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MgCr$_{2}$O$_{4}$ is one of the best-known realizations of the pyrochlore-lattice Heisenberg antiferromagnet. The strong antiferromagnetic exchange interactions are perturbed by small further-neighbor exchanges such that this compound may in principle realize a spiral spin liquid (SSL) phase in the zero-temperature limit. However, a spin Jahn-Teller transition below $T_{\rm N} \approx 13$ K yields a complicated long-range magnetic order with multiple coexisting propagation vectors. We present neutron scattering and thermo-magnetic measurements of MgCr$_{2}$O$_{4}$ samples under applied hydrostatic pressure up to $P=1.7$ GPa demonstrating the existence of multiple close-lying nearly degenerate magnetic ground states. We show that the application of hydrostatic pressure increases the ordering temperature by around 0.8 K per GPa and increases the bandwidth of the magnetic excitations by around 0.5 meV per GPa. We also evidence a strong tendency for the preferential occupation of a subset of magnetic domains under pressure. In particular, we show that the $\boldsymbol{k}=(0,0,1)$ magnetic phase, which is almost negligible at ambient pressure, dramatically increases in spectral weight under pressure. This modifies the spectrum of magnetic excitations, which we interpret unambiguously as spin waves from multiple magnetic domains. Moreover, we report that the application of pressure reveals a feature in the magnetic susceptibility above the magnetostructural transition. We interpret this as the onset of a short-range ordered phase associated with $\boldsymbol{k}=(0,0,1)$, previously not observed in magnetometry measurements., Comment: 19 pages including supplementary information, 10 main figures 6 supplementary figures

The study of CoNb$_2$O$_6$ sits at the confluence of simplicity and complexity: on one hand, the model for Ising chains -- the building blocks of CoNb$_2$O$_6$ -- in a transverse field, can be exactly solved and, thus, serves as an archetype of quantum criticality; on the other hand, the weak but nonzero interchain coupling adds geometric frustration to the stage, substantially complicating the phase diagram. Here we utilize low-temperature specific heat and thermal conductivity measurements to study the low-lying magnetic excitations in CoNb$_2$O$_6$ and its spin-1 analogue NiNb$_2$O$_6$. The thermal conductivity is found to be suppressed around the quantum critical point, where the specific heat is enhanced due to gapless magnetic excitations, pointing to the localized nature of the latter. These results highlight the predominant role of frustration in determining the quantum critical magnetic excitations of spin chains, which may furthermore underlie the remarkable similarities between the phenomenology of these spin-1/2 and spin-1 systems., Comment: 9 pages, 3 figures

The true topological nature of the Kondo insulator SmB$_6$ remains to be unveiled. Our previous tunneling study not only found evidence for the existence of surface Dirac fermions, but it also uncovered that they inherently interact with the spin excitons, collective excitations in the bulk. We have extended such a spectroscopic investigation into crystals containing a Sm deficiency. The bulk hybridization gap is found to be insensitive to the deficiency up to 1% studied here, but the surface states in Sm-deficient crystals exhibit quite different temperature evolutions from those in stoichiometric ones. We attribute this to the topological surface states remaining incoherent down to the lowest measurement temperature due to their continued interaction with the spin excitons that remain uncondensed. This result shows that the detailed topological nature of SmB$_6$ could vary drastically in the presence of disorder in the lattice. This sensitiveness to disorder is seemingly contradictory to the celebrated topological protection, but it can be understood as being due to the intimate interplay between strong correlations and topological effects., Comment: 17 pages, 4 figures

When degenerate states are separated by large energy barriers, the approach to thermal equilibrium can be slow enough that physical properties are defined by the thermalization process rather than the equilibrium. The exploration of thermalization pushes experimental boundaries and provides refreshing insights into atomic scale correlations and processes that impact steady state dynamics and prospects for realizing solid state quantum entanglement. We present a comprehensive study of magnetic relaxation in Ho$_2$Ti$_2$O$_7$ based on frequency-dependent susceptibility measurements and neutron diffraction studies of the real-time atomic-scale response to field quenches. Covering nearly ten decades in time scales, these experiments uncover two distinct relaxation processes that dominate in different temperature regimes. At low temperatures (0.6K

The Ising chain in transverse field is a paradigmatic model for a host of physical phenomena, including spontaneous symmetry breaking, topological defects, quantum criticality, and duality. Although the quasi-1D ferromagnet CoNb$_2$O$_6$ has been put forward as the best material example of the transverse field Ising model, it exhibits significant deviations from ideality. Through a combination of THz spectroscopy and theory, we show that CoNb$_2$O$_6$ in fact is well described by a different model with strong bond dependent interactions, which we dub the {\it twisted Kitaev chain}, as these interactions share a close resemblance to a one-dimensional version of the intensely studied honeycomb Kitaev model. In this model the ferromagnetic ground state of CoNb$_2$O$_6$ arises from the compromise between two distinct alternating axes rather than a single easy axis. Due to this frustration, even at zero applied field domain-wall excitations have quantum motion that is described by the celebrated Su-Schriefer-Heeger model of polyacetylene. This leads to rich behavior as a function of field. Despite the anomalous domain wall dynamics, close to a critical transverse field the twisted Kitaev chain enters a universal regime in the Ising universality class. This is reflected by the observation that the excitation gap in CoNb$_2$O$_6$ in the ferromagnetic regime closes at a rate precisely twice that of the paramagnet. This originates in the duality between domain walls and spin-flips and the topological conservation of domain wall parity. We measure this universal ratio `2' to high accuracy -- the first direct evidence for the Kramers-Wannier duality in nature., Comment: 6 pages, 3 figures; 5 pages, 7 figures (Supplemental)

The pyrochlore magnet Tb$_{2}$Ti$_{2}$O$_{7}$ shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform time-domain THz spectroscopy on high quality Tb$_{2}$Ti$_{2}$O$_{7}$ crystal and study the low energy excitations as a function of [111] magnetic field with high energy resolution. The low energy crystal field excitations change their energies anomalously under magnetic field. Despite several sharp field dependent changes, we show that the material's spectrum can be described not by a phase transitions, but by field dependent hybridization between the low energy crystal field levels. We highlight the strong coupling between spin and lattice degrees of freedom in Tb$_{2}$Ti$_{2}$O$_{7}$ as evidenced by the magnetic field tunable crystal field environment. Calculations based on single ion physics with field induced symmetry reduction of the crystal field environment can reproduce our data., Comment: 11 pages, 10 figures

In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb$_2$Ti$_2$O$_7$ as a putative U(1) QSL but a new class of ultra-pure single crystals make it abundantly clear the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for $\vec{B}\,\parallel\,\langle 100\rangle$, while field parallel to $\langle 110 \rangle$ and $\langle 111 \rangle$ enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb$_2$Ti$_2$O$_7$ in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of $T_{\mathrm{C}}$ and reentrant phase boundary by fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.

We document the coexistence of ferro- and anti-ferromagnetism in pyrochlore $\rm Yb_2Ti_2O_7$ using three neutron scattering techniques on stoichiometric crystals: elastic neutron scattering shows a canted ferromagnetic ground state, neutron scattering shows spin wave excitations from both a ferro-and an antiferro-magnetic state, and field and temperature dependent small angle neutron scattering reveals the corresponding anisotropic magnetic domain structure. High-field $\langle 111 \rangle$ spin wave fits show that $\rm Yb_2Ti_2O_7$ is extremely close to an antiferromagnetic phase boundary. Classical Monte Carlo simulations based on the interactions inferrred from high field spin wave measurements confirm $\psi_2$ antiferromagnetism is metastable within the FM ground state., Comment: 11 pages and 11 figures, 8 pages appendices with 10 additional figures

NiNb$_{2}$O$_{6}$ is an almost ideal realization of a 1D spin-1 ferromagnetic Heisenberg chain compound with weak unidirectional anisotropy. Using time-domain THz spectroscopy, we measure the low-energy electrodynamic response of NiNb$_{2}$O$_{6}$ as a function of temperature and external magnetic field. At low temperatures, we find a magnon-like spin-excitation, which corresponds to the lowest energy excitation at $q\sim0$. At higher temperatures, we unexpectedly observe a temperature-dependent renormalization of the spin-excitation energy, which has a strong dependence on field direction. Using theoretical arguments, exact diagonalizations and finite temperature dynamical Lanczos calculations, we construct a picture of magnon-magnon interactions that naturally explains the observed renormalization. This unique scenario is a consequence of the spin-1 nature and has no analog in the more widely studied spin-1/2 systems., Comment: Includes both main text and supplementary

In this letter, we report the gigantic thermal Hall effect $\kappa_{xy}$ in the low-field correlated-paramagnetic state of the frustrated pyrochlore Yb$_2$Ti$_2$O$_7$. We observed a record magnitude for the thermal Hall angle in an insulator, $\left|\kappa_{xy}\right|/\kappa_{xx}\sim 2\,\%$. The signal onsets at $T\sim 3\,$K and is severely weakened around the transition to canted ferromagnetic order at $T_\text{CFM}=0.275\,$K. Besides the large $\kappa_{xy}>0$ of the fluctuating regime, a sign change towards negative $\kappa_{xy}$ occurs at the lowest temperatures and in moderate field, where sharp magnon excitations appear in the inelastic neutron scattering spectra. We analyze the magnon-Hall signal and its suppression with field semi-quantitatively. A contribution of phonon skew scattering to $\kappa_{xy}$ is ruled out by a comparison of $\kappa_{xx}$ for Tb-, Yb-, and Y-based rare earth pyrochlore titanates. These results represent the first report of non-vanishing $\kappa_{xy}$ measured in a dilution refrigerator ($T < 0.29\,$K)., Comment: 13 pages, 4 figures

Pyrochlore erbium titanate samples in the form of powders (with nominal compositions of Er2Ti2+xO7) and bulk single crystals were prepared to elucidate the effect of synthesis and growth conditions on their composition, structure, and transition temperature. All samples were characterized using X-ray diffraction and specific heat measurements. Larger lattice parameters were measured for the Ti-deficient stuffed powders, Er2(Ti2-xErx)O7-{\delta}, while Ti-rich anti-stuffed powders, (Er2-xTix)Ti2O7+{\delta}, showed a decrease in lattice parameter. The lattice parameter for the stoichiometrically synthesized powder, Er2Ti2O7, was measured to be a = 10.07522(9) {\AA}. Single crystals grown by the conventional floating zone (FZ) technique were Ti deficient (stuffed) and darker in color due to oxygen vacancies. Using the traveling solvent floating zone (TSFZ), a high structural quality, transparent and stoichiometric Er2Ti2O7 single crystal was grown at a lower temperature using the TiO2 solvent. Heat capacity measurements of the TSFZ grown Er2Ti2O7 crystal and stoichiometric powder exhibited a very close magnetic transition temperature TC ~1.23K that was suppressed in off-stoichiometric samples, demonstrating correlations between stoichiometry and ground state magnetism in Er2Ti2O7.

We report a comprehensive inelastic neutron-scattering study of the frustrated pyrochlore antiferromagnet MgCr2O4 in its cooperative paramagnetic regime. Theoretical modeling yields a microscopic Heisenberg model with exchange interactions up to third-nearest neighbors, which quantitatively explains all the details of the dynamic magnetic response. Our work demonstrates that the magnetic excitations in paramagnetic MgCr2O4 are faithfully represented in the entire Brillouin zone by a theory of magnons propagating in a highly-correlated paramagnetic background. Our results also suggest that MgCr2O4 is proximate to a spiral spin-liquid phase distinct from the Coulomb phase, which has implications for the magneto-structural phase transition in MgCr2O4 ., Comment: 6 pages, 3 figures; supplementary material: 12 pages with figures

Samarium hexaboride is a topological Kondo insulator, with metallic surface states manifesting from its insulating band structure. Since the insulating bulk itself is driven by strong correlations, both the bulk and surface host compelling magnetic and electronic phenomena. We employed X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Sm M$_{4,5}$ edges to measure surface and bulk magnetic properties of Sm$^{2+}$ and Sm$^{3+}$ within SmB$_6$. We observed anti-alignment to the applied field of the Sm$^{3+}$ magnetic dipole moment below $T = 75$ K and of the total orbital moment of samarium below 30 K. The induced Sm$^{3+}$ moment at the cleaved surface at 8 K and 6 T implies 1.5% of the total Sm as magnetized Sm$^{3+}$. The field dependence of the Sm$^{3+}$ XMCD dichorism at 8 K is diamagnetic and approximately linear. The bulk magnetization at 2 K is however driven by Sm$^{2+}$ Van Vleck susceptibility as well as 1% paramagnetic impurities with $\mu_{\rm Eff} = 5.2(1)~\mu_{\rm B}$. This indicates diamagnetic Sm$^{3+}$ is compensated within the bulk. The XAS and XMCD spectra are weakly affected by Sm vacancies and carbon doping while XAS is strongly affected by polishing.

We report the detailed optical properties of Cd$_3$As$_2$ crystals in a wide parameter space: temperature, magnetic field, carrier concentration and crystal orientation. We investigate high-quality crystals synthesized by three different techniques. In all the studied samples, independently of how they were prepared and how they were treated before the optical experiments, our data indicate conspicuous fluctuations in the carrier density (up to 30%). These charge puddles have a characteristic scale of 100 $\mu$m, they become more pronounced at low temperatures, and possibly, they become enhanced by the presence of crystal twinning. The Drude response is characterized by very small scattering rates ($\sim 1$ meV) for as-grown samples. Mechanical treatment, such as cutting or polishing, influences the optical properties of single crystals, by increasing the Drude scattering rate and also modifying the high frequency optical response. Magneto-reflectivity and Kerr rotation are consistent with electron-like charge carriers and a spatially non-uniform carrier density., Comment: Accepted in Physical Review B

The thermal conductivity $\kappa$ of the Kondo insulator SmB$_6$ was measured at low temperature, down to 70 mK, in magnetic fields up to 15 T, on single crystals grown using both the floating-zone and the flux methods. The residual linear term $\kappa_0/T$ at $T \to 0$ is found to be zero in all samples, for all magnetic fields, in agreement with previous studies. There is therefore no clear evidence of fermionic heat carriers. In contrast to some prior data, we observe a large enhancement of $\kappa(T)$ with increasing field. The effect of field is anisotropic, depending on the relative orientation of field and heat current (parallel or perpendicular), and with respect to the cubic crystal structure. We interpret our data in terms of heat transport predominantly by phonons, which are scattered by magnetic impurities., Comment: publish version

Recent experiments have uncovered evidence of low energy excitations in the bulk of SmB$_6$ that are perhaps associated with unconventional quasiparticles, bringing into question whether this Kondo "insulator" is truly insulating in the bulk. Recently, we demonstrated that SmB$_6$ possesses significant in-gap bulk ac conduction far in excess of typical disordered semiconductors. Whether such conduction is an intrinsic feature of SmB$_6$, suggesting the formation of an exotic state, or residual conduction from impurities continues to be a topic of debate. Here, we further examine the origin of the ac optical conductivity of SmB$_6$ in light of recent experimental and theoretical developments. The optical conductivity of SmB$_6$ is shown to possess distinct regimes of either dominant free carrier or $localized$ response contributions. The free carrier response is found to be in good qualitative agreement with previous literature, although quantitative differences are revealed and discussed. The localized response, which dominates at the lowest temperatures, is analyzed in the context of models of either in-gap impurity states or an exotic neutral Fermi surface. The charge density or effective mass of this low temperature in-gap conductivity is extracted through a conductivity sum rule analysis and found to be in general alignment with both models in the appropriate limits. Our results shed further light on the nature of the in-gap states of this remarkable material., Comment: Submitted for the proceedings of the 2017 Strongly Correlated Electron Symposium

We present a magnetic phase diagram of rare-earth pyrochlore $\rm{Yb_2Ti_2O_7}$ in a $\langle 111 \rangle$ magnetic field. Using heat capacity, magnetization, and neutron scattering data, we show an unusual field-dependence of a first-order phase boundary, wherein a small applied field increases the ordering temperature. The zero-field ground state has ferromagnetic domains, while the spins polarize along $\langle 111 \rangle$ above 0.65T. A classical Monte Carlo analysis of published Hamiltonians does account for the critical field in the low T limit. However, this analysis fails to account for the large bulge in the reentrant phase diagram, suggesting that either long-range interactions or quantum fluctuations govern low field properties., Comment: 5 pages, 5 pages supplementary information

A series of Yb2Ti2O7 doped samples demonstrates the effects of off-stoichiometry on Yb2Ti2O7's structure, properties, and magnetic ground state via x-ray diffraction, specific heat, and magnetization measurements. A stoichiometric single crystal of Yb2Ti2O7 grown by the traveling solvent floating zone technique (solvent = 30 wt% rutile TiO2 and 70 wt% Yb2Ti2O7) is characterized and evaluated in light of this series. Our data shows that upon positive x doping, the cubic lattice parameter a increases and the Curie-Weiss temperature decreases. Heat capacity measurements of stoichiometric Yb2Ti2O7 samples exhibit a sharp, first-order peak at T = 268(4) mK that is suppressed in magnitude and temperature in samples doped off ideal stoichiometry. The full entropy recovered per Yb ion is 5.7 J/K ~ Rln2. Our work establishes the effects of doping on Yb2Ti2O7's physical properties, which provides further evidence indicating that previous crystals grown by the traditional floating zone method are doped off ideal stoichiometry. Additionally, we present how to grow high-quality colorless single crystals of Yb2Ti2O7 by the traveling solvent floating zone growth method., Comment: 11 pages, 6 figures, 1 table (including Appendix - 2 pages, 1 figure, 1 table)

Inelastic neutron scattering reveals a broad continuum of excitations in Pr$_2$Zr$_2$O$_7$, the temperature and magnetic field dependence of which indicate a continuous distribution of quenched transverse fields ($\Delta$) acting on the non-Kramers Pr$^{3+}$ crystal field ground state doublets. Spin-ice correlations are apparent within 0.2 meV of the Zeeman energy. A random phase approximation provides an excellent account of the data with a transverse field distribution $\rho(\Delta)\propto (\Delta^2+\Gamma^2)^{-1}$ where $ \Gamma=0.27(1)$ meV. Established during high temperature synthesis due to an underlying structural instability, it appears disorder in Pr$_2$Zr$_2$O$_7$ actually induces a quantum spin liquid., Comment: 5 pages, 4 figures

The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures, T$<5$ K. In this temperatures range, a plateau is observed in the dc resistivity, contrary to the exponential divergence expected for a gapped system. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator (TKI) and propose that the residual conductivity is due to topological surface states which reside within the Kondo gap. Since the TKI prediction many experiments have claimed to observe high mobility surface states within a perfectly insulating hybridization gap. Here, we investigate the low energy optical conductivity within the hybridization gap of single crystals of SmB$_6$ via time domain terahertz spectroscopy. Samples grown by both optical floating zone and aluminum flux methods are investigated to probe for differences originating from sample growth techniques. We find that both samples display significant 3D bulk conduction originating within the Kondo gap. Although SmB$_6$ may be a bulk dc insulator, it shows significant bulk ac conduction that is many orders of magnitude larger than any known impurity band conduction. The nature of these in-gap states and their coupling with the low energy spin excitons of SmB$_6$ is discussed. Additionally, the well defined conduction path geometry of our optical experiments allows us to show that any surface states, which lie below our detection threshold if present, must have a sheet resistance of R$/\square \ge$ 1000 $\Omega$., Comment: 10 pages, 3 figures

A description is given of the traveling solvent technique, which has been used for the crystal growth of both congruently and incongruently melting materials of many classes of intermetallic, chalcogenide, semiconductor and oxide materials. The use of a solvent, growth at lower temperatures and the zoning process, that are inherent ingredients of the method, can help to grow large, high structural quality, high purity crystals. In order to optimize this process, careful control of the various growth variables is imperative; however, this can be difficult to achieve due to the large number of independent experimental parameters that can be grouped under the broad headings growth conditions, characteristics of the material being grown, and experimental configuration, setup and design.This review attempts to describe the principles behind the traveling solvent technique and the various experimental variables. Guidelines are detailed to provide the information necessary to allow closer control of the crystal growth process through a systematic approach. Comparison is made between the traveling solvent technique and other crystal growth methods, in particular the more conventional stationary flux method. The use of optical heating is described in detail and successful traveling solvent growth by optical heating is reported for the first time for crystals of $Tl_5Te_3$, $Cd_3As_2$, and $FeSc_2S_4$ (using Te, Cd and FeS fluxes, respectively)., Comment: 13 pages, 5 figures, in press in Progress in Crystal Growth and Characterization of Materials, 06 May 2016

Recent theoretical and experimental findings suggest that the long-known but not well understood low temperature resistance plateau of SmB6 may originate from protected surface states arising from a topologically non-trivial bulk band structure having strong Kondo hybridization. Yet other studies have ascribed this feature to impurity phases, sample vacancies, and surface reconstructions. Given the typical methods used to prepare SmB6 single crystals, the flux and floating zone procedures, these ascriptions should not be taken lightly. Here, we demonstrate how compositional variations and observable amounts of impurity phases in SmB6 crystals grown by floating zone and flux affect the physical properties. From neutron diffraction and X-ray computed tomography experiments, we observe that a 154Sm11B6 crystal prepared using aluminum flux contains co-crystallized, epitaxial aluminum. A large, nearly stoichiometric crystal of SmB6 was successfully grown using the float-zone technique; upon continuing the zone melting, samarium vacancies are introduced. These samarium vacancies drastically alter the resistance and plateauing magnitude of the low temperature resistance compared to stoichiometric SmB6. These results highlight that small presences of impurity phases and compositional variations must be considered when collecting and analyzing physical property data of SmB6. Finally, a more accurate samarium-154 coherent neutron scattering length value, 8.9(1) fm, is reported.

We performed ultrafast time-resolved near-infrared pump, resonant soft X-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 +- 1.1 ps, which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wavevector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wavevector on these time scales., Comment: 9 pages, 4 figures

Using inelastic neutron scattering, we map a 14 meV coherent resonant mode in the topological Kondo insulator SmB6 and describe its relation to the low energy insulating band structure. The resonant intensity is confined to the X and R high symmetry points, repeating outside the first Brillouin zone and dispersing less than 2 meV, with a 5d-like magnetic form factor. We present a slave-boson treatment of the Anderson Hamiltonian with a third neighbor dominated hybridized band structure. This approach produces a spin exciton below the charge gap with features that are consistent with the observed neutron scattering. We find that maxima in the wave vector dependence of the inelastic neutron scattering indicate band inversion.

Pyrochlore $\rm Pr^{3+}_{2+x}Zr^{4+}_{2-x}O_{7-x/2}$ samples in the form of both powders $(-0.02 \le x \le 0.02)$ and bulk single crystals have been studied to elucidate the dependence of their magnetic, compositional and structural properties on synthesis and growth conditions. All samples were characterized using X-ray diffraction, specific heat, and DC magnetization measurements. The crystals were also studied using the X-ray Laue technique and scanning electron microscopy. Increasing the Pr content for the $\rm Pr_{2+x}Zr_{2-x}O_{7-x/2}$ powders enlarged the lattice parameter, and resulted in systematic changes in magnetic susceptibility and specific heat. Stoichiometric and high quality single crystals of $\rm Pr_2Zr_2O_7$ were grown using the optical floating zone technique under a high purity static argon atmosphere, to avoid inclusions of Pr$^{4+}$ and limit Pr vaporization. Increasing the growth speed was found to significantly reduce Pr vaporization for better control of stoichiometry. Scanning electron microscopy provided direct evidence of spinodal decomposition during growth that is controllable via rotation rate. An intermediate rotation rate of 3-6 rpm was found to produce the best microstructure. The magnetic susceptibility of crystals grown at rates from 1-20 mm/hr revealed changes that were consistent with Pr vaporization. Further, we report indications of local off-centering of Pr$^{3+}$ ions from the ideal pyrochlore sites, similar to what is known for the trivalent cation in $\rm Bi_2Ti_2O_7$ and $\rm La_2Zr_2O_7$. The effect varies with Pr content and radically modulates the low temperature specific heat. Overall, the results clearly demonstrate important correlations between the growth conditions and physical properties of $\rm Pr_2Zr_2O_7$ crystals.

We describe why Ising spin chains with competing interactions in $\rm SrHo_2O_4$ segregate into ordered and disordered ensembles at low temperatures ($T$). Using elastic neutron scattering, magnetization, and specific heat measurements, the two distinct spin chains are inferred to have N\'eel ($\uparrow\downarrow\uparrow\downarrow$) and double-N\'eel ($\uparrow\uparrow\downarrow\downarrow$) ground states respectively. Below $T_\mathrm{N}=0.68(2)$~K, the N\'eel chains develop three dimensional (3D) long range order (LRO), which arrests further thermal equilibration of the double-N\'eel chains so they remain in a disordered incommensurate state for $T$ below $T_\mathrm{S}= 0.52(2)$~K. $\rm SrHo_2O_4$ distills an important feature of incommensurate low dimensional magnetism: kinetically trapped topological defects in a quasi$-d-$dimensional spin system can preclude order in $d+1$ dimensions., Comment: 10 pages, 10 figures

In condensed matter systems, the formation of long range order (LRO) with broken symmetry is often accompanied by new types of excitations. However, in many magnetic pyrochlore oxides, geometrical frustration suppresses conventional LRO while at the same time non-trivial spin correlations are observed. For such materials, a natural question to ask then is what is the nature of the excitations in this highly correlated state without broken symmetry? Frequently the application of a symmetry breaking field can stabilize excitations whose properties still reflect certain aspects of the anomalous state without long-range order. Here we report evidence of novel magnetic excitations in the quantum spin ice material Yb$_2$Ti$_2$O$_7$, obtained from time-domain terahertz spectroscopy (TDTS). At large fields, both magnon and two-magnon-like excitations are observed in a $<$001$>$ directed magnetic field illustrating the stabilization of a field induced LRO state. The unique ability of TDTS to measure complex response functions allows a direct study of magnetic responses in different polarization channels, revealing the existence of an unusual left-hand polarized magnon. The g-factors of these excitations are dramatically enhanced in the low-field limit, showing a cross-over of these one- and two-magnon states into features consistent with quantum string-like excitations proposed to exist in quantum spin ice in a small $<$001$>$ applied field., Comment: 15 pages

The transverse Ising magnet Hamiltonian describing the Ising chain in a transverse magnetic field is the archetypal example of a system that undergoes a transition at a quantum critical point (QCP). The columbite CoNb$_2$O$_6$ is the closest realization of the transverse Ising magnet found to date. At low temperatures, neutron diffraction has observed a set of discrete collective spin modes near the QCP. We ask if there are low-lying spin excitations distinct from these relatively high energy modes. Using the heat capacity, we show that a significant band of gapless spin excitations exists. At the QCP, their spin entropy rises to a prominent peak that accounts for 30$\%$ of the total spin degrees of freedom. In a narrow field interval below the QCP, the gapless excitations display a fermion-like, temperature-linear heat capacity below 1 K. These novel gapless modes are the main spin excitations participating in, and affected, by the quantum transition., Comment: 14 pages total, 8 figures

Topological insulators are materials characterized by dissipationless, spin-polarized surface states resulting from non-trivial band topologies. Recent theoretical models and experiments suggest that SmB6 is the first topological Kondo insulator, in which the topologically non-trivial band structure results from electron-electron interactions via Kondo hybridization. Here, we report that the surface conductivity of SmB6 increases systematically with bulk carbon content. Further, addition of carbon is linked to an increase in n-type carriers, larger low temperature electronic contributions to the specific heat with a characteristic temperature scale of T* = 17 K, and a broadening of the crossover to the insulating state. Additionally, X-ray absorption spectroscopy shows a change in Sm valence at the surface. Our results highlight the importance of phonon dynamics in producing a Kondo insulating state and demonstrate a correlation between the bulk thermodynamic state and low temperature resistance of SmB6.

Kink bound states in the one dimensional ferromagnetic Ising chain compound CoNb$_2$O$_6$ have been studied using high resolution time-domain terahertz spectroscopy in zero applied magnetic field. When magnetic order develops at low temperature, nine bound states of kinks become visible. Their energies can be modeled exceedingly well by the Airy function solutions to a 1D Schr\"odinger equation with a linear confining potential. This sequence of bound states terminates at a threshold energy near two times the energy of the lowest bound state. Above this energy scale we observe a broad feature consistent with the onset of the two particle continuum. At energies just below this threshold we observe a prominent excitation that we interpret as a novel bound state of bound states -- two pairs of kinks on neighboring chains.

Resonant Bragg diffraction of soft, circularly polarized x-rays has been used to observe directly the temperature dependence of chiral-order melting in a motif of Mn ions in terbium manganate. The underlying mechanism uses the b-axis component of a cycloid, which vanishes outside the polar phase. Melting is witnessed by the first and second harmonics of a cycloid, and we explain why the observed temperature dependence is different in the two harmonics. Our direct observation of melting is supported by a solid foundation of evidence, derived from extensive studies of the azimuthal-angle dependence of intensities with both linear and circular polarization.

Materials with strong spin-orbit coupling have attracted attention following the prediction and subsequent discovery of strong two- and three-dimensional topological insulators in which a topological property of the bulk band structure of an insulator results in metallic surface states with Dirac-like dispersion. Here we report the discovery of Dirac-like surface states in the perovskite superconductor [Tl$_4$]TlTe$_3$ (Tl5Te3) and its non-superconducting tin-doped derivative, [Tl4](Tl$_{0.4}$Sn$_{0.6}$)Te$_3$, as observed by angle-resolved photoemission spectroscopy (ARPES). Density functional theory (DFT) calculations predict a single spin-orbit driven band parity inversion at the $Z$ point above the Fermi level of Tl5Te3, suggesting the surface states are protected by Z$_2$ topology. Calculations on [Tl$_4$]SnTe$_3$ show no parity inversions, implying that a topological transition from non-trivial to trivial must occur upon doping with tin, i.e., [Tl$_4$](Tl$_{1-x}$Sn$_{x}$)Te$_3$. Thus [Tl$_4$]{\it M}Te$_3$ perovskites are a possible new, non-trigonal class of Z$_2$ topological compounds. Additionally, as Tl5Te3 is a stoichiometric bulk superconductor, these perovskites are ideal materials in which to study the interplay between surface states and bulk superconductivity., Comment: Submitted to Nature Communications