arXiv subjects

J. T. Haraldsen

Publications and source records attributed to J. T. Haraldsen.

27 records · Page 2Linked to original sources

arXiv2011

Induced polarization at a paraelectric/superconducting interface

We examine the modified electronic states at the interface between superconducting and ferro(para)-electric heterostructures. We find that electric polarization $P$ and superconducting $ψ$ order parameters can be significantly modified due to coupling through linear terms brought about by explicit symmetry breaking at the interface. Using an effective action and a Ginzburg-Landau formalism, we show that an interaction term linear in the electric polarization will modify the superconducting order parameter $ψ$ at the interface. This also produces modulation of a ferroelectric polarization. It is shown that a paraelectric-superconductor interaction will produce an interface-induced ferroelectric polarization.

cond-mat.supr-con↗

arXiv2010

Identifying the multiferroic phase of doped CuFeO_2 using inelastic neutron scattering

We report inelastic neutron scattering measurements that provide a distinct dynamical "fingerprint" for the multiferroic ground state of 3.5% Ga-doped CuFeO_2. The complex ground state is stabilized by the displacement of the oxygen atoms, which are also responsible for the multiferroic coupling predicted by Arima. By comparing the observed and calculated spectrum of spin excitations, we conclude that the magnetic ground state is a distorted screw-type spin configuration that requires a mechanism for magnetoelectric coupling different from the generally accepted spin-current model.

cond-mat.str-el↗

arXiv2009

Spin Rotation Technique for Non-Collinear Magnetic Systems: Application to the Generalized Villain Model

This work develops a new generalized technique for determining the static and dynamic properties of any non-collinear magnetic system. By rotating the spin operators into the local spin reference frame, we evaluate the zeroth, first, and second order terms in a Holstein-Primakoff expansion, and through a Green's functions approach, we determine the structure factor intensities for the spin-wave frequencies. To demonstrate this technique, we examine the spin-wave dynamics of the generalized Villain model with a varying interchain interaction. The new interchain coupling expands the overall phase diagram with the realization of two non-equivalent canted spin configurations. The rotational Holstein-Primakoff expansion provides both analytical and numerical results for the spin dynamics and intensities of these phases.

cond-mat.str-el↗

arXiv2009

Critical Anisotropies of a Geometrically-Frustrated Triangular-Lattice Antiferromagnet

This work examines the critical anisotropy required for the local stability of the collinear ground states of a geometrically-frustrated triangular-lattice antiferromagnet (TLA). Using a Holstein-Primakoff expansion, we calculate the spin-wave frequencies for the 1, 2, 3, 4, and 8-sublattice (SL) ground states of a TLA with up to third neighbor interactions. Local stability requires that all spin-wave frequencies are real and positive. The 2, 4, and 8-SL phases break up into several regions where the critical anisotropy is a different function of the exchange parameters. We find that the critical anisotropy is a continuous function everywhere except across the 2-SL/3-SL and 3-SL/4-SL phase boundaries, where the 3-SL phase has the higher critical anisotropy.

cond-mat.str-el↗

arXiv2009

Spin-Wave Instabilities and Non-Collinear Magnetic Phases of a Geometrically-Frustrated Triangular-Lattice Antiferromagnet

This paper examines the relation between the spin-wave instabilities of collinear magnetic phases and the resulting non-collinear phases for a geometrically-frustrated triangular-lattice antiferromagnet in the high spin limit. Using a combination of phenomenological and Monte-Carlo techniques, we demonstrate that the instability wave-vector with the strongest intensity in the collinear phase determines the wave-vector of a cycloid or the dominant elastic peak of a more complex non-collinear phase. Our results are related to the observed multi-ferroic phase of Al-doped CuFeO$_2$.

cond-mat.str-el↗

arXiv2008

Spin-lozenge thermodynamics and magnetic excitations in Na$_3$RuO$_4$

We report inelastic and elastic neutron scattering, magnetic susceptibility, and heat capacity measurements of polycrystalline sodium ruthenate (Na$_3$RuO$_4$). Previous work suggests this material consists of isolated tetramers of $S=3/2$ Ru$^{5+}$ ions in a so-called "lozenge" configuration. Using a Heisenberg antiferromagnet Hamiltonian, we analytically determine the energy eigenstates for general spin $S$. From this model, the neutron scattering cross-sections for excitations associated with spin-3/2 spin-tetramer configurations is determined. Comparison of magnetic susceptibility and inelastic neutron scattering results shows that the proposed "lozenge" model is not distinctly supported, but provides evidence that the system may be better described as a pair of non-interacting inequivalent dimers, \textit{i.e} double dimers. However, the existence of long-range magnetic order below $T_c \approx 28$ K immediately questions such a description. Although no evidence of the lozenge model is observed, future studies on single crystals may further clarify the appropriate magnetic Hamiltonian.

cond-mat.mtrl-sci↗

arXiv2008

The Collinear Magnetic Phases of the Geometrically-Frustrated Antiferromagnet CuFeO$_2$: The Importance of Stacking

The correct stacking of hexagonal layers is used to obtain accurate estimates for the exchange and anisotropy parameters of the geometrically-frustrated antiferromagnet CuFeO$_2$. Those parameters are highly constrained by the stability of a collinear metamagnetic phase between fields of 13.5 and 20 T. Constrained fits of the spin-wave frequencies of the collinear $\uudd $ phase below 7 T are used to identify the magnetic unit cell of the metamagnetic $\uuudd $ phase, which contains two hexagonal layers and 10 Fe$^{3+}$ spins.

cond-mat.str-el↗

arXiv2006

Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We measured the magneto-optical response of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ in order to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. We discuss field- and temperature-induced trends within the framework of the Tomioka-Tokura global electronic phase diagram picture and suggest that controlled disorder near a phase boundary can be used to tune the magnetodielectric response. Remnants of the spin-glass insulator to ferromagnetic metallic transition can also drive 300 K color changes in (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$.

cond-mat.str-el↗

arXiv2004

Neutron Scattering and Magnetic Observables for S = 1/2 Molecular Magnets

In this paper we report results for magnetic observables of finite spin clusters composed of S=1/2 ions. We consider clusters of two, three and four spins in distinct spatial arrangements, with isotropic Heisenberg interactions of various strengths between ion pairs. In addition to the complete set of energy eigenvalues and eigenvectors, specific heat and magnetic susceptibility, we also quote results for the single crystal and powder average inelastic neutron scattering structure factors. Examples of the application of these results to experimental systems are also discussed.

cond-mat.str-el↗