The exchange interaction and magnetization reversal processes in exchange-biased heterostructures consisting of amorphous ferrimagnetic Fe-Tb alloy films and ferromagnetic Co/Pt multilayers were investigated. Both film systems exhibit an easy axis magnetization perpendicular to the film plane. The dependence of the interfacial exchange coupling on the stoichiometry of the Fe-Tb layer as well as the number of repetitions in the Co/Pt multilayers were analyzed. The net magnetization and effective magnetic anisotropy of the Fe-Tb alloy films have an influence on the exchange energy per unit area. A large exchange-bias field up to 8 kOe is found to be accompanied by an interfacial domain wall as probed by element specific x-ray magnetic circular dichroism absorption measurements. This domain wall exhibits a total thickness between 3 and 4 nm and affects strongly the magnetization reversal in the heterostructure. Furthermore, a linear relation between the thickness of the ferromagnetic Co/Pt multilayers and the exchange-bias field is observed and can be attributed to a change of the location where the interfacial domain wall nucleates in the heterostructure.

We use broadband ferromagnetic resonance spectroscopy and x-ray diffraction to investigate the fundamental origin of perpendicular anisotropy in Co${}_{90}$Fe${}_{10}$/Ni multilayers. By careful evaluation of the spectroscopic $g$-factor, we determine the orbital moment along the out-of-plane and in-plane directions. For the multilayers, we find a direct relationship between the orbital moment asymmetry and the perpendicular anisotropy, consistent with the theory of Bruno [Bruno, Phys. Rev. B 39, 865 (1989)]. A systematic x-ray diffraction study revealed the presence of a trigonal strain as high as 0.7$\%$ in some samples. However, we found no direct correlation between the strain and the anisotropy, indicating that the anisotropy is not dominated by magnetoelastic effects. To further study the interface structure on the anisotropy, we prepared a set of equivalent alloy samples. The strain in the alloy samples was comparable to that of the multilayer samples; however, the orbital moment asymmetry in the alloy samples showed a very different trend, allowing us to isolate the effect of the interfaces in the multilayers.

The specific heat of improved quality polycrystalline LiFeP, $T_c\approx 5$ K, was measured down to 0.4 K. The results indicate that this is another iron-based nodal superconductor (in addition to KFe${}_2$As${}_2$, $T_c\approx 4$ K) where the discontinuity in the specific heat $\Delta C$ at $T_c$ violates the global trend in $\Delta C/T_c$ with $T_c$ for the iron-based superconductors (IBSs), first pointed out by Bud'ko, Ni, and Canfield [Phys. Rev. B 79, 220516(R) (2009)]. Thus, it may be possible that there exists a minimum of two different kinds of IBSs distinguished by their low $T_c$, nodal, and $\Delta C/T_c$ behavior.

NiO${}_6$ octahedral tilts in the LaNiO${}_3$/SrTiO${}_3$ superlattices are quantified using position averaged convergent beam electron diffraction in scanning transmission electron microscopy. It is shown that maintaining oxygen octahedra connectivity across the interface controls the octahedral tilts in the LaNiO${}_3$ layers, their lattice parameters, and their transport properties. Unlike films and layers that are connected on one side to the substrate, subsequent LaNiO${}_3$ layers in the superlattice exhibit a relaxation of octahedral tilts towards bulk values. This relaxation is facilitated by tilts in the SrTiO${}_3$ layers and is correlated with the conductivity enhancement of the LaNiO${}_3$ layers in the superlattices relative to individual films.

Morphotropic phase boundary (MPB) systems are characterized by the coexistence of two ferroelectric phases and are associated with anomalous piezoelectric properties. In general, such coexistence is brought about by composition induced ferroelectric-ferroelectric instability. Here we demonstrate that a pure ferroelectric compound Na${}_{1/2}$Bi${}_{1/2}$TiO${}_3$ (NBT) exhibits the coexistence of two ferroelectric phases, rhombohedral ($R$3$c$) and monoclinic ($Cc$), in its equilibrium state at room temperature. This was unravelled by adopting a unique strategy of comparative structural analysis of electrically poled and thermally annealed specimens using high resolution synchrotron x-ray powder diffraction data. The relative fraction of the coexisting phases was found to be highly sensitive to thermal, mechanical, and electrical stimuli. The coexistence of ferroelectric phases in the ground state of the pure compound will have significant bearing on the way MPB is induced in NBT-based lead-free piezoceramics.

We present magnetization and Hall effect measurements on the pyrochlore iridate Nd${}_2$Ir${}_2$O${}_7$. Previous muon spin rotation measurements have shown that the system undergoes an unusual transition at $T_M\approx 110$ K into a magnetic phase lacking long-range order, followed by a transition at $T_{\mathrm{LRO}}\approx 6$ K into a state with long-range magnetic order. We observe a small remnant magnetization when cycling through a zero magnetic field at temperatures below $T_M$. Below $T_{\mathrm{LRO}}$, this remnant magnetization increases sharply, and additional hysteresis effects appear at a higher field $B_c$ $=$ 2.5 T, while the Hall resistance develops a nonmonotonic and hysteretic magnetic field dependence, with a maximum at $B_c$ and signatures of an anomalous Hall effect. The dependence on field sweep direction demonstrates a nontrivial transition into a magnetically ordered state with properties paralleling those of known spin-ice systems and suggests a spin reorientation transition across the metal-insulator transition in the $A$-227 series.

We describe a “disordered local moment” first-principles electronic structure theory which demonstrates that tricritical metamagnetism can arise in an antiferromagnetic metal due to the dependence of local moment interactions on the magnetization state. Itinerant electrons can therefore play a defining role in metamagnetism in the absence of large magnetic anisotropy. Our model is used to accurately predict the temperature dependence of the metamagnetic critical fields in CoMnSi-based alloys, explaining the sensitivity of metamagnetism to Mn-Mn separations and compositional variations found previously. We thus provide a finite-temperature framework for modeling and predicting different metamagnets of interest in applications such as magnetic cooling.

Through partial substitution of Sm by U in SmFeAsO, a different member of the family of iron-based superconductors was successfully synthesized. X-ray diffraction measurements show that the lattice constants along the $a$ and $c$ axes are both squeezed through U doping, indicating a successful substitution of U at the Sm site. The parent compound shows a strong resistivity anomaly near 150 K, associated with spin-density-wave instability. U doping suppresses this instability and leads to a transition to the superconducting state at temperatures up to 49 K. Magnetic measurements confirm the bulk superconductivity in this system. For the sample with a doping level of $x=0.2$, the external magnetic field suppresses the onset temperature very slowly, indicating a rather high upper critical field. In addition, the Hall effect measurements show that U clearly dopes electrons into the material.

We investigate the oxygen octahedral rotations that occur in two perovskites, SrTiO${}_3$ and PbTiO${}_3$, as a function of applied three-dimensional electric displacement field, allowing us to map out the phase diagram of rotations in both the paraelectric and ferroelectric regions of the polar response. First-principles calculations at fixed electric displacement field are used to extract parameters of a Landau-Devonshire model that is analyzed to identify the phase boundaries between different rotational states. The calculations reveal a rich phase diagram of rotations versus applied field in both SrTiO${}_3$ and PbTiO${}_3$, although the details are quite different in the two cases.

First-order antiferromagnetic insulating (AFI)-ferromagnetic metallic (FMM) transition as a function of temperature and magnetic field has been imaged using magnetic force microscopy (MFM) in La${}_{5/8-y}$Pr${}_y$Ca${}_{3/8}$MnO${}_3$ ($y=0.45$) thin film grown on ${\text{NdGaO}}_3$ substrate. These images, showing AFI to FMM transformation during in-field cooling and reverse transformation during warming of the same region, cannot be described solely in terms of the broad first-order transition due to quench disorder. Growth morphology of the FMM phase is observed to be path dependent as larger size FMM regions (but with smaller net fraction) are observed for isothermal field induced transition when compared to isofield thermal transition. This is shown to be a consequence of the glasslike arrested AFI state even at $\approx$100 K. Apparent variable range hopping like transport near $T_{\text{MI}}$ during cooling is shown to be a consequence of growth of the FMM phase in an AFI matrix.

In the low-temperature regime where the thermal polarization $P$ is of order unity and spin-lattice relaxation is “frozen out,” resonator-induced relaxation can be used to polarize a nuclear-spin sample for optimal detection sensitivity. We characterize the potential of resonator-induced polarization for enhancing the sensitivity of nuclear-magnetic-resonance spectroscopy. The sensitivities of two detection schemes are compared, one involving detection of a polarized sample dipole and the other involving detection of spin-noise correlations in an unpolarized sample. In the case where the dominant noise source is instrument noise associated with resonator fluctuations and with detection of the mechanical motion, a simple criterion can be used to compare the two schemes. Polarizing the sample improves sensitivity when $P$ is larger than the signal-to-noise ratio for detection of a fully-polarized spin during a single transient. Even if the instrument noise is decreased to a level near the quantum-mechanical limit, it is larger than spin noise for unpolarized samples containing up to a few tens of nuclei. Under these conditions, spin polarization of order unity would enhance spectroscopic detection sensitivity by an order of magnitude or more. In the limiting case where signal decay is due to resonator-induced dissipation during ideal spin locking, and where resonator fluctuations are the noise source, the only parameter of the spin-resonator system that affects the sensitivity per spin is the ratio of frequency to temperature. A balance between the coupling strength, the noise power, and the signal lifetime causes the cancellation of other parameters from the sensitivity formula. Partial cancellation of parameters, associated with a balance between the same three quantities, occurs more generally when the resonator is both the dominant noise source and the dominant source of signal decay. An intrinsic sensitivity limit exists for resonant detection of coherent spin evolution, due to the fact that the detector causes signal decay by enhancing the spins' spontaneous emission. For a single-spin sample, the quantum-limited signal-to-noise ratio for resonant detection is $1/3$. In contrast to the sensitivity, the time required for sample polarization between transients depends strongly on resonator parameters. We discuss resonator design and show that for a torsional resonator, the coupling is optimal when the resonator's magnetization remains aligned with the applied field during the mechanical oscillations.

We consider the appearance of vortices in a two-dimensional (2D) superconducting film exposed to a nonuniform magnetic field ${\mathit{B}}_a$ produced by a nearby coil. The film has “infinite” radius $R_f$ and thickness $t$ about equal to the coherence length ξ. The coil is approximated as a point dipole. We find that the first vortex-bearing state to appear has both a vortex and an antivortex. The Gibbs free energy of this state is lower than the vortex-free state when the maximum applied perpendicular field, i.e., the applied field $B_0$ at the origin, exceeds the external critical field $B_{c1}^0=\frac{4\sqrt{2}\Lambda }{R}\frac{{\Phi }_0}{4\pi {\Lambda }^2}\ln \left(\frac{\Lambda }{\xi }\right)$, where $\frac{{\Phi }_0}{4\pi {\Lambda }^2}\ln \left(\frac{\Lambda }{\xi }\right)\equiv B_{c1}^{2D}$ is the intrinsic critical field in 2D, $\Lambda \equiv 2{\lambda }^2/t$ is the 2D penetration depth introduced by Pearl, and λ is the bulk penetration depth. The prefactor $4\sqrt{2}\Lambda /R$ is calculated in the strong-screening regime $\Lambda /R\ll 1$. $R$ is the radial distance at which the applied perpendicular field $B_{a,z}\left(\rho \right)$ changes sign. In the lab, the onset of vortex effects generally occurs at a field much higher than $B_{c1}^0$, indicating that vortices are inhibited by the vortex-antivortex unbinding barrier, or by pinning.

The mixed-state Hall effect was examined in a Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ epitaxial film with a high critical current density. The transverse resistivity ${\rho }_{xy}$ and the longitudinal resistivity ${\rho }_{xx}$ follow power law scaling ${\rho }_{xy}$ $=$ $A{\rho }_{xx}$${}^{\beta }$. In the temperature sweep with a fixed field ($T$ sweep), all of the β values are independent of magnetic field up to 9 T, and are lower than 2.0 (around 1.8). In contrast, the β values in the magnetic-field sweep with a fixed temperature ($H$ sweep) change from 1.8 to 2.0 as the temperature increases from 13 to 16 K even in the $T$/$H$ region that overlaps with the $T$ sweep measurements. These results indicate that the vortices introduced at low temperatures are trapped by strong pinning centers, but a portion of the vortices introduced at high temperatures are not strongly trapped by the pinning centers. The sign of ${\rho }_{xy}$ is negative, and a sign reversal is not detected. These distinct scaling behaviors, which sharply contrast cuprates and MgB${}_2$, are explained by high-density pinning centers in the Ba(Fe${}_{1-x}$Co${}_x$)${}_2$As${}_2$ epitaxial film and are consistent with a wider vortex liquid phase.

The angular dependence of the x-ray absorption near-edge structure (XANES) was recorded at the calcium $K$ edge in three graphite intercalated compounds: CaC${}_6$, Li${}_{3.1}$Ca${}_{2.1}$C${}_6$ (phase $\beta$), and Li${}_{0.4}$Ca${}_{2.7}$C${}_6$ (phase $\alpha$). The polarized XANES spectra provide experimental pictures of the Ca $p_{x,y}$ and $p_z$ empty states of these three materials. In the case of CaC${}_6$, first-principles calculations based on density functional theory were performed, including XANES modeling and projected density-of-state simulations. A good agreement is obtained between theoretical and experimental polarized spectra. This enables us to assign the main XANES peaks in terms of orbital hybridization. The thorough analysis of the CaC${}_6$ polarized XANES spectra is used to interpret the spectral differences observed between the three compounds. The absence of pre-edge for all these phases indicates that Ca atoms are in locally centrosymmetric sites. The presence of Li in the graphitic interlayer drastically reduces the anisotropy of the Ca $p$ empty states, as compared to CaC${}_6$. Structural and electronic information is extracted from the spectra of the $\alpha$ and $\beta$ phases, leading to substantial results, in absence of fully determined crystallographic structures.

Stimulated by the small/large Fermi surface controversy in the cuprates, we consider a small number of holes injected into the bilayer antiferromagnet. The system has an $O\left(3\right)$ quantum critical point (QCP) separating the magnetically ordered and the magnetically disordered phases. We demonstrate that nearly critical quantum magnetic fluctuations can change the Fermi surface topology and also lead to spin charge separation (SCS) in two dimensions. We demonstrate that in the physically interesting regime there is a magnetically driven Lifshitz point (LP) inside the magnetically disordered phase. At the LP the topology of the hole Fermi surface is changed. The position of the LP, while being close to the position of the QCP, generally differs. Dependent on the additional hole hopping integrals $t^{\prime }$ and $t^{\prime \prime }$, the LP can be located in the magnetically ordered phase and/or in the magnetically disordered phase. We also demonstrate that in this regime the hole spin and charge necessarily separate when approaching the QCP. The considered model sheds light on generic problems concerning the physics of the cuprates.

The availability of novel technologies that enable the fabrication of bulk metamaterials made it necessary to develop a theoretical language to discuss their properties appropriately. Here we use the fundamental Bloch mode approximation to explore bulk properties of metamaterials that operate at terahertz frequencies. We demonstrate that this approximation is valid in the case of terahertz meta-foil metamaterials. While relying on theoretical and experimental results, we show that the fundamental Bloch mode approximation can explain minuscule details that are unique to the bulk metamaterial. Various applications such as cylindrical lenses, terahertz cloaks, and sensors will benefit from our analysis.

The random phase approximation (RPA) is attracting renewed interest as a universal and accurate method for first-principles total energy calculations. The RPA naturally accounts for long-range dispersive forces without compromising accuracy for short-range interactions making the RPA superior to semilocal and hybrid functionals in systems dominated by weak van der Waals or mixed covalent-dispersive interactions. In this work, we present plane-wave-based RPA calculations for a broad collection of systems with bond types ranging from strong covalent to van der Waals. Our main result is the RPA potential energy surfaces of graphene on the Cu(111), Ni(111), Co(0001), Pd(111), Pt(111), Ag(111), Au(111), and Al(111) metal surfaces, which represent archetypical examples of metal-organic interfaces. Comparison with semilocal density approximations and a nonlocal van der Waals functional show that only the RPA captures both the weak covalent and dispersive forces, which are equally important for these systems. We benchmark our implementation in the gpaw electronic structure code by calculating cohesive energies of graphite and a range of covalently bonded solids and molecules as well as the dissociation curves of H2 and H2$+$. These results show that the RPA with orbitals from the local density approximation suffers from delocalization errors and systematically underestimates covalent bond energies yielding similar or lower accuracy than the Perdew-Burke-Ernzerhof (PBE) functional for molecules and solids, respectively.

Electronic structures of $5d$ transition-metal oxide BaIrO${}_3$ are investigated by using a first-principles method. The system is found to be an exotic spin-orbital Mott insulator in multiple $J_{\text{eff}}=1/2$ states, achieved only after spin-orbit coupling and a modest on-site Coulomb interaction are taken into account simultaneously. The multiplicity of $J_{\text{eff}}=1/2$ states is associated with the unique Ir${}_3$O${}_{12}$ structural units in the crystal. These results explain well the nonmetallic behavior of the system observed in experiments. The magnetic state is found to be weak and noncollinear. Our work provides essential understandings on the electronic states of this type of $5d$ transition-metal oxide.

We present angle-resolved photoemission data from Cu(111). Using a focused 6 eV continuous-wave laser for photoexcitation, we achieve a high effective momentum resolution, enabling detection of the Rashba spin splitting in the Shockley surface state on Cu(111). The magnitude of the spin splitting of $\Delta k\sim 0.006$ Å${}^{-1}$ is surprisingly large and exceeds values predicted for the analogous surface state on Ag(111), but is reproduced by first-principles calculations. We further resolve a kink in the dispersion, which we attribute to electron-phonon coupling.

We report a series of electronic-structure calculations for CrN films within the LDA + $U$ method. In the bulk, it was found previously that with the onset of antiferromagnetic order, a gap opens and an insulating state appears. However, for thin films with increasing thickness (4–10 layers), we find that starting with a critical thickness of 10 (cubic symmetry) or 6 layers (orthorhombic) the gap closes and conducting states appear. The appearance of metallic states is connected with a structural relaxation at the surface, where Cr (N) atoms buckle inside (outside), forming an effective dipole moment. With CrN being a low-gap system, the electric dipoles at the surface caused by the Cr atoms displacing inwards shift the bands around the Fermi level significantly enough to drive those thin films metallic. The potential shift due to these surface dipoles is also visible in Cr and O core-level shifts.