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Ruddlesden–Popper halide perovskites are 2D solution-processed quantum wells with a general formula A 2 A’ n -1 M n X 3 n +1 , where optoelectronic properties can be tuned by varying the perovskite layer thickness ( n -value), and have recently emerged as efficient semiconductors with technologically relevant stability. However, fundamental questions concerning the nature of optical resonances (excitons or free carriers) and the exciton reduced mass, and their scaling with quantum well thickness, which are critical for designing efficient optoelectronic devices, remain unresolved. Here, using optical spectroscopy and 60-Tesla magneto-absorption supported by modeling, we unambiguously demonstrate that the optical resonances arise from tightly bound excitons with both exciton reduced masses and binding energies decreasing, respectively, from 0.221  m 0 to 0.186  m 0 and from 470 meV to 125 meV with increasing thickness from n equals 1 to 5. Based on this study we propose a general scaling law to determine the binding energy of excitons in perovskite quantum wells of any layer thickness. Hybrid 2D layered perovskites are solution-processed quantum wells whose optoelectronic properties are tunable by varying the thickness of the inorganic slab. Here Blancon et al. work out a general behavior for dependence of the excitonic properties in layered 2D perovskites.

Understanding and controlling charge and energy flow in state-of-the-art semiconductor quantum wells has enabled high-efficiency optoelectronic devices. Two-dimensional (2D) Ruddlesden-Popper perovskites are solution-processed quantum wells wherein the band gap can be tuned by varying the perovskite-layer thickness, which modulates the effective electron-hole confinement. We report that, counterintuitive to classical quantum-confined systems where photogenerated electrons and holes are strongly bound by Coulomb interactions or excitons, the photophysics of thin films made of Ruddlesden-Popper perovskites with a thickness exceeding two perovskite-crystal units (>1.3 nanometers) is dominated by lower-energy states associated with the local intrinsic electronic structure of the edges of the perovskite layers. These states provide a direct pathway for dissociating excitons into longer-lived free carriers that substantially improve the performance of optoelectronic devices.

Reducing the dimensionality of three-dimensional hybrid metal halide perovskites can improve their optoelectronic properties. Here, we show that the third-order optical nonlinearity, n 2 , of hybrid lead iodide perovskites is enhanced in the two-dimensional Ruddlesden-Popper series, (CH 3 (CH 2 ) 3 NH 3 ) 2 (CH 3 NH 3 ) n -1 Pb n I 3 n +1 ( n  = 1–4), where the layer number ( n ) is engineered for bandgap tuning from E g  = 1.60 eV ( n  = ∞; bulk) to 2.40 eV ( n  = 1). Despite the unfavorable relation, n 2 ∝ E g - 4 , strong quantum confinement causes these two-dimensional perovskites to exhibit four times stronger third harmonic generation at mid-infrared when compared with the three-dimensional counterpart, (CH 3 NH 3 )PbI 3 . Surprisingly, however, the impact of dimensional reduction on two-photon absorption, which is the Kramers-Kronig conjugate of n 2 , is rather insignificant as demonstrated by broadband two-photon spectroscopy. The concomitant increase of bandgap and optical nonlinearity is truly remarkable in these novel perovskites, where the former increases the laser-induced damage threshold for high-power nonlinear optical applications. Hybrid metal halide perovskites can exhibit improved optoelectronic properties when their dimensionality is reduced. Here, Saouma et al. study the enhancement of third-order nonlinearities in two-dimensional lead iodide perovskites in the Ruddlesden-Popper series.

Elucidating the nature of the magnetic ground state of iron-based superconductors is of paramount importance in unveiling the mechanism behind their high-temperature superconductivity. Until recently, it was thought that superconductivity emerges only from an orthorhombic antiferromagnetic stripe phase, which can in principle be described in terms of either localized or itinerant spins. However, we recently reported that tetragonal symmetry is restored inside the magnetically ordered state of certain hole-doped compounds, revealing the existence of a new magnetic phase at compositions close to the onset of superconductivity. Here, we present Mössbauer data that show that half of the iron sites in this tetragonal phase are non-magnetic, establishing conclusively the existence of a novel magnetic ground state with a non-uniform magnetization that is inconsistent with localized spins. Instead, this state is naturally explained as the interference between two commensurate spin-density waves, a rare example of collinear double- Q magnetic order. Our results demonstrate the itinerant character of the magnetism of the iron pnictides, and the primary role played by magnetic degrees of freedom in determining their phase diagram. A combination of neutron scattering, X-ray scattering and Mössbauer spectroscopy experiments reveal the existence of a collinear double- Q magnetic ordering in an iron arsenide superconductor.

A charge-density wave (CDW) state has a broken symmetry described by a complex order parameter with an amplitude and a phase. The conventional view, based on clean, weak-coupling systems, is that a finite amplitude and long-range phase coherence set in simultaneously at the CDW transition temperature T cdw . Here we investigate, using photoemission, X-ray scattering and scanning tunnelling microscopy, the canonical CDW compound 2 H -NbSe 2 intercalated with Mn and Co, and show that the conventional view is untenable. We find that, either at high temperature or at large intercalation, CDW order becomes short-ranged with a well-defined amplitude, which has impacts on the electronic dispersion, giving rise to an energy gap. The phase transition at T cdw marks the onset of long-range order with global phase coherence, leading to sharp electronic excitations. Our observations emphasize the importance of phase fluctuations in strongly coupled CDW systems and provide insights into the significance of phase incoherence in ‘pseudogap’ states. Charge density waves are described by a complex order parameter whose amplitude is expected to vanish at the transition temperature. This study shows that the transition in 2 H -NbSe 2 is driven by fluctuations of the phase of the order parameter, with a finite amplitude surviving in the disordered state.

Lead halide perovskites exhibit structural instabilities and large atomic fluctuations thought to impact their optical and thermal properties, yet detailed structural and temporal correlations of their atomic motions remain poorly understood. Here, these correlations are resolved in CsPbBr 3 crystals using momentum-resolved neutron and X-ray scattering measurements as a function of temperature, complemented with first-principles simulations. We uncover a striking network of diffuse scattering rods, arising from the liquid-like damping of low-energy Br-dominated phonons, reproduced in our simulations of the anharmonic phonon self-energy. These overdamped modes cover a continuum of wave vectors along the edges of the cubic Brillouin zone, corresponding to two-dimensional sheets of correlated rotations in real space, and could represent precursors to proposed two-dimensional polarons. Further, these motions directly impact the electronic gap edge states, linking soft anharmonic lattice dynamics and optoelectronic properties. These results provide insights into the highly unusual atomic dynamics of halide perovskites, relevant to further optimization of their optical and thermal properties. Neutron and X-ray scattering measurements provide further insight into the anharmonic behaviour of lead halide perovskites, revealing that rotations of PbBr 6 octahedra in CsPbBr 3 crystals occur in a correlated fashion along two-dimensional planes.

Thermoelectrics interconvert heat to electricity and are of great interest in waste heat recovery, solid-state cooling and so on. The efficiency of thermoelectric materials depends directly on the average ZT (dimensionless figure of merit) over a certain temperature range, which historically has been challenging to increase. Here we report that 2.5% K-doped PbTe 0.7 S 0.3 achieves a ZT of >2 for a very wide temperature range from 673 to 923 K and has a record high average ZT of 1.56 (corresponding to a theoretical energy conversion efficiency of ~20.7% at the temperature gradient from 300 to 900 K). The PbTe 0.7 S 0.3 composition shows spinodal decomposition with large PbTe-rich and PbS-rich regions where each region exhibits dissimilar types of nanostructures. Such high average ZT is obtained by synergistically optimized electrical- and thermal-transport properties via carrier concentration tuning, band structure engineering and hierarchical architecturing, and highlights a realistic prospect of wide applications of thermoelectrics. Obtaining highly efficient thermoelectric materials relies on a high ZT, and on this value being consistently high over a wide temperature range. Here, the authors demonstrate a phase-separated PbTe-based material that exhibits a ZT of >2 from 673 to 923 K, and a resultantly high average ZT of 1.56 between 300 and 900 K.

As a generic property, all substances transfer heat through microscopic collisions of constituent particles1. A solid conducts heat through both transverse and longitudinal acoustic phonons, but a liquid employs only longitudinal vibrations2,3. As a result, a solid is usually thermally more conductive than a liquid. In canonical viewpoints, such a difference also serves as the dynamic signature distinguishing a solid from a liquid. Here, we report liquid-like thermal conduction observed in the crystalline AgCrSe2. The transverse acoustic phonons are completely suppressed by the ultrafast dynamic disorder while the longitudinal acoustic phonons are strongly scattered but survive, and are thus responsible for the intrinsically ultralow thermal conductivity. This scenario is applicable to a wide variety of layered compounds with heavy intercalants in the van der Waals gaps, manifesting a broad implication on suppressing thermal conduction. These microscopic insights might reshape the fundamental understanding on thermal transport properties of matter and open up a general opportunity to optimize performances of thermoelectrics.

In the PDF version of this article, Eq. 5 is missing all elements after the equals sign. The correct version of Eq. 5 is given below. The HTML version of the paper was correct from the time of publication. G 2 x = 1 ( 2 x ) 6 4 [ 1 - ( 1 - x ) 3 ∕ 2 - ( 1 + x ) 3 ∕ 2 ] - 3 4 x 2 [ ( 1 - x ) - 1 ∕ 2 + ( 1 + x ) - 1 ∕ 2 ] + 6 x { ( 1 - x ) 1 ∕ 2 - ( 1 + x ) 1 ∕ 2 } + 2 [ H ( 1 - 2 x ) ( 1 - 2 x ) 3 ∕ 2 + ( 1 + 2 x ) 3 ∕ 2 ] + 1 ( 2 10 x 5 ) 70 x 2 + 3 x [ ( 1 - x ) - 1 ∕ 2 - ( 1 + x ) - 1 ∕ 2 ] - 1 2 x 2 [ ( 1 - x ) - 3 ∕ 2 + ( 1 + x ) - 3 ∕ 2 ]