arXiv subjects

Marc Janoschek

Publications and source records attributed to Marc Janoschek.

13 recordsLinked to original sources

arXiv2026

Field-direction sensitivity of Kondo hybridization in UTe$_2$

Neutron scattering experiments on the spin-triplet superconductor UTe$_2$ have established that the dominant low-energy magnetic response is along Brillouin zone boundaries, resembling the magnetic susceptibility of narrow-gap interband excitations. We report a study of the sensitivity of these excitations to magnetic field along the crystallographic $\hat{a}$-axis. Up to fields of $\mu_0 H$=13 T, the maximal inelastic neutron spectral weight increases in energy transfer, with a pronounced increase in $d\hbar\omega_{peak}/dH$ near $\mu_0 H$=7 T. This behavior parallels the field and temperature dependent features of the electrical resistivity that are associated with Kondo hybridization. Our measurements suggest that $\hat{a}$-axis fields near $\mu_0 H$=7~T induce a change in the hybridization between heavy $f$-electrons and the bare conduction band.

cond-mat.str-el↗

arXiv2024

Magnetic Phase Diagram of ErB$_4$ as Explored by Neutron Scattering

The tetragonal $4f$-electron intermetallic ErB$_4$ is characterized by strong Ising anisotropy along the tetragonal $c$ axis. The magnetic moments on the erbium sites can be mapped onto a Shastry-Sutherland (SSL) lattice resulting in geometrical frustration. At zero magnetic field ErB$_4$ exhibits collinear columnar antiferromagnetic (CAFM) order below $T_\text{N} = 15.4$ K. In the presence of a magnetic field parallel to the $c$ axis, ErB$_4$ exhibits a plateau at $1/2$ of the saturation magnetization $M_\text{S}$, which arises at a spin flip transition at $H_1$ $=$ 1.9 T. Fractional magnetization plateaus and other exotic spin phases are a well-established characteristic feature of frustrated spin systems. Monte Carlo simulations propose that ErB$_4$ is an ideal candidate to feature a spin supersolid phase in close vicinity of $H_1$ between the CAFM and $M/M_\text{S}=1/2$ plateau (HP) phase. Here we combine single-crystal neutron diffraction and inelastic neutron scattering to study the magnetic phase diagram and the crystal electric field (CEF) ground state of ErB$_4$. Our measurements as a function of magnetic field find no signature of the spin supersolid phase but allow us to determine the magnetic structure of the HP phase to be of the uuud type consistent with an Ising material. The magnetic moment $\mu_{\mathrm{CEF}}$ $=$ 8.96 $\mu_B$ expected from the CEF configuration determined by our inelastic neutron scattering measurements is also consistent with the ordered moment observed in neutron diffraction showing that the moments are fully ordered and close to the Er$^{3+}$ free ion moment (9.6 $\mu_B$).

cond-mat.str-el↗

arXiv2024

Connection between f-electron correlations and magnetic excitations in UTe2

The detailed anisotropy of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe$_2$ is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetry $Y$ and $T$ points and disperse along the crystallographic $\hat{b}$-axis. In applied magnetic fields to at least $\mu_0 H=11$~T along the $\hat{c}-$axis, the magnetism is found to be field-independent in the $(hk0)$ plane. The scattering intensity is consistent with that expected from U$^{3+}$/U$^{4+}$ $f$-electron spins with preferential orientation along the crystallographic $\hat{a}$-axis, and a fluctuating magnetic moment of 2.3(7) $\mu_B$. These characteristics indicate that the excitations are due to intraband spin excitons arising from $f$-electron hybridization.

cond-mat.str-el↗

arXiv2024

Absence of bulk charge density wave order in the normal state of UTe$_2$

A spatially modulated superconducting state, known as pair density wave (PDW), is a tantalizing state of matter with unique properties. Recent scanning tunneling microscopy (STM) studies revealed that spin-triplet superconductor UTe$_2$ hosts an unprecedented spin-triplet, multi-component PDW whose three wavevectors are indistinguishable from a preceding charge-density wave (CDW) order that survives to temperatures well above the superconducting critical temperature, $T_{c}$. Whether the PDW is the mother or a subordinate order remains unsettled. Here, based on a systematic search for bulk charge order above $T_{c}$ using resonant elastic X-ray scattering (REXS), we show that the structure factor of charge order previously identified by STM is absent in the bulk within the sensitivity of REXS. Our results invite two scenarios: either the density-wave orders condense simultaneously at $T_{c}$ in the bulk, in which case PDW order is likely the mother phase, or the charge modulations are restricted to the surface.

cond-mat.str-el↗

arXiv2024

Tuning of Charge Order by Uniaxial Stress in a Cuprate Superconductor

Strongly correlated electron materials are often characterized by competition and interplay of multiple quantum states. For example, in high-temperature cuprate superconductors unconventional superconductivity, spin- and charge-density wave orders coexist. A key question is whether competing states coexist on the atomic scale or if they segregate into distinct 'islands'. Using X-ray diffraction, we investigate the competition between charge order and superconductivity in the archetypal cuprate La(2-x)BaxCuO4, around the x = 1/8-doping, where uniaxial stress restores optimal 3D superconductivity at approximately 0.06 GPa. We find that the charge order peaks and the correlation length along the stripe are strongly reduced up to the critical stress, above which they stay constant. Simultaneously, the charge order onset temperature only shows a modest decrease. Our findings suggest that optimal 3D superconductivity is not linked to the absence of charge stripes but instead requires their arrangement into smaller 'islands'. Our results provide insight into the length scales over which the interplay between superconductivity and charge order takes place.

cond-mat.supr-con↗

arXiv2022

Coexisting Kondo hybridization and itinerant f-electron ferromagnetism in UGe2

Kondo hybridization in partially filled f-electron systems conveys significant amount of electronic states sharply near the Fermi energy leading to various instabilities from superconductivity to exotic electronic orders. UGe2 is a 5f heavy fermion system, where the Kondo hybridization is interrupted by the formation of two ferromagnetic phases below a 2nd order transition Tc ~ 52 K and a crossover transition Tx ~ 32 K. These two ferromagnetic phases are concomitantly related to a spin-triplet superconductivity that only emerges and persists inside the magnetically ordered phase at high pressure. The origin of the two ferromagnetic phases and how they form within a Kondo-lattice remain ambiguous. Using scanning tunneling microscopy and spectroscopy, we probe the spatial electronic states in the UGe2 as a function of temperature. We find a Kondo resonance and sharp 5f-electron states near the chemical potential that form at high temperatures above Tc in accordance with our density functional theory (DFT) + Gutzwiller calculations. As temperature is lowered below Tc, the resonance narrows and eventually splits below Tx dumping itinerant f-electron spectral weight right at the Fermi energy. Our findings suggest a Stoner mechanism forming the highly polarized ferromagnetic phase below Tx that itself sets the stage for the emergence of unconventional superconductivity at high pressure.

cond-mat.str-el↗

arXiv2022

Crystal Symmetry of Stripe Ordered La1.88Sr0.12CuO4

We present a combined x-ray and neutron diffraction study of the stripe ordered superconductor \lscox{0.12}. The average crystal structure is consistent with the orthorhombic $Bmab$ space group as commonly reported in the literature. This structure however is not symmetry compatible with a second order phase transition into the stripe order phase, and, as we report here numerous Bragg peaks forbidden in the $Bmab$ space group are observed. We have studied and analysed these $Bmab$-forbidden Bragg reflections. Fitting of the diffraction intensities yields monoclinic lattice distortions that are symmetry consistent with charge stripe order.

cond-mat.supr-con↗

arXiv2017

Realization of the Axial Next-Nearest-Neighbor Ising model in U$_3$Al$_2$Ge$_3$

Here we report small-angle neutron scattering (SANS) measurements and theoretical modeling of U$_3$Al$_2$Ge$_3$. Analysis of the SANS data reveals a phase transition to sinusoidally modulated magnetic order, at $T_{\mathrm{N}}=63$~K to be second order, and a first order phase transition to ferromagnetic order at $T_{\mathrm{c}}=48$~K. Within the sinusoidally modulated magnetic phase ($T_{\mathrm{c}} < T < T_{\mathrm{N}}$), we uncover a dramatic change, by a factor of three, in the ordering wave-vector as a function of temperature. These observations all indicate that U$_3$Al$_2$Ge$_3$ is a close realization of the three-dimensional Axial Next-Nearest-Neighbor Ising model, a prototypical framework for describing commensurate to incommensurate phase transitions in frustrated magnets.

cond-mat.str-el↗

arXiv2016

Electronic Correlation and Magnetism in the Ferromagnetic Metal Fe3GeTe2

Motivated by the search for design principles of rare-earth-free strong magnets, we present a study of electronic structure and magnetic properties of the ferromagnetic metal Fe3GeTe2 within local density approximation (LDA) of the density functional theory, and its combination with dynamical mean-field theory (DMFT). For comparison to these calculations, we have measured magnetic and thermodynamic properties as well as X-ray magnetic circular dichroism and the photoemission spectrum of single crystal Fe3GeTe2. We find that the experimentally determined Sommerfeld coefficient is enhanced by an order of magnitude with respect to the LDA value. This enhancement can be partially explained by LDA+DMFT. In addition, the inclusion of dynamical electronic correlation effects provides the experimentally observed magnetic moments, and the spectral density is in better agreement with photoemission data. These results establish the importance of electronic correlations in this ferromagnet.

cond-mat.str-el↗

arXiv2015

Symmetry and correlations underlying Hidden Order in URu2Si2

We experimentally investigate the symmetry in the Hidden Order (HO) phase of intermetallic URu2Si2 by mapping the lattice and magnetic excitations via inelastic neutron and x-ray scattering measurements in the HO and high-temperature paramagnetic phases. At all temperatures the excitations respect the zone edges of the body-centered tetragonal paramagnetic phase, showing no signs of reduced spatial symmetry, even in the HO phase. The magnetic excitations originate from transitions between hybridized bands and track the Fermi surface, whose feature are corroborated by the phonon measurements. Due to a large hybridization energy scale, a full uranium moment persists in the HO phase, consistent with a lack of observed crystal-field-split states. Our results are inconsistent with local order parameter models and the behavior of typical density waves. We suggest that an order parameter that does not break spatial symmetry would naturally explain these characteristics.

cond-mat.str-el↗

arXiv2014

LDA+DMFT Approach to Magnetocrystalline Anisotropy of Strong Magnets

The new challenges posed by the need of finding strong rare-earth free magnets demand methods that can predict magnetization and magnetocrystalline anisotropy energy (MAE). We argue that correlated electron effects, which are normally underestimated in band structure calculations, play a crucial role in the development of the orbital component of the magnetic moments. Because magnetic anisotropy arises from this orbital component, the ability to include correlation effects has profound consequences on our predictive power of the MAE of strong magnets. Here we show that incorporating the local effects of electronic correlations with dynamical mean-field theory provides reliable estimates of the orbital moment, the mass enhancement and the MAE of YCo5.

cond-mat.str-el↗

arXiv2012

Fluctuation-induced first-order phase transition in Dzyaloshinskii-Moriya helimagnets

Two centuries of research on phase transitions have repeatedly highlighted the importance of critical fluctuations that abound in the vicinity of a critical point. They are at the origin of scaling laws obeyed by thermodynamic observables close to second-order phase transitions resulting in the concept of universality classes, that is of paramount importance for the study of organizational principles of matter. Strikingly, in case such soft fluctuations are too abundant they may alter the nature of the phase transition profoundly; the system might evade the critical state altogether by undergoing a discontinuous first-order transition into the ordered phase. Fluctuation-induced first-order transitions have been discussed broadly and are germane for superconductors, liquid crystals, or phase transitions in the early universe, but clear experimental confirmations remain scarce. Our results from neutron scattering and thermodynamics on the model Dzyaloshinskii-Moriya (DM) helimagnet (HM) MnSi show that such a fluctuation-induced first-order transition is realized between its paramagnetic and HM state with remarkable agreement between experiment and a theory put forward by Brazovskii. While our study clarifies the nature of the HM phase transition in MnSi that has puzzled scientists for several decades, more importantly, our conclusions entirely based on symmetry arguments are also relevant for other DM-HMs with only weak cubic magnetic anisotropies. This is in particular noteworthy in light of a wide range of recent discoveries that show that DM helimagnetism is at the heart of problems such as topological magnetic order, multiferroics, and spintronics.

cond-mat.str-el↗

arXiv2011

Search for pressure induced superconductivity in CeFeAsO and CeFePO iron pnictides

The CeFeAsO and CeFePO iron pnictide compounds were studied via electrical transport measurements under high-pressure. In CeFeAsO polycrystals, the magnetic phases involving the Fe and Ce ions coexist up to 15 GPa, with no signs of pressure-induced superconductivity up to 50 GPa. For the CeFePO single crystals, pressure stabilizes the Kondo screening of the Ce 4f-electron magnetic moments.

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