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We present a single crystal neutron diffraction study of the magnetic short-range correlations in Tb$_5$Ge$_4$ which orders antiferromagnetically below the Neel temperature $T_N$ $approx$ 92 K. Strong diffuse scattering arising from magnetic short-range correlations was observed in wide temperature ranges both below and above $T_N$. The antiferromagnetic ordering in Tb$_5$Ge$_4$ can be described as strongly coupled ferromagnetic block layers in the $ac$-plane that stack along the b-axis with weak antiferromagnetic inter-layer coupling. Diffuse scattering was observed along both $a^*$ and $b^*$ directions indicating three-dimensional short-range correlations. Moreover, the $q$-dependence of the diffuse scattering is Squared-Lorentzian in form suggesting a strongly clustered magnetic state that may be related to the proposed Griffiths-like phase in Gd$_5$Ge$_4$.
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The magnetic ground state of the hyper-kagome lattice in Na4Ir3O8 is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen, state comprised of quasi-static moments develops below a characteristic temperature of T_F=6 K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasi-static, short-range, spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state arises in Na4Ir3O8 driven either by disorder inherent to the creation of the hyper-kagome lattice itself or stabilized via quantum fluctuations.
We present experimental results for the heavy-electron compound CeCu$_{4}$Ga which show that it possesses short-range magnetic correlations down to a temperature of $T = 0.1$ K. Our neutron scattering data show no evidence of long-range magnetic order occurring despite a peak in the specific heat at $T^{*} =1.2$ K. Rather, magnetic diffuse scattering occurs which corresponds to short-range magnetic correlations occurring across two unit cells. The specific heat remains large as $Tsim0$ K resulting in a Sommerfeld coefficient of $gamma_{0} = 1.44(2)$ J/mol-K$^{2}$, and, below $T^{*}$, the resistivity follows $T^{2}$ behavior and the ac magnetic susceptibility becomes temperature independent. A magnetic peak centered at an energy transfer of $E_{rm{c}}=0.24(1)$ meV is seen in inelastic neutron scattering data which shifts to higher energies and broadens under a magnetic field. We discuss the coexistence of large specific heat, magnetic fluctuations, and short-range magnetic correlations at low temperatures and compare our results to those for materials possessing spin-liquid behavior.
Angle-resolved photoemission spectroscopy (ARPES) is one of most powerful techniques to unravel the electronic properties of layered materials and in the last decades it has lead to a significant progress in the understanding of the band structures of cuprates, pnictides and other materials of current interest. On the other hand, its application to Mott-Hubbard insulating materials where a Fermi surface is absent has been more limited. Here we show that in these latter materials, where electron spins are localized, ARPES may provide significant information on the spin correlations which can be complementary to the one derived from neutron scattering experiments. Sr$_2$Cu$_{1-x}$Zn$_x$O$_2$Cl$_2$, a prototype of diluted spin $S=1/2$ antiferromagnet (AF) on a square lattice, was chosen as a test case and a direct correspondence between the amplitude of the spectral weight beyond the AF zone boundary derived from ARPES and the spin correlation length $xi$ estimated from $^{35}$Cl NMR established. It was found even for correlation lengths of a few lattice constants a significant spectral weight in the back-bended band is present which depends markedly on $xi$. Moreover the temperature dependence of that spectral weight is found to scale with the $x$ dependent spin-stiffness. These findings prove that ARPES technique is very sensitive to short-range correlations and its relevance in the understanding of the electronic correlations in cuprates is discussed.
We report on the electronic ground state of a layered perovskite vanadium oxide Sr$_2$VO$_4$ studied by the combined use of synchrotron radiation x-ray diffraction (SR-XRD) and muon spin rotation/relaxation ($mu$SR) techniques, where $mu$SR measurements were extended down to 30 mK. We found an intermediate orthorhombic phase between $T_{rm c2} sim$~130 K and $T_{rm c1} sim$~100 K, whereas a tetragonal phase appears for $T > T_{rm c2}$ and $T < T_{rm c1}$. The absence of long-range magnetic order was confirmed by $mu$SR at the reentrant tetragonal phase below $T_{rm c1}$, where the relative enhancement in the $c$-axis length versus that of the $a$-axis length was observed. However, no clear indication of the lowering of the tetragonal lattice symmetry with superlattice modulation, which is expected in the orbital order state with superstructure of $d_{yz}$ and $d_{zx}$ orbitals, was observed by SR-XRD below $T_{rm c1}$. Instead, it was inferred from $mu$SR that a magnetic state developed below $T_{rm c0} sim$~10 K, which was characterized by the highly inhomogeneous and fluctuating local magnetic fields down to 30 mK. We argue that the anomalous magnetic ground state below $T_{rm c0}$ originates from the coexistence of ferromagnetic and antiferromagnetic correlations.
Investigations of magnetically ordered phases on the femtosecond timescale have provided significant insights into the influence of charge and lattice degrees of freedom on the magnetic sub-system. However, short-range magnetic correlations occurring in the absence of long-range order, for example in spin-frustrated systems, are inaccessible to many ultrafast techniques. Here, we show how time-resolved resonant inelastic X-ray scattering (trRIXS) is capable of probing such short-ranged magnetic dynamics in a charge-transfer insulator through the detection of a Zhang-Rice singlet exciton. Utilizing trRIXS measurements at the O K-edge, and in combination with model calculations, we probe the short-range spin-correlations in the frustrated spin chain material CuGeO3 following photo-excitation, revealing a strong coupling between the local lattice and spin sub-systems.
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