Switches made up of single molecules form the basis for the concept of molecular electronics. Miyamachi et al. demonstrate that an iron-based spin crossover molecule can be switched between different spin states, provided it is decoupled from a metallic substrate by a thin insulating layer.

/pdf/robust-spin-crossover-and-memristance-across-a-single-5fzsm1y5sm.pdf

Robust spin crossover and memristance across a single molecule

DOI: 10.1002/aelm.201900818 phase transitions by Ginzburg et al.[7] In the 1950s, ferroelectric thin films of perovskite structures were first deposited. After successful integration into a semiconductor, ferroelectric thin films exhibit as an important component in the application of electronics and microtransducers. However, most early experiments were performed using a range of film thickness from 100 nm to several micrometers. It has been known since the late 1950s that ferroelectric thin films exhibit quite different phase transition characteristics compared with those of bulk materials. The finitesize effect in ferroelectric films was analyzed and applied to ferroelectrics based on a transverse Ising model.[8] When the thickness of a thin film is below the critical value of around several to tens of nanometers, the spontaneous polarization for traditional perovskite oxide usually disappears with a dropped transition temperature due to the reduced longrange Coulomb coupling, significantly enhanced depolarizing electrostatic field, surface-reconstruction caused by the surface energy, electron screening and interfacial bonding related strain, the chemical environment, etc.[9,10] However, with the rapid development of microelectronics, higher density electronic devices are urgently needed and the scaling trends continue to shrink the feature size of materials. Controllable thinner, that is, nanoscale ferroelectric materials, are essential not only for understanding the size effect but also for electronic applications. Although the dilemma has posed unprecedented challenges to the entire industry, many efforts have been put forward to overcome the difficulties. As early as the 1980s, ferroelectricity was introduced in polyvinylidene fluoride[11] and its copolymers with trifluoroethylene P(VDFTrFE).[12] In 1986, P(VDF-TrFE) films as thin as 60 nm with a saturated polarization of 100 mC m–2 were reported.[13] Later in 1991, Scott reported the phase transitions in ferroelectric films with a thickness of 70–400 nm both theoretically and experimentally.[14] Significantly, in 1998, a ferroelectric transition in a two monolayers thick random copolymer of vinylidene fluoride and trifluoroethylene was observed.[15] Then, in 2000, Blinov et al. discussed the unique ferroelectric properties in two monolayers (≈1 nm) of P(VDF-TrFE), which were fabricated by the Langmuir–Blodgett technique and demonstrated a breakthrough in original critical thickness, thus encouraging researchers to further explore The investigation of two-dimensional (2D) ferroelectrics has attracted significant interest in recent years for applications in functional electronics. Without the limitation of a finite size effect, 2D materials with stable layered structures and reduced surface energy may go beyond the presence of an enhanced depolarization field in ultrathin ferroelectrics, thereby opening a pathway to explore low-dimensional ferroelectricity, making ultra-high-density devices possible and maintaining Moore’s Law. Although many theoretical works on potential 2D ferroelectric materials have been conducted, much still needs to be accomplished experimentally, as it is rare for 2D ferroelectric materials to be proven and plenty of 2D ferroelectrics are waiting to be discovered. First, experimental and theoretical progress on 2D ferroelectric materials, including in-plane and out-of-plane, is reviewed, followed by a general introduction to various characterization methods. Intrinsic mechanisms associated with promising 2D ferroelectric materials, together with related applications, are also discussed. Finally, an outlook for future trends and development in 2D ferroelectricity are explored. Researchers can use this to obtain a basic understanding of 2D ferroelectric materials and to build a database of progress of 2D ferroelectrics.

Recent Progress in Two‐Dimensional Ferroelectric Materials

The spin excitation energy and the magnetic anisotropy of individual atoms can be modified by varying the exchange coupling of the atomic spin to metallic leads.

/pdf/control-of-single-spin-magnetic-anisotropy-by-exchange-533ohp7iql.pdf

Control of single-spin magnetic anisotropy by exchange coupling

Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking Nevertheless, experimental evidence for magnetically induced WSMs is scarce Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd2As2 We attribute this effect to the itinerant electrons experiencing quasi-static and quasi-long-range ferromagnetic fluctuations Moreover, the spin-nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not essential requirements for WSM states in centrosymmetric systems and that WSM states can emerge in a wider range of condensed matter systems than previously thought

/pdf/spin-fluctuation-induced-weyl-semimetal-state-in-the-50lz01iqro.pdf

Spin fluctuation induced Weyl semimetal state in the paramagnetic phase of EuCd2As2

Under certain conditions, a fermion in a superconductor can separate in space into two parts known as Majorana zero modes, which are immune to decoherence from local noise sources and are attractive building blocks for quantum computers. Promising experimental progress has been made to demonstrate Majorana zero modes in materials with strong spin-orbit coupling proximity coupled to superconductors. Here we report signatures of Majorana zero modes in a material platform utilizing the surface states of gold. Using scanning tunneling microscope to probe EuS islands grown on top of gold nanowires, we observe two well-separated zero-bias tunneling conductance peaks aligned along the direction of the applied magnetic field, as expected for a pair of Majorana zero modes. This platform has the advantage of having a robust energy scale and the possibility of realizing complex designs using lithographic methods.

https://www.pnas.org/content/pnas/117/16/8775.full.pdf

Signature of a pair of Majorana zero modes in superconducting gold surface states.

The local in-plane strain of Ag(111) islands on the Nb(110) surface has been derived from the lateral variation of the electronic density of states measured by scanning tunneling spectroscopy. The onset energy Ess of the Shockley-type surface-state band is shifted to higher energies compared to the bulk due to thermal strain arising from the difference in thermal expansion between Nb and Ag and cooling by 565 K. A quantitative dependence of Ess from the strain is obtained by density-functional theory calculations. This allows a mapping of the local strain of individual Ag islands on the nanometer scale.

/pdf/local-strain-mapping-on-ag-111-islands-on-nb-110-1axf4knwgh.pdf

Local-strain mapping on Ag(111) islands on Nb(110)

We use epitaxial strain to spatially tune the bottom of the surface-state band ${E}_{\mathrm{SS}}$ of Ag(111) islands on Nb(110). Bulk and surface-state contributions to the Ag(111) local density of states (LDOS) can be separated with scanning tunneling spectroscopy. For thick islands ($\ensuremath{\approx}20$ nm), the Ag surface states are decoupled from the Ag bulk states and the superconductive gap induced by proximity to Nb is due to bulk states only. However, for thin islands (3--4 nm), surface-state electrons develop superconducting correlations as identified by a complete energy gap in the LDOS when ${E}_{\mathrm{SS}}$ is smaller than but close to the Fermi level. The induced superconductivity in this case is of a two-band nature and appears to occur when the surface-state wave function reaches down to the Ag/Nb interface.

Two-band superconductivity of bulk and surface states in Ag thin films on Nb

The electrical resistance $R$ of metallic nanocontacts subjected to controlled cyclic electromigration in ultrahigh vacuum has been investigated in situ as a function of applied voltage $V$. For sufficiently small contacts, i.e., large resistance, a decrease of $R(V)$ while increasing $V$ is observed. This effect is tentatively attributed to the presence of contacts separated by thin vacuum barriers in parallel to ohmic nanocontacts. Simple model calculations indicate that both thermal activation or tunneling can lead to this unusual behavior. We describe our data by a tunneling model whose key parameter, i.e., the tunneling distance, changes because of thermal expansion due to Joule heating and/or electrostatic strain arising from the applied voltage. Oxygen exposure during electromigration prevents the formation of negative $R(V)$ slopes, and at the same time enhances the probability of uncontrolled melting, while other gases show little effects. In addition, indication for field emission has been observed in some samples.

/pdf/resistance-voltage-dependence-of-nanojunctions-during-5bqh1nvhlj.pdf

Resistance-voltage dependence of nanojunctions during electromigration in ultrahigh vacuum

We report the specific heat, magnetic susceptibility, and magnetization as well as resistivity of hexagonal $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Yb}\mathrm{Pd}\mathrm{Sn}$ polycrystals down to temperatures $T$ of $50\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ and fields up to $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. In the susceptibility $\ensuremath{\chi}(T)$, several different temperature regimes can be distinguished. For $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}gTg170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a Curie-Weiss-like behavior is obtained with the effective moments close to that of ${\mathrm{Yb}}^{3+}$. The large Weiss temperature ${\ensuremath{\Theta}}_{H}\ensuremath{\approx}150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and the leveling off of $\ensuremath{\chi}$ toward low $T$ are indicative of valence fluctuations. Around $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, crystal-field excitations manifest themselves in features of $\ensuremath{\chi}(T)$ and the resistivity $\ensuremath{\rho}(T)$. For $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}gTg0.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a Curie-Weiss behavior with ${\ensuremath{\mu}}_{\mathit{eff}}=1.27{\ensuremath{\mu}}_{B}∕\mathrm{Yb}$ atom and ${\ensuremath{\Theta}}_{L}=0.41\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is found, signaling the occupation of the lowest crystal-field doublet. A sharp maximum in $\ensuremath{\chi}(T)$ and in the specific heat $C(T)$ indicates antiferromagnetic ordering at $250\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. The specific-heat peak develops into a broadened Schottky anomaly in moderate magnetic fields. The sizable linear specific-heat coefficient $\ensuremath{\gamma}=68\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕\text{mole}\phantom{\rule{0.2em}{0ex}}{\mathrm{K}}^{2}$ is attributed to valence fluctuations. Likewise, the magnetization at 10 and $2.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ measured up to $12\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ can be decomposed into a contribution of delocalized electrons and localized magnetic moments. The data can be consistently interpreted in terms of a two-fluid model with $\ensuremath{\sim}6%$ localized and $\ensuremath{\sim}94%$ delocalized moments derived from (nearly) trivalent Yb.

Coexistence of localized and delocalized f-electrons in α − Yb Pd Sn from thermodynamic and transport measurements

We report on the low-temperature dc magnetic susceptibility of CeCu 6 - x Au x ( x = 0.1 ) in very small fields B = 100 μ T and 1 mT. For x = 0.1 , i.e. for the quantum critical concentration, we confirm the anomalous exponent α of the susceptibility along the easy c-direction, χ c - 1 ∼ θ + T α with α = 0.8 observed previously in moderate fields B = 100 mT (see, A. Schroder, et al., Nature 407 (2000) 351). However, below 240 mK we see a clear additional contribution to χ c . At the lowest measuring temperature T = 40 mK , the ratio χ c : χ a : χ b = 4 : 1.5 : 1 of the susceptibility anisotropy is weaker than at higher T where the ratio is 10:2:1. This is caused by a steeper increase of χ a and χ b as compared to χ c .

Anisotropy of the magnetic susceptibility of CeCu6-xAux near the quantum phase transition

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