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Understanding the physics of phase transitions in two-dimensional (2D) systems underpins the research in diverse fields including statistical mechanics, quantum systems, nanomagnetism, and soft condensed matter. However, many fundamental aspects of 2D phase transitions are still not well understood, including the effects of interparticle potential, polydispersity, and particle shape. Magnetic skyrmions, which are non-trivial chiral spin structures, can be considered as quasi-particles that form two-dimensional lattices. Here we show, by real-space imaging using in situ cryo-Lorentz transmission electron microscopy coupled with machine learning, the ordering behavior of N\'eel skyrmion lattices in van der Waals Fe3GeTe2. We demonstrate a distinct change in the skyrmion size distribution during field-cooling, which leads to a loss of lattice order and an evolution of the skyrmion liquid phase. Remarkably, the lattice order is restored during field heating and demonstrates a thermal hysteresis. Our quantitative analysis explains this behavior based on the energy landscape of skyrmions and demonstrates the potential to control the lattice order in 2D phase transitions.

Low-valent nickelates R$_{n+1}$Ni$_n$O$_{2n+2}$ (R = rare earth) containing Ni$^{1+}$ (d$^{9}$) with a quasi-two-dimensional (quasi-2D) square planar coordination geometry possess structural and electronic properties that are similar to those of high-Tc cuprates, including superconductivity itself in the doped infinite layer ($n = \infty$) RNiO$_2$ system. Within this R$_{n+1}$Ni$_n$O$_{2n+2}$ nickelate family, the crystallographic isomorphs Pr$_4$Ni$_3$O$_8$ and La$_4$Ni$_3$O$_8$ exhibit singularly different ground states: Pr$_4$Ni$_3$O$_8$ is metallic and La$_4$Ni$_3$O$_8$ is a charge- and spin-stripe ordered insulator. To explore and understand the ground state evolution from metallic Pr$_4$Ni$_3$O$_8$ to stripe-ordered La$_4$Ni$_3$O$_8$ in the R$_4$Ni$_3$O$_8$ family, we have grown a series of isovalent-substituted single crystals (Pr$_{1-x}$La$_x$)$_4$Ni$_3$O$_8$. Combining thermodynamic, transport, magnetic, and synchrotron X-ray single crystal diffraction measurements, we reveal a transition between metallic and stripe-insulator phase regions, with a putative quantum phase transition at x = 0.4. We propose two possible models for (Pr$_{1-x}$La$_x$)$_4$Ni$_3$O$_8$: an electronically inhomogeneous system that could serve as a candidate for exploring quantum Griffiths phase physics and a homogeneous system with a putative quantum critical point at the phase boundary.

Doped perovskite cobaltites (e.g., La$_{1-x}$Sr$_x$CoO$_3$) have been extensively studied for their spin-state physics, electronic inhomogeneity, and insulator-metal transitions. Ferromagnetically-interacting spin-state polarons emerge at low $x$ in the phase diagram of these compounds, eventually yielding long-range ferromagnetism. The onset of long-range ferromagnetism ($x \approx 0.18$) is substantially delayed relative to polaron percolation ($x \approx 0.05$), however, generating a troubling inconsistency. Here, Monte-Carlo simulations of a disordered classical spin model are used to establish that previously ignored magnetic frustration is responsible for this effect, enabling faithful reproduction of the magnetic phase diagram., Comment: 6 pages, 4 figures; includes Supplemental Material (4 pages, 2 figures)

The information content of crystalline materials becomes astronomical when collective electronic behavior and their fluctuations are taken into account. In the past decade, improvements in source brightness and detector technology at modern x-ray facilities have allowed a dramatically increased fraction of this information to be captured. Now, the primary challenge is to understand and discover scientific principles from big data sets when a comprehensive analysis is beyond human reach. We report the development of a novel unsupervised machine learning approach, XRD Temperature Clustering (X-TEC), that can automatically extract charge density wave (CDW) order parameters and detect intra-unit cell (IUC) ordering and its fluctuations from a series of high-volume X-ray diffraction (XRD) measurements taken at multiple temperatures. We apply X-TEC to XRD data on a quasi-skutterudite family of materials, (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$, where a quantum critical point arising from charge order is observed as a function of Ca concentration. We further apply X-TEC to XRD data on the pyrochlore metal, Cd$_2$Re$_2$O$_7$, to investigate its two much debated structural phase transitions and uncover the Goldstone mode accompanying them. We demonstrate how unprecedented atomic scale knowledge can be gained when human researchers connect the X-TEC results to physical principles. Specifically, we extract from the X-TEC-revealed selection rule that the Cd and Re displacements are approximately equal in amplitude, but out of phase. This discovery reveals a previously unknown involvement of $5d^2$ Re, supporting the idea of an electronic origin to the structural order. Our approach can radically transform XRD experiments by allowing in-operando data analysis and enabling researchers to refine experiments by discovering interesting regions of phase space on-the-fly.

Single crystals of PrNiO3 were grown under an oxygen pressure of 295 bar using a unique high-pressure optical-image floating zone furnace. The crystals, with volume in excess of 1 mm3, were characterized structurally using single crystal and powder X-ray diffraction. Resistivity, specific heat, and magnetic susceptibility were measured, all of which evidenced an abrupt, first order metal-insulator transition (MIT) at ~130 K, in agreement with previous literature reports on polycrystalline specimens. Temperature-dependent single crystal diffraction was performed to investigate changes through the MIT. Our study demonstrates the opportunity space for high fugacity, reactive environments for single crystal growth specifically of perovskite nickelates but more generally to correlated electron oxides., Comment: 19 pages, 6 figures, Supporting Information included

We report, for the first time, measurements of the third order, $\chi_3$ and fifth order, $\chi_5$, susceptibilities in an itinerant oxide metamagnet, Sr$_3$Ru$_2$O$_7$ for magnetic fields both parallel and perpendicular to the c-axis. These susceptibilities exhibit maxima in their temperature dependence such that $T_1 \approx 2T_3 \approx 4T_5$ where the $T_i$ are the position in temperature where a peak in the $i$-th order susceptibility occurs. These features taken together with the scaling of the critical field with the temperature $T_1$ observed in a diverse variety of itinerant metamagnets find a natural explanation in a single band model with one Van Hove singularity (VHS) and onsite repulsion $U$. The separation of the VHS from the Fermi energy $\Delta$, sets a single energy scale, which is the primary driver for the observed features of itinerant metamagnetism at low temperatures., Comment: 8 pages, 7 figures

A polycrystalline sample of FeSe, which adopts the tetragonal PbO-type structure (P4/nmm) at room temperature, has been prepared using solid state reaction. We have investigated pressure-induced structural changes in tetragonal FeSe at varying hydrostatic pressures up to 0.6 GPa in the orthorhombic (T = 50 K) and tetragonal (T = 190 K) phases using high resolution neutron powder diffraction. We report that the structure is quite compressible with a Bulk modulus around 31 GPa to 33 GPa and that the pressure response is anisotropic with a larger compressibility along the c-axis. Key bond angles of the SeFe4 pyramids and FeSe4 tetrahedra are also determined as a function of pressure.

This paper, which focuses on access and enrollment at community colleges, is part of a series published by the Center for Community College Policy, designed to support state and local policymakers, as well as educational leaders who are interested in policy issues related to the two-year postsecondary sector. While some students plan on earning a terminal associate degree and others plan to transfer to a four-year institution, there are other students for whom the community college represents their last best hope for postsecondary education. Community colleges offer these students an opportunity to start over and improve their employability by learning specific skills. Ideally, community colleges serve as a vehicle leading to self-sufficiency for those on welfare or otherwise in need of support. The "open door" policy of community colleges is threatened due to declining investments and concurrent rising enrollment. Some of the policy recommendations explored in this paper address enrollment, student outcomes, funding policies, remedial education, and restricting enrollment in favor of those students who do not already possess a degree. Information about contacting specific community colleges cited as examples in this document are included along with references. (Contains 10 references.) (AF)

In 1989, North Iowa Area Community College (NIACC) conducted a study of transfer students to examine the relative effect of college attendance on academic achievement, transfer students' satisfaction with their college experience, and the college's fulfillment of general education goals. The study examined the length of exposure to college effects, as measured by total semester credit hours (SCH) earned, and student satisfaction with NIACC as determined through a mailed survey. Study findings, based on a 61.24% response rate, included the following: (1) a significant relationship existed between SCH earned at NIACC and the level of transfer student satisfaction with NIACC preparation for the workplace; (2) no significant difference or relationship was observed between SCH earned at NIACC and grade point average at graduation from a four-year institution; (3) no difference was observed between SCH and student satisfaction with the extent to which NIACC had prepared them as individuals, citizens, or family members; (4) the number of SCH earned at NIACC had little or no influence to student outcomes. However, the data did prove that the length of community college attendance was not negatively related to selected student outcomes. Appendixes state the purposes and hypotheses of the study, list general education goals, report findings from previous studies on community college effects, present the demographic characteristics of NIACC students, and offer a summary and conclusions. (JMC)

Attitudes concerning faculty unionization have an impact on its rejection or acceptance and its subsequent implementation. This study was conducted to determine the level of agreement among faculty, student services staff, and students before major collective bargaining activities, such as a run-off election, take place. Assessed are attitudes held by faculty, students who are active and assumedly concerned with the welfare and the dynamics of the University, and staff members under the Office of Student Services toward aspects of collective bargaining at the Ohio State University, Columbia campus. Findings point out a variety of areas, such as fringe benefits, working conditions, and student participation that have the potential to become areas of conflict. In the event of collective bargaining at Ohio State, the question whether decisions will be made in the distributive model as a result of power, coercion, and formalization, or in the integrative model as a result of trust, reason, and shared goals will be partly dependent upon a change in the attitudes of the parties involved. Those parties must understand that they are not simply interest groups but that they constitute the University and share a variety of common goals and interests. (Author/KE)