npj Quantum Materials最新文献

Electronic liquid crystal (ELC) phases are spontaneous symmetry breaking states believed to arise from strong electron correlation in quantum materials such as cuprates and iron pnictides. Here, we report a direct observation of a smectic phase in a weakly correlated nonsymmorphic square-net semimetal GdSb_xTe_2-x. Incommensurate smectic charge modulation and intense local unidirectional nanostructure, which coexist with Dirac fermions across Fermi level, are visualized by using spectroscopic imaging—scanning tunneling microscopy. As materials with highly mobile carriers are mostly weakly correlated, the discovery of such an ELC phase are anomalous and raise questions on the origin of their emergence. Specifically, we demonstrate how chemical substitution generates these symmetry breaking phases before the system undergoes a charge density wave (CDW)—orthorhombic structural transition. Our results highlight the importance of impurities in realizing ELC phases and present a new material platform for exploring the interplay among quenched disorder, Dirac fermions and electron correlation.

The Hall effect is one of the most fundamental but elusive phenomena in condensed matter physics due to the rich variety of underlying mechanisms. Here we report an exceptionally large Hall effect in a frustrated magnet GdCu₂ with high conductivity. The Hall conductivity at the base temperature is as high as the order of 10⁴–10⁵ Ω⁻¹cm⁻¹ and shows abrupt sign changes under magnetic fields. Remarkably, the giant Hall effect is rapidly suppressed as the longitudinal conductivity is lowered upon increasing temperature or introducing tiny amount of quenched disorder. Our systematic transport measurements combined with neutron scattering measurements, ab initio band calculations and spin model calculations indicate that the unusual Hall effect can be understood in terms of spin-splitting induced emergence/disappearance of Fermi pockets as well as skew scattering from spin-chiral cluster fluctuations in a field-polarized state. The present study demonstrates complex interplay among magnetization, spin-dependent electronic structure, and spin fluctuations in producing the giant Hall effect in highly conductive frustrated magnets with a distorted triangular lattice.

Quantum anomalous Hall (QAH) insulators are characterized by vanishing longitudinal resistance and quantized Hall resistance in the absence of an external magnetic field. Among them, high-Chern-number QAH insulators offer multiple nondissipative current channels, making them crucial for the development of low-power-consumption electronics. Using first-principles calculations, we propose that high-Chern-number (C > 1) QAH insulators can be realized in MnBi₂Te₄ (MBT) multilayer films through the combination of mixed stacking orders, eliminating the need for additional buffer layers. The underlying physical mechanism is validated by calculating real-space-resolved anomalous Hall conductivity (AHC). Local AHC is found to be predominantly located in regions with consecutive correct stacking orders, contributing to quasi-quantized AHC. Conversely, regions with consecutive incorrect stacking contribute minimally to the total AHC, which can be attributed to the varied interlayer coupling in different stacking configurations. Our work provides valuable insights into the design principle for achieving large Chern numbers, and highlights the role of stacking configurations in manipulating electronic and topological properties in MBT films and its derivatives.

The nonlinear driving of collective modes in quantum materials can lead to a number of striking non-equilibrium functional responses, which merit a comprehensive exploration of underlying dynamics. However, the coherent coupling between nonlinearly-driven modes frequently involves multiple mode coordinates at once, and is often difficult to capture by one-dimensional pump probe spectroscopy. One example is phonon-mediated amplification of Josephson plasmons in YBa₂Cu₃O_6+x, a phenomenon likely associated with the mysterious superconducting-like optical response observed in this material. Here, we report two-dimensional nonlinear spectroscopy measurements in driven YBa₂Cu₃O_6+x. We excite apical oxygen phonons with pairs of mutually-delayed carrier envelope phase stable mid-infrared pump pulses, and detect time-modulated second-order nonlinear optical susceptibility. We find that the driven phonons parametrically amplify coherent pairs of fluctuating opposite-momentum Josephson plasma polaritons, corresponding to a squeezed state of the Josephson plasma.

Two-dimensional (2D) layered material grey arsenic exhibits great potential for electronic and optoelectronics devices. Identifying the orbital texture in the electronic energy bands close to Fermi level is crucial for understanding and further manipulating the optoelectronic properties of grey arsenic. In this work, we investigate the orbital properties from bulk-state and surface-state of grey arsenic by using multidimensional angle-resolved photoemission spectroscopy, under different light polarization and crystal orientation conditions. Furthermore, by combining the experimental results with first-principles calculations based on density functional theory (DFT), we reveal that both the surface and bulk states of grey arsenic contain 4 s, 4p_x, 4p_y and 4p_z orbitals, but the orbital ratios are different. Our study offers new insight into the orbital nature of grey arsenic and also paves the way for investigation of orbital properties in other 2D materials.

Magnet/superconductor hybrid systems have been put forward as a platform for realizing topological superconductivity. We investigated the heterostructure of ferromagnetic monolayer CrCl₃ and superconducting NbSe₂. Using low-temperature scanning tunneling microscopy, we observe topologically trivial Yu-Shiba-Rusinov (YSR) states localized at the edge of CrCl₃ islands. DFT simulations reveal that the Cr atoms at the edge have an enhanced d-orbital DOS close to E_F. This leads to an exchange coupling between these atoms and the substrate that rationalizes the edge-localization of the YSR states.

Chromium ditelluride, CrTe₂, is an attractive candidate van der Waals material for hosting 2D magnetism. However, how the room-temperature ferromagnetism of the bulk evolves as the sample is thinned to the single-layer limit has proved controversial. This, in part, reflects its metastable nature, vs. a series of more stable self-intercalation compounds with higher relative Cr:Te stoichiometry. Here, exploiting a recently developed method for enhancing nucleation in molecular-beam epitaxy growth of transition-metal chalcogenides, we demonstrate the selective stabilisation of high-coverage CrTe₂ and Cr_2+εTe₃ epitaxial monolayers. Combining X-ray magnetic circular dichroism, scanning tunnelling microscopy, and temperature-dependent angle-resolved photoemission, we demonstrate that both compounds order magnetically with a similar T_C. We find, however, that monolayer CrTe₂ forms as an antiferromagnetic metal, while monolayer Cr_2+εTe₃ hosts an intrinsic ferromagnetic semiconducting state. This work thus demonstrates that control over the self-intercalation of metastable Cr-based chalcogenides provides a powerful route for tuning both their metallicity and magnetic structure, establishing the Cr_xTe_y system as a flexible materials class for future 2D spintronics.

We present numerical simulations of X-ray magnetic circular dichroism (XMCD) at the L_2,3 edge of Ni in the weakly ferromagnetic altermagnet NiF₂. Our results predict a significant XMCD signal for light propagating perpendicular to the magnetic moments, which are approximately aligned along the [010] easy-axis direction. The analysis shows that the altermagnetic and ferromagnetic contributions to the XMCD signal can be uniquely distinguished by their dependence on an applied magnetic field. By varying the angle of the field relative to the easy axis, the in-plane orientation of both the Néel vector and the net magnetization can be systematically controlled. We further demonstrate that the XMCD signal, even under fields as strong as 40 T and for any in-plane orientation, can be accurately described as a linear combination of two spectral components, with geometrical prefactors determined by the field’s magnitude and direction. This insight enables experimental validation of the distinctive relationship between the Néel vector orientation and the X-ray Hall vector in the rutile structure. Quantitative simulations supporting these findings are provided.

In two-dimensional topological nodal superconductors, Majorana edge states have been conventionally believed to exhibit spin-triplet pairing correlations. Here, we show that a substantial spin-singlet pairing component is present in Majorana edge states of antiferromagnetic topological nodal-point superconductors. We reveal that this unexpected phenomenon emerges from the interplay of antiferromagnetic order and symmetry, which leads to Majorana edge states manifesting with nearly flat bands instead of strictly flat bands. Crucially, it can be detectable by means of spin-dependent Andreev reflection, where the zero-bias conductance peaks are maximized when the spin of incident electrons is nearly antiparallel to that of the Majorana edge excitations. Our findings unveil a unique spin signature for Andreev reflection resonances, thus advancing our fundamental understanding of spin-dependent mechanisms in topological superconductivity and representing a significant step towards the experimental detection of Majorana fermions.

Altermagnets are predicted to exhibit anomalous transport phenomena, such as the anomalous Hall and Nernst effects, as observed in ferromagnets but with a vanishing net magnetic moment, akin to antiferromagnets. Despite their potential, progress has been limited due to the scarcity of metallic altermagnets. Motivated by the recent discovery of the altermagnetic metal CrSb, we conducted a systematic study of its electrical and thermoelectric transport properties, using first-principles calculations. CrSb exhibits low magnetocrystalline anisotropy energy, enabling the manipulation of the Néel vector in CrSb films through a suitable ferromagnetic substrate. The anomalous Hall and Nernst conductivities reach their maximum when the Néel vector is aligned along (frac{1}{2}{boldsymbol{a}}+{boldsymbol{b}}). The origins of both conductivities were analyzed in terms of Berry curvature distribution. Our results demonstrate that CrSb provides a good platform for investigating the Néel vector-dependent anomalous transport in altermagnetic metals.