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G. Sala

Publications and source records attributed to G. Sala.

At least 37 records · Page 2Linked to original sources
arXiv2019

Crystal field splitting, local anisotropy, and low energy excitations in the quantum magnet YbCl$_3$

We study the correlated quantum magnet, YbCl$_3$, with neutron scattering, magnetic susceptibility, and heat capacity measurements. The crystal field Hamiltonian is determined through simultaneous refinements of the inelastic neutron scattering and magnetization data. The ground state doublet is well isolated from the other crystal field levels and results in an effective spin-1/2 system with local easy plane anisotropy at low temperature. Cold neutron spectroscopy shows low energy excitations that are consistent with nearest neighbor antiferromagnetic correlations of reduced dimensionality.

cond-mat.str-el↗
arXiv2019

Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb$_2$Si$_2$O$_7$

The quantum dimer magnet (QDM) is the canonical example of quantum magnetism. The QDM state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced triplon Bose-Einstein condensate (BEC) phase. We report on a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb$_2$Si$_2$O$_7$. Our single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet ground state at zero field with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of $H_{c1} \sim 0.4$ T and $H_{c2} \sim 1.4$ T. Using inelastic neutron scattering in an applied magnetic field we observe a Goldstone mode (gapless to within $δE$ = 0.037 meV) that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well-known cases of this phase, the high-field ($μ$$_0$$H\geq1.2$T) part of the phase diagram in Yb$_2$Si$_2$O$_7$ is interrupted by an unusual regime signaled by a change in the field dependence of the ultrasound velocity and magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs.

cond-mat.str-el↗
arXiv2019

Antiferromagnetic ordering and dipolar interactions of YbAlO$_3$

In this paper we report low-temperature magnetic properties of the rare-earth perovskite material YbAlO$_3$. Results of elastic and inelastic neutron scattering experiment, magnetization measurements along with the crystalline electrical field (CEF) calculations suggest that the ground state of Yb moments is a strongly anisotropic Kramers doublet, and the moments are confined in the $ab$-plane, pointing at an angle of $φ= \pm 23.5^{\circ}$ to the $a$-axis. With temperature decreasing below $T_{\rm N}=0.88$ K, Yb moments order into the coplanar, but non-collinear antiferromagnetic (AFM) structure $AxGy$, where the moments are pointed along their easy-axes. In addition, we highlight the importance of the dipole-dipole interaction, which selects the type of magnetic ordering and may be crucial for understanding magnetic properties of other rare-earth orthorhombic perovskites. Further analysis of the broad diffuse neutron scattering shows that one-dimensional interaction along the $c$-axis is dominant, and suggests YbAlO$_3$ as a new member of one dimensional quantum magnets.

cond-mat.str-el↗
arXiv2019

Tomonaga-Luttinger Liquid Behavior and Spinon Confinement in YbAlO$_3$

Low dimensional quantum magnets are interesting because of the emerging collective behavior arising from strong quantum fluctuations. The one-dimensional (1D) S = 1/2 Heisenberg antiferromagnet is a paradigmatic example, whose low-energy excitations, known as spinons, carry fractional spin S = 1/2. These fractional modes can be reconfined by the application of a staggered magnetic field. Even though considerable progress has been made in the theoretical understanding of such magnets, experimental realizations of this low-dimensional physics are relatively rare. This is particularly true for rare-earth based magnets because of the large effective spin anisotropy induced by the combination of strong spin-orbit coupling and crystal field splitting. Here, we demonstrate that the rare-earth perovskite YbAlO$_3$ provides a realization of a quantum spin S = 1/2 chain material exhibiting both quantum critical Tomonaga-Luttinger liquid behavior and spinon confinement-deconfinement transitions in different regions of magnetic field-temperature phase diagram.

cond-mat.str-el↗
arXiv2018

Physical properties of the trigonal binary compound Nd$_2$O$_3$

We have studied the physical properties of Nd$_2$O$_3$ with neutron diffraction, inelastic neutron scattering, heat capacity, and magnetic susceptibility measurements. Nd$_2$O$_3$ crystallizes in a trigonal structure, with Nd$^{3+}$ ions surrounded by cages of 7 oxygen anions. The crystal field spectrum consists of four excitations spanning the energy range 3-60 meV. The refined eigenfunctions indicate XY-spins in the $ab$ plane. The Curie-Weiss temperature of $θ_{CW}=-23.7(1)$ K was determined from magnetic susceptibility measurements. Heat capacity measurements show a sharp peak at 550 mK and a broader feature centered near 1.5 K. Neutron diffraction measurements show that the 550 mK transition corresponds to long-range anti-ferromagnetic order implying a frustration index of $θ_{CW}/T_N\approx43$. These results indicate that Nd$_2$O$_3$ is a structurally and chemically simple model system for frustration caused by competing interactions with moments with predominate XY anisotropy.

cond-mat.str-el↗
arXiv2018

What we learn from the X-ray grating spectra of Nova SMC 2016

Nova SMC 2016 has been the most luminous nova known in the direction of the Magellanic Clouds. It turned into a very luminous supersoft X-ray source between day 16 and 28 after the optical maximum. We observed it with Chandra, the HRC-S camera and the Low Energy Transmission Grating (LETG) on 2016 November and 2017 January (days 39 and 88 after optical maximum), and with XMM-Newton on 2016 December (day 75). We detected the compact white dwarf (WD) spectrum as a luminous supersoft X-ray continuum with deep absorption features of carbon, nitrogen, magnesium, calcium, probably argon and sulfur on day 39, and oxygen, nitrogen and carbon on days 75 and 88. The spectral features attributed to the WD atmosphere are all blue-shifted, by about 1800 km/s on day 39 and up to 2100 km/s in the following observations. Spectral lines attributed to low ionization potential transitions in the interstellar medium are also observed. Assuming the distance of the Small Magellanic Cloud, the bolometric luminosity exceeded Eddington level for at least three months. A preliminary analysis with atmospheric models indicates effective temperature around 700,000 K on day 39, peaking at the later dates in the 850,000-900,000 K range, as expected for a 1.25 m(sol) WD. We suggest a possible classification as an oxygen-neon WD, but more precise modeling is needed to accurately determine the abundances. The X-ray light curves show large, aperiodic ux variability, not associated with spectral variability. We detected red noise, but did not find periodic or quasi-periodic modulations.

astro-ph.HE↗
arXiv2018

Spin gaps in the ordered states of La$_2$LiXO$_6$ (X = Ru, Os) and their relation to distortion of the cubic double perovskite structure in 4$d^3$ and 5$d^3$ magnets

Time-of-flight inelastic neutron scattering measurements have been carried out on polycrystalline samples of the 4$d^3$ and 5$d^3$ double pervoskite antiferromagnets La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$. These reveal the development of an inelastic spin gap in La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$ of $\sim$ 1.8(8) meV and 6(1) meV, below their respective ordering temperatures, $T_N$, $\sim$ 23.8 K and 30 K. The bandwidth of the spin excitations is shown to be $\sim$ 5.7(9) to 12(1) meV, respectively, at low temperatures. Spin gaps are surprising in such magnets as the t$_{2g}$ levels of Ru$^{5+}$ or Os$^{5+}$ are expected to be half-filled, resulting in an anticipated orbital singlet for both materials. We compare these results in monoclinic double perovskites La$_2$LiRuO$_6$ and La$_2$LiOsO$_6$ with those in cubic Ba$_2$YRuO$_6$ and Ba$_2$YOsO$_6$, as well as with those in the more strongly monoclinic La$_2$NaRuO$_6$, La$_2$NaOsO$_6$, and Sr$_2$ScOsO$_6$, and model the inelastic magnetic scattering with linear spin wave theory using minimal anisotropic exchange interactions. We discuss the possible role of the distortion of the face-centered cubic, double perovskite structure on the spin gap formation and geometric frustration in these materials, and show that $T_N$ scales with the top of the spin wave band in all members of these families that display long range order.

cond-mat.str-el↗
arXiv2018

Dipolar-Octupolar Ising Antiferromagnetism in Sm$_2$Ti$_2$O$_7$: A Moment Fragmentation Candidate

Over the past two decades, the magnetic ground states of all rare earth titanate pyrochlores have been extensively studied, with the exception of Sm$_2$Ti$_2$O$_7$. This is, in large part, due to the very high absorption cross-section of naturally-occurring samarium, which renders neutron scattering infeasible. To combat this, we have grown a large, isotopically-enriched single crystal of Sm$_2$Ti$_2$O$_7$. Using inelastic neutron scattering, we determine that the crystal field ground state for Sm$^{3+}$ is a dipolar-octupolar doublet with Ising anisotropy. Neutron diffraction experiments reveal that Sm$_2$Ti$_2$O$_7$ orders into the all-in, all-out magnetic structure with an ordered moment of 0.44(7) $μ_B$ below $T_N=0.35$ K, consistent with expectations for antiferromagnetically-coupled Ising spins on the pyrochlore lattice. Zero-field muon spin relaxation measurements reveal an absence of spontaneous oscillations and persistent spin fluctuations down to 0.03 K. The combination of the dipolar-octupolar nature of the Sm$^{3+}$ moment, the all-in, all-out ordered state, and the low-temperature persistent spin dynamics make this material an intriguing candidate for moment fragmentation physics.

cond-mat.str-el↗
arXiv2018

Magneto-elastic induced vibronic bound state in the spin ice pyrochlore Ho$_2$Ti$_2$O$_7$

The single ion physics of Ho$_2$Ti$_2$O$_7$ is well-understood to produce strong Ising anisotropy, which is an essential ingredient to its low-temperature spin ice state. We present inelastic neutron scattering measurements on Ho$_2$Ti$_2$O$_7$ that reveal a clear inconsistency with its established single ion Hamiltonian. Specifically, we show that a crystal field doublet near 60~meV is split by approximately 3~meV. Furthermore, this crystal field splitting is not isolated to Ho$_2$Ti$_2$O$_7$ but can also be found in its chemical pressure analogs, Ho$_2$Ge$_2$O$_7$ and Ho$_2$Sn$_2$O$_7$. We demonstrate that the origin of this effect is a vibronic bound state, resulting from the entanglement of a phonon and crystal field excitation. We derive the microscopic Hamiltonian that describes the magneto-elastic coupling and provides a quantitative description of the inelastic neutron spectra.

cond-mat.str-el↗
arXiv2018

Breaking the habit - the peculiar 2016 eruption of the unique recurrent nova M31N 2008-12a

Since its discovery in 2008, the Andromeda galaxy nova M31N 2008-12a has been observed in eruption every single year. This unprecedented frequency indicates an extreme object, with a massive white dwarf and a high accretion rate, which is the most promising candidate for the single-degenerate progenitor of a type-Ia supernova known to date. The previous three eruptions of M31N 2008-12a have displayed remarkably homogeneous multi-wavelength properties: (i) From a faint peak, the optical light curve declined rapidly by two magnitudes in less than two days; (ii) Early spectra showed initial high velocities that slowed down significantly within days and displayed clear He/N lines throughout; (iii) The supersoft X-ray source (SSS) phase of the nova began extremely early, six days after eruption, and only lasted for about two weeks. In contrast, the peculiar 2016 eruption was clearly different. Here we report (i) the considerable delay in the 2016 eruption date, (ii) the significantly shorter SSS phase, and (iii) the brighter optical peak magnitude (with a hitherto unobserved cusp shape). Early theoretical models suggest that these three different effects can be consistently understood as caused by a lower quiescence mass-accretion rate. The corresponding higher ignition mass caused a brighter peak in the free-free emission model. The less-massive accretion disk experienced greater disruption, consequently delaying re-establishment of effective accretion. Without the early refueling, the SSS phase was shortened. Observing the next few eruptions will determine whether the properties of the 2016 outburst make it a genuine outlier in the evolution of M31N 2008-12a.

astro-ph.SR↗
arXiv2017

The supersoft X-ray source in V5116 Sgr I. The high resolution spectra

Classical novae occur on the surface of an accreting white dwarf in a binary system. After ejection of a fraction of the envelope and when the expanding shell becomes optically thin to X-rays, a bright source of supersoft X-rays arises, powered by residual H burning on the surface of the white dwarf. While the general picture of the nova event is well established, the details and balance of accretion and ejection processes in classical novae are still full of unknowns. The long-term balance of accreted matter is of special interest for massive accreting white dwarfs, which may be promising supernova Ia progenitor candidates. V5116 Sgr was observed as a bright and variable supersoft X-ray source by XMM-Newton 610~days after outburst. The light curve showed a periodicity consistent with the orbital period. During one third of the orbit the luminosity was a factor of seven brighter than during the other two thirds of the orbital period. In the present work we aim to disentangle the X-ray spectral components of V5116 Sgr and their variability. We present the high resolution spectra obtained with XMM-Newton RGS and Chandra LETGS/HRC-S in March and August 2007. The grating spectrum during the periods of high-flux shows a typical hot white dwarf atmosphere dominated by absorption lines of N VI and N VII. During the low-flux periods, the spectrum is dominated by an atmosphere with the same temperature as during the high-flux period, but with several emission features superimposed. Some of the emission lines are well modeled with an optically thin plasma in collisional equilibrium, rich in C and N, which also explains some excess in the spectra of the high-flux period. No velocity shifts are observed in the absorption lines, with an upper limit set by the spectral resolution of 500 km/s, consistent with the expectation of a non-expanding atmosphere so late in the evolution of the post-nova.

astro-ph.HE↗
arXiv2016

M31N 2008-12a - the remarkable recurrent nova in M31: Pan-chromatic observations of the 2015 eruption

The Andromeda Galaxy recurrent nova M31N 2008-12a had been observed in eruption ten times, including yearly eruptions from 2008-2014. With a measured recurrence period of $P_\mathrm{rec}=351\pm13$ days (we believe the true value to be half of this) and a white dwarf very close to the Chandrasekhar limit, M31N 2008-12a has become the leading pre-explosion supernova type Ia progenitor candidate. Following multi-wavelength follow-up observations of the 2013 and 2014 eruptions, we initiated a campaign to ensure early detection of the predicted 2015 eruption, which triggered ambitious ground and space-based follow-up programs. In this paper we present the 2015 detection; visible to near-infrared photometry and visible spectroscopy; and ultraviolet and X-ray observations from the Swift observatory. The LCOGT 2m (Hawaii) discovered the 2015 eruption, estimated to have commenced at Aug. $28.28\pm0.12$ UT. The 2013-2015 eruptions are remarkably similar at all wavelengths. New early spectroscopic observations reveal short-lived emission from material with velocities $\sim13000$ km s$^{-1}$, possibly collimated outflows. Photometric and spectroscopic observations of the eruption provide strong evidence supporting a red giant donor. An apparently stochastic variability during the early super-soft X-ray phase was comparable in amplitude and duration to past eruptions, but the 2013 and 2015 eruptions show evidence of a brief flux dip during this phase. The multi-eruption Swift/XRT spectra show tentative evidence of high-ionization emission lines above a high-temperature continuum. Following Henze et al. (2015a), the updated recurrence period based on all known eruptions is $P_\mathrm{rec}=174\pm10$ d, and we expect the next eruption of M31N 2008-12a to occur around mid-Sep. 2016.

astro-ph.SR↗
arXiv2015

Neutron Spectroscopic Study of Crystalline Electric Field Excitations in Stochiometric and Lightly Stuffed Yb2Ti2O7

Time-of-flight neutron spectroscopy has been used to determine the crystalline electric field (CEF) Hamiltonian, eigenvalues and eigenvectors appropriate to the $J$ = 7/2 Yb$^{3+}$ ion in the candidate quantum spin ice pyrochlore magnet $\rm Yb_2Ti_2O_7$. The precise ground state (GS) of this exotic, geometrically-frustrated magnet is known to be sensitive to weak disorder associated with the growth of single crystals from the melt. Such materials display weak "stuffing" wherein a small proportion, $\approx$ 2\%, of the non-magnetic Ti$^{4+}$ sites are occupied by excess Yb$^{3+}$. We have carried out neutron spectroscopic measurements on a stoichiometric powder sample of Yb$_2$Ti$_2$O$_7$, as well as a crushed single crystal with weak stuffing and an approximate composition of Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ with $x$ = 0.046. All samples display three CEF transitions out of the GS, and the GS doublet itself is identified as primarily composed of m$_J$ = $\pm$1/2, as expected. However,"stuffing" at low temperatures in Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ induces a similar finite CEF lifetime as is induced in stoichiometric Yb$_2$Ti$_2$O$_7$ by elevated temperature. We conclude that an extended strain field exists about each local "stuffed" site, which produces a distribution of random CEF environments in the lightly stuffed Yb$_{2+x}$Ti$_{2-x}$O$_{7+y}$, in addition to producing a small fraction of Yb-ions in defective environments with grossly different CEF eigenvalues and eigenvectors.

cond-mat.str-el↗
arXiv2015

A remarkable recurrent nova in M31: Discovery and optical/UV observations of the predicted 2014 eruption

The Andromeda Galaxy recurrent nova M31N 2008-12a had been caught in eruption eight times. The inter-eruption period of M31N 2008-12a is ~1 year, making it the most rapidly recurring system known, and a strong single-degenerate Type Ia Supernova progenitor candidate. Following the 2013 eruption, a campaign was initiated to detect the predicted 2014 eruption and to then perform high cadence optical photometric and spectroscopic monitoring using ground-based telescopes, along with rapid UV and X-ray follow-up with the Swift satellite. Here we report the results of a high cadence multicolour optical monitoring campaign, the spectroscopic evolution, and the UV photometry. We also discuss tantalising evidence of a potentially related, vastly-extended, nebulosity. The 2014 eruption was discovered, before optical maximum, on October 2, 2014. We find that the optical properties of M31N 2008-12a evolve faster than all Galactic recurrent novae known, and all its eruptions show remarkable similarity both photometrically and spectroscopically. Optical spectra were obtained as early as 0.26 days post maximum, and again confirm the nova nature of the eruption. A significant deceleration of the inferred ejecta expansion velocity is observed which may be caused by interaction of the ejecta with surrounding material, possibly a red giant wind. We find a low ejected mass and low ejection velocity, which are consistent with high mass-accretion rate, high mass white dwarf, and short recurrence time models of novae. We encourage additional observations, especially around the predicted time of the next eruption, towards the end of 2015.

astro-ph.SR↗
arXiv2015

Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores

The idea of magnetic monopoles in spin ice has enjoyed much success at intermediate temperatures, but at low temperatures a description in terms of monopole dynamics alone is insufficient. Recently, numerical simulations were used to argue that magnetic impurities account for this discrepancy by introducing a magnetic equivalent of residual resistance in the system. Here we propose that oxygen deficiency is the leading cause of magnetic impurities in as-grown samples, and we determine the defect structure and magnetism in Y2Ti2O(7-δ) using diffuse neutron scattering and magnetization measurements. These defects are eliminated by oxygen annealing. The introduction of oxygen vacancies causes Ti4+ to transform to magnetic Ti3+ with quenched orbital magnetism, but the concentration is anomalously low. In the spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects suppress moments on neighbouring rare-earth sites, and that these magnetic distortions dramatically slow down the long-time monopole dynamics at sub-Kelvin temperatures.

cond-mat.str-el↗
arXiv2015

A remarkable recurrent nova in M 31: The predicted 2014 outburst in X-rays with Swift

The M 31 nova M31N 2008-12a was recently found to be a recurrent nova (RN) with a recurrence time of about 1 year. This is by far the fastest recurrence time scale of any known RNe. Our optical monitoring programme detected the predicted 2014 outburst of M31N 2008-12a in early October. We immediately initiated an X-ray/UV monitoring campaign with Swift to study the multiwavelength evolution of the outburst. We monitored M31N 2008-12a with daily Swift observations for 20 days after discovery, covering the entire supersoft X-ray source (SSS) phase. We detected SSS emission around day six after outburst. The SSS state lasted for approximately two weeks until about day 19. M31N 2008-12a was a bright X-ray source with a high blackbody temperature. The X-ray properties of this outburst were very similar to the 2013 eruption. Combined X-ray spectra show a fast rise and decline of the effective blackbody temperature. The short-term X-ray light curve showed strong, aperiodic variability which decreased significantly after about day 14. Overall, the X-ray properties of M31N 2008-12a are consistent with the average population properties of M 31 novae. The optical and X-ray light curves can be scaled uniformly to show similar time scales as those of the Galactic RNe U Sco or RS Oph. The SSS evolution time scales and effective temperatures are consistent with a high-mass WD. We predict the next outburst of M31N 2008-12a to occur in autumn 2015.

astro-ph.HE↗
arXiv2014

X-ray monitoring of classical novae in the central region of M31. III. Autumn and winter 2009/10, 2010/11 and 2011/12

[Abridged] Classical novae (CNe) represent the major class of supersoft X-ray sources (SSSs) in the central region of our neighbouring galaxy M31. We performed a dedicated monitoring of the M31 central region, aimed to detect SSS counterparts of CNe, with XMM-Newton and Chandra between Nov and Mar of the years 2009/10, 2010/11 and 2011/12. In total we detected 24 novae in X-rays. Seven of these sources were known from previous observations, including the M31 nova with the longest SSS phase, M31N~1996-08b, which was found to fade below our X-ray detection limit 13.8 yr after outburst. Of the new discoveries several novae exhibit significant variability in their short-term X-ray light curves with one object showing a suspected period of about 1.3 h. We studied the SSS state of the most recent outburst of a recurrent nova which had previously shown the shortest time ever observed between two outbursts (about 5 yr). The total number of M31 novae with X-ray counterpart was increased to 79 and we subjected this extended catalogue to detailed statistical studies. Four previously indicated correlations between optical and X-ray parameters could be confirmed and improved. We found indications that the multi-dimensional parameter space of nova properties might be dominated by a single physical parameter. We discuss evidence for a different X-ray behaviour of novae in the M31 bulge and disk. Exploration of the multi-wavelength parameter space of optical and X-ray measurements is shown to be a powerful tool for examining properties of extragalactic nova populations. While there are hints that the different stellar populations of M31 (bulge vs disk) produce dissimilar nova outbursts, there is also growing evidence that the overall behaviour of an average nova might be understood in surprisingly simple terms.

astro-ph.HE↗