anisotropy in detwinned BaFe2As2 and compare the results to the nonmagnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, BaFe2As2 exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge -spin interaction in BaFe2As2 that is not present in the Ni-doped material.

Understanding the magnetoresistance (MR) of a magnetic material forms the basis for uncovering the orbital mechanisms and charge-spin interactions in the system. Although the parent state of iron-based high-temperature superconductors, including ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$, exhibits unusual electron transport properties resulting from spin and charge correlations, there is still valuable insight to be gained by understanding the in-plane MR effect due to twin domains in the orthorhombic antiferromagnetic ordered state. Here, we study the in-plane magnetoresistance anisotropy in detwinned ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ and compare the results to the nonmagnetic Ni-doped sample. We find that in the antiferromagnetically ordered state, ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ exhibits anisotropic MR that becomes large at low temperatures and high fields. Both transverse and longitudinal MRs are highly anisotropic and dependent on the field and current orientations. These results cannot be fully explained by calculations considering only the anisotropic Fermi surface. Instead, the spin orientation of the ordered moment also affects the MR effect, suggesting the presence of a large charge-spin interaction in ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ that is not present in the Ni-doped material.

We use transport measurements to determine the in-plane anisotropy of the upper critical field Hc2 in detwinned superconducting Ba(Fe1-xMx)2As2 (M = Co, Ni) single crystals. In previous measurements on twinned single crystals, the charge carrier doping dependence (x) of the upper critical field anisotropy for fields along the inter-planar (c-axis) and in-plane field directions was found to increase in the overdoped regime. For underdoped samples, which exhibit a spin nematic phase below the tetragonal to orthorhombic structural transition temperature Ts , we find that Hc2 along the a-axis is considerably lower than that along the b-axis. The upper critical field anisotropy disappears in the over-doped regime when the system becomes tetragonal. By combining these results with inelastic neutron scattering studies of spin excitations, and angle-resolved photoemission spectroscopy, we conclude that superconductivity in under-doped iron pnictides is orbital selective - with a dominant contribution from electrons with the dyz orbital character and being intimately associated with spin excitations.

The kagome metals display an intriguing variety of electronic and magnetic phases arising from the connectivity of atoms on a kagome lattice. A growing number of these materials with vanadium-kagome nets host charge-density waves (CDWs) at low temperatures, including ScV6Sn6, CsV3Sb5, and V3Sb2. Curiously, only the Sc version of the RV6Sn6 materials with a HfFe6Ge6-type structure hosts a CDW (R = Gd-Lu, Y, Sc). In this study, we investigate the role of rare earth size in CDW formation in the RV6Sn6 compounds. Magnetization measurements on our single crystals of (Sc,Lu)V6Sn6 and (Sc,Y)V6Sn6 establish that the CDW is suppressed by substituting Sc by larger Lu or Y. Single-crystal X-ray diffraction reveals that compressible Sn-Sn bonds accommodate the larger rare earth atoms within loosely packed R-Sn-Sn chains without significantly expanding the lattice. We propose that Sc provides extra room in these chains crucial to CDW formation in ScV6Sn6. Our rattling chain model explains why both physical pressure and substitution by larger rare earth atoms hinder CDW formation despite opposite impacts on lattice size. We emphasize the cooperative effect of pressure and rare earth size by demonstrating that pressure further suppresses the CDW in a Lu-doped ScV6Sn6 crystal. Our model not only addresses why a CDW only forms in the RV6Sn6 materials with tiny Sc but also advances our understanding of why unusual CDWs form in the kagome metals.

Kagome metals are widely recognized as versatile platforms for exploring novel topological properties, unconventional electronic correlations, magnetic frustration, and superconductivity. In the RV_6Sn_6 family of materials (R = Sc, Y, Lu), ScV_6Sn_6 hosts an unusual charge density wave ground state as well as structural similarities with the AV_3Sb_5 system (A = K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with first-principles lattice dynamics calculations to reveal the charge density wave state in ScV_6Sn_6. In the low temperature phase, we find a five-fold splitting of the V-containing totally symmetric mode near 240 cm^-1 suggesting that the density wave acts to mix modes of P6/mmm and R3̅m symmetry - an effect that we quantify by projecting phonons of the high symmetry state onto those of the lower symmetry structure. We also test the stability of the density wave state under compression and find that both physical and chemical pressure act to quench the effect. We discuss these findings in terms of symmetry and the structure-property trends that can be unraveled in this system.

AbstractKagomé metals are widely recognized, versatile platforms for exploring topological properties, unconventional electronic correlations, magnetic frustration, and superconductivity. In the RV6Sn6 family of materials (R = Sc, Y, Lu), ScV6Sn6 hosts an unusual charge density wave ground state as well as structural similarities with the AV3Sb5 system (A = K, Cs, Rb). In this work, we combine Raman scattering spectroscopy with first-principles lattice dynamics calculations to reveal phonon mixing processes in the charge density wave state of ScV6Sn6. In the low temperature phase, we find at least four new peaks in the vicinity of the V-containing totally symmetric mode near 240 cm−1 suggesting that the density wave acts to mix modes of P6/mmm and $$R\bar{3}m$$ R 3 ¯ m symmetry - a result that we quantify by projecting phonons of the high symmetry state onto those of the lower symmetry structure. We also test the stability of the short-range ordered density wave state under compression and propose that both physical and chemical pressure quench the effect. We discuss these findings in terms of symmetry and the structure-property trends that can be unraveled in this system.

Thermal and thermoelectric measurements are known as powerful tools to uncover the physical properties of quantum materials due to their sensitivity towards the scattering and chirality of heat carriers. We use these techniques to confirm the presence of momentum and real-space topology in ScMn$_6$Sn$_6$. There is an unconventional dramatic increase in the Seebeck coefficient on entering the transverse conical spiral (TCS) below $T$ = 200 K suggesting an unusual scattering of heat carriers. In addition, the observed anomalous thermal Hall effect and the anomalous Nernst effect indicates non-zero Berry curvature in $k$-space. Furthermore, we identify a significant topological contribution to the thermal Hall and Nernst signals in the TCS phase revealing the impacts of real-space Berry curvature. We discuss the presence of topological thermal Hall effect and topological Nernst effect for the first time in the diverse HfFe$_6$Ge$_6$ family. This study illustrates the importance of transverse thermal and thermoelectric measurements to investigate the origin of topological transport in the non-coplanar magnetic phases in this family of kagome metals.

Since the discovery of superconductivity at ~ 200 K in H3S [1], similar or higher transition temperatures, Tcs, have been reported for various hydrogen-rich compounds under ultra-high pressures [2]. Superconductivity was experimentally proved by different methods, including electrical resistance, magnetic susceptibility, optical infrared, and nuclear resonant scattering measurements. The crystal structures of superconducting phases were determined by X-ray diffraction. Numerous electrical transport measurements demonstrate the typical behavior of a conventional phonon-mediated superconductor: zero resistance below Tc, shift of Tc to lower temperatures under external magnetic fields, and pronounced isotope effect. Remarkably, the results are in good agreement with the theoretical predictions, which describe superconductivity in hydrides within the framework of the conventional BCS theory. However, despite this acknowledgement, experimental evidences for the superconducting state in these compounds have recently been treated with criticism [3–7], which apparently stems from misunderstanding and misinterpretation of complicated experiments performed under very high pressures. Here, we describe in greater detail the experiments revealing high-temperature superconductivity in hydrides under high pressures. We show that the arguments against superconductivity [3–7] can be either refuted or explained. The experiments on the high-temperature superconductivity in hydrides clearly contradict the theory of hole superconductivity [8] and eliminate it [3].