arXiv subjects

I. I. Mazin

Publications and source records attributed to I. I. Mazin.

At least 37 records · Page 2Linked to original sources

arXiv2021

Ising superconductivity in monolayer niobium dichalcogenide alloys

NbSe$_{2}$ and NbS$_{2}$ are isostructural two-dimensional materials that exhibit contrasting superconducting properties when reduced to the single monolayer limit. Monolayer NbSe$_{2}$ is an Ising superconductor, while there have been no reports of superconductivity in monolayer NbS$_{2}$. NbS$_{x}$Se$_{2-x}$ alloys exhibit an intriguing non-monotonic dependence of the superconducting transition temperature with sulfur content, which has been interpreted as a manifestation of fractal superconductivity. However, several key questions about this result are not known: (1) Does the electronic structure of the alloy differ from the parent compounds, (2) Are spin fluctuations which have been shown to be prominent in monolayer NbSe$_{2}$ also present in the alloys? Using first-principles calculations, we show that the density of states at the Fermi level and the proximity to magnetism in NbS$_{x}$Se$_{2-x}$ alloys are both reduced compared to the parent compound; the former would decrease the transition temperature while the latter would increase it. We also show that Se vacancies, which are likely magnetic pair-breaking defects, may form in large concentrations in NbSe$_{2}$. Based on our results, we suggest an alternative explanation of the non-monotonic behavior the superconducting transition temperature in NbS$_{x}$Se$_{2-x}$ alloys, which does not require the conjecture of multifractality.

cond-mat.supr-con↗

arXiv2021

Magnetism-driven unconventional effects in Ising superconductors: role of proximity, tunneling, and nematicity

Hybrid Ising superconductor-ferromagnetic insulator heterostructures provide a unique opportunity to explore the interplay between proximity-induced magnetism, spin-orbit coupling and superconductivity. Here we use a combination of first-principles calculations of NbSe$_{2}$/CrBr$_{3}$ heterostructures and an analytical theory of Ising superconductivity to analyze the existing experiments and provide a complete explanation of highly nontrivial and largely counterintuitive effects: an increase in the magnitude of the superconducting gap accompanied by the broadening of the tunneling peaks; hysteretic behavior of the tunneling conductance that sets in $\approx 2$ K below $T_c$; and nematic symmetry breaking in the superconducting state. The microscopic reason in all three cases appears to be the interplay between the proximity-induced exchange splitting and intrinsic defects. Finally, we predict additional interesting effects that at the moment cannot be addressed experimentally: spin-filtering when tunneling across CrBr$_{3}$ and tunneling ``hot spots'' in momentum space that are anticorrelated with regions where the spin-orbit splitting is maximum.

cond-mat.supr-con↗

arXiv2021

Strongly anisotropic antiferromagnetic coupling in EuFe2As2 revealed by stress detwinning

Of all parent compounds of iron-based high-temperature superconductors, EuFe2As2 exhibits by far the largest magnetostructural coupling due to the sizable biquadratic interaction between Eu and Fe moments. While the coupling between Eu antiferromagnetic order and Fe structural/antiferromagnetic domains enables rapid field detwinning, this prevents simple magnetometry measurements from extracting the critical fields of the Eu metamagnetic transition. Here we measure these critical fields by combining x-ray magnetic circular dichroism spectroscopy with in-situ tunable uniaxial stress and applied magnetic field. The combination of two tuning knobs allows us to separate the stress-detwinning of structural domains from the field-induced reorientation of Eu moments. Intriguingly, we find a spin-flip transition which can only result from a strongly anisotropic interaction between Eu planes. We argue that this anisotropic exchange is a consequence of the strong anisotropy in the magnetically ordered Fe layer, which presents a new form of higher-order coupling between Eu and Fe magnetism.

cond-mat.supr-con↗

arXiv2021

Prediction of unconventional magnetism in doped FeSb2

It is commonly believed that in typical collinear antiferromagnets, with no net magnetization, the energy bands are spin-(Kramers-degenerate. The opposite case is usually associated with a global time-reversal symmetry breaking (e.g., via ferro(i)magnetism), or with the spin-orbit interaction is combined with the broken spatial inversion symmetry. Recently, another type of spin splitting was demonstrated to emerge in some fully compensated by symmetry, nonrelativistic, collinear magnets, and not even necessarily non-centrosymmetric. These materials feature non-zero spin density staggered not only in real, but also in momentum space. This duality results in a combination of characteristics typical of ferro- and antiferromagnets. Here we discuss this novel concept in application to a well-known semiconductor, FeSb2, and predict that upon certain alloying it becomes magnetic, and features such magnetic duality. The calculated energy bands split antisymmetrically with respect to spin degenerate nodal surfaces (and not nodal points, as in the case of spin-orbit splitting. This combination of a large (0.2 eV) spin splitting, compensated net magnetization and metallic ground-state, and a particular magnetic easy axis generate a large anomalous Hall conductivity (~150 S/cm) and a sizable magneto-optical Kerr effect, all deemed to be hallmarks of nonzero net magnetization. We identify a large contribution to the anomalous response originating from the spin-orbit interaction gapped anti-Kramers nodal surfaces, a mechanism distinct from the nodal lines and Weyl {\it points} in ferromagnets.

cond-mat.mtrl-sci↗

arXiv2021

Magnetization process of atacamite: a case of weakly coupled $S = 1/2$ sawtooth chains

We present a combined experimental and theoretical study of the mineral atacamite Cu$_2$Cl(OH)$_3$. Density functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateau-like magnetization at about $M_{\rm sat}/2$. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.

cond-mat.str-el↗

arXiv2020

Chiral charge-density wave in TiSe$_2$ due to photo-induced structural distortions

A variety of experiments have been carried out to establish the origin of the chiral charge-density wave transition in 1T-TiSe$_2$, which in turn has led to contradictory conclusions on the origin of this transition. Some studies suggest the transition is a phonon-driven structural distortion while other studies suggest it is an excitonic insulator phase transition that is accompanied by a lattice distortion. First, we propose these interpretations can be reconciled if one analyzes the available experimental and theoretical data within a formal definition of what constitutes an excitonic insulator as initially proposed by Keldysh and Kopaev. Next, we present pump-probe measurements of circularly polarized optical transitions and first-principles calculations where we highlight the importance of accounting for structural distortions to explain the finite chirality of optical transitions in the CDW phase. We show that at the elevated electronic temperature that occurs upon photoexcitation, there is a non-centrosymmetric structure that is near-degenerate in energy with the centrosymmetric charge density wave structure, which explains the finite chirality of the optical transitions observed in the CDW phase of TiSe$_2$.

cond-mat.mtrl-sci↗

arXiv2020

Ising superconductivity and magnetism in NbSe$_2$

Recent studies on superconductivity in NbSe$_2$ have demonstrated a large anisotropy in the superconducting critical field when the material is reduced to a single monolayer. Motivated by this recent discovery, we use density functional theory (DFT) calculations to quantitatively address the superconducting properties of bulk and monolayer NbSe$_2$. We demonstrate that NbSe$_2$ is close to a ferromagnetic instability, and analyze our results in the context of experimental measurements of the spin susceptibility in NbSe$_2$. We show how this magnetic instability, which is pronounced in a single monolayer, can enable sizeable singlet-triplet mixing of the superconducting order parameter, contrary to contemporary considerations of the pairing symmetry in monolayer NbSe$_2$, and discuss approaches as to how this degree of mixing can be addressed quantitatively within our DFT framework. Our calculations also enable a quantitative description of the large anisotropy of the superconducting critical field, using DFT calculations of monolayer NbSe$_2$ in the normal state

cond-mat.supr-con↗

arXiv2020

Reduction of the spin susceptibility in the superconducting state of Sr2RuO4 observed by polarized neutron scattering

Recent observations [A.~Pustogow et al. Nature 574, 72 (2019)] of a drop of the $^{17}$O nuclear magnetic resonance (NMR) Knight shift in the superconducting state of Sr$_2$RuO$_4$ challenged the popular picture of a chiral odd-parity paired state in this compound. Here we use polarized neutron scattering to show that there is a $34 \pm 6$ % drop in the magnetic susceptibility at the ruthenium site below the superconducting transition temperature. Measurements are made at lower fields $H \sim \tfrac{1}{3} H_{c2}$ than a previous study allowing the suppression to be observed. Our results are consistent with the recent NMR observations and rule out the chiral odd-parity $\mathbf{d}=\hat{\mathbf{z}}(k_x\pm ik_y)$ state. The observed susceptibility is consistent with several recent proposals including even-parity $B_{1g}$ and odd-parity helical states.

cond-mat.supr-con↗

arXiv2020

Novel magnetic states and nematic spin chirality in the kagome lattice metal YMn$_{6}$Sn$_{6}$

Identification, understanding, and manipulation of novel magnetic textures is essential for the discovery of new quantum materials for future spin-based electronic devices. In particular, materials that manifest a large response to external stimuli such as a magnetic field are subject to intense investigation. Here, we study the kagome-net magnet YMn$_{6}$Sn$_{6}$ by magnetometry, transport, and neutron diffraction measurements combined with first principles calculations. We identify a number of nontrivial magnetic phases, explain their microscopic nature, and demonstrate that one of them hosts a large topological Hall effect (THE). We propose a new nematic chirality mechanism, reminiscent of the nematicity in Fe-based superconductors, which leads to the THE at elevated temperatures. This interesting physics comes from parametrically frustrated interplanar exchange interactions that trigger strong magnetic fluctuations. Our results pave a path to new chiral spin textures, promising for novel spintronics.

cond-mat.str-el↗

arXiv2020

Ultrafast dynamics in normal and Charge Density Wave phase of 2H-NbSe2

We investigate carrier and collective mode dynamics in 2H-NbSe$_2$ using time-resolved optical pump-probe spectroscopy and compare the results with first-principle calculations. Broadband ultrafast reflectivity studies of 2H-NbSe$_2$ in a wide temperature interval covering the normal, charge density wave (CDW) and superconducting phase were performed. Spectral features observed in the transient reflectivity experiment were associated with specific optical transitions obtained from band structure calculations. Displacive excitation of coherent phonons showed CDW-associated coherent oscillations of the soft phonon mode across the whole spectral range. Temperature evolution of this coherent phonon mode in the low-excitation linear regime shows softening of the mode down to the CDW transition temperature T$_{CDW}$ with subsequent hardening below T$_{CDW}$. The global fit of the broadband probe data reveals four different relaxation times associated with characteristic electron-electron, electron-phonon and phonon-phonon relaxation processes. From first principle calculations of electron-phonon coupling we associate the few picosecond electron-phonon relaxation time $τ_2$ with a specific group of phonons with frequencies around 20 meV. On the other hand, the anomalously long relaxation time of $τ_3$~100 ps is associated with anharmonicity-driven phonon-phonon scattering. All relaxation processes result from anomalies near the second order CDW phase transition that are reflected in the temperature dependencies of the characteristic relaxation times and amplitudes of optical densities. At highest fluences we observe electronic melting of the CDW and disappearance of the mode hardening below T$_{CDW}$.

cond-mat.str-el↗

arXiv2020

SrRuO$_3$-SrTiO$_3$ heterostructure as a possible platform for studying unconventional superconductivity in Sr$_{2}$RuO$_{4}$

There is intense controversy around the unconventional superconductivity in strontium ruthenate, where the various theoretical and experimental studies suggest diverse and mutually exclusive pairing symmetries. Currently, the investigation is solely focused on only one material, Sr2RuO4, and the field suffers from the lack of comparison targets. Here, employing a density functional theory based analysis, we show that the heterostructure composed of SrRuO3 and SrTiO3 is endowed with all the key characteristics of Sr2RuO4, and, in principle, can host superconductivity. Furthermore, we show that competing magnetic phases and associated frustration, naturally affecting the superconducting state, can be tuned by epitaxial strain engineering. This system thus offers an excellent platform for gaining more insight into superconductivity in ruthenates.

cond-mat.supr-con↗

arXiv2019

$\textit{Ab initio}$ prediction of a two-dimensional variant of the iridate IrO$_2$

We propose an insulating 2D phase of IrO$_{2}$, predicted by $\textit{ab initio}$ evolutionary algorithms. The predicted phase is a van der Waals crystal, in which Ir forms a triangular lattice, and is energetically competitive with the metastable spinel phase, observed experimentally. Electronic structure calculations show that the magnetic properties of this phase are highly nontrivial, with an almost perfect degeneracy of 120$^{\circ}$ $\textit{Neel}$ and $Y$-stripe orders, and unusually soft magnetic moments. The resulting behavior, which we term $\textit{easy plane anisotropy}$, is entirely different from what is realized in previously-explored Kitaev honeycomb lattices. Our results thus suggest that IrO$_{2}$ may be an ideal candidate to realize highly unusual magnetic properties.

cond-mat.str-el↗

arXiv2019

Impact of biaxial and uniaxial strain on V$_2$O$_3$

Using first-principles calculations we determine the role of compressive and tensile uniaxial and equibiaxial strain on the structural, electronic and magnetic properties of V$_2$O$_3$. We find that compressive strain increases the energy cost to transition from the high-temperature paramagnetic metallic phase to the low-temperature antiferromagnetic insulating phase. This shift in the energy difference can be explained by changes in the V-V bond lengths that are antiferromagnetically aligned in the low temperature structure. The insights that we have obtained provide a microscopic explanation for the shifts in the metal-insulator transition temperature that have been observed in experiments of V$_2$O$_3$ films grown on different substrates.

cond-mat.mtrl-sci↗

arXiv2019

Why band theorists have been so successful in explaining and predicting novel superconductors?

In this contribution to the J. Phys. memorial issue in honor of Sandro Massidda I reflect on a phenomenon Sandro had been a part of. While theoretical condensed matter physicists have made, over the years, exciting and most elegant contributions to the theory of superconductivity (which, in and by itself, is one of the most beautiful constructs in theoretical physics), some of them of utmost importance, they have had less success in predicting and explaining superconducting states and mechanisms in specific materials. More down-to-earth computational materials scientists, who often go by the moniker "band theorists", have been much more successful in applying (usually other people's) ideas in such circumstances. In this essay I give some examples, largely drawn from my own experience, and speculate on their meaning.

cond-mat.supr-con↗

arXiv2019

Normal state $^{17}$O NMR studies of Sr$_{2}$RuO$_{4}$ under uniaxial stress

The effects of uniaxial compressive stress on the normal state $^{17}$O nuclear magnetic resonance properties of the unconventional superconductor Sr$_{2}$RuO$_{4}$ are reported. The paramagnetic shifts of both planar and apical oxygen sites show pronounced anomalies near the nominal $\mathbf{a}$-axis strain $\varepsilon_{aa}\equiv\varepsilon_v$, that maximizes the superconducting transition temperature, $T_{c}$. The spin susceptibility weakly increases on lowering the temperature below $T$$\simeq$10 K, consistent with an enhanced density of states associated with passing the Fermi energy through a van Hove singularity. Although such a Lifshitz transition occurs in the $γ$ band, formed by the Ru $d_{xy}$ states hybridized with in-plane O $p_π$ orbitals, the large Hund's coupling renormalizes the uniform spin susceptibilty, which, in turn, affects the hyperfine fields of all nuclei. We estimate this \textquotedblleft Stoner\textquotedblright\ renormalization, $S,$ by combining the data with first-principles calculations and conclude that this is an important part of the strain effect, with implications for superconductivity.

cond-mat.str-el↗

arXiv2019

Novel Fe-based superconductor LaFe2As2 in comparison with traditional pnictides

The recently discovered Fe-based superconductor (FeBS) LaFe2As2 seems to break away from an established pattern that doping FeBS beyond 0.2e/Fe destroys superconductivity. LaFe2As2 has an apparent doping of 0.5e, yet superconducts at 12.1 K. Its Fermi surface bears no visual resemblance with the canonical FeBS Fermiology. It also exhibits two phases, none magnetic and only one superconducting. We show that the difference between them has nonetheless magnetic origin, the one featuring disordered moments, and the other locally nonmagnetic. We find that La there assumes an unusual valence of +2.6 to +2.7, so that the effective doping is reduced to 0.30-0.35e. A closer look reveals the same key elements: hole Fermi surfaces near Gamma-Z and electron ones near the X-P lines, with the corresponding peak in susceptibility, and a strong tendency to stripe magnetism. The physics of LaFe2As2 is thus more similar to the FeBS paradigm than hitherto appreciated.

cond-mat.supr-con↗

arXiv2019

The role of defects in the metal-insulator transition in VO$_{2}$ and V$_2$O$_3$

The vanadates VO$_2$ and V$_2$O$_3$ are prototypical examples of strongly correlated materials that exhibit a metal-insulator transition. While the phase transitions in these materials have been studied extensively, there is a limited understanding of how the properties of these materials are affected by the presence of defects and doping. In this study we investigate the impact of native point defects in the form of Frenkel defects on the structural, magnetic and electronic properties of VO$_2$ and V$_2$O$_3$, using first-principles calculations. In VO$_2$ the vanadium Frenkel pairs lead to a non-trivial insulating state. The unpaired vanadium interstitial bonds to a single dimer, which leads to a trimer that has one singlet state and one localized single-electron $S=1/2$ state. The unpaired broken dimer created by the vanadium vacancy also has a localized $S=1/2$ state. Thus, the insulating state is created by the singlet dimers, the trimer and the two localized $S=1/2$ states. Oxygen Frenkel pairs, on the other hand, lead to a metallic state in VO$_2$, but are expected to be present in much lower concentrations. In contrast, the Frenkel defects in V$_2$O$_3$ do not directly suppress the insulating character of the material. However, the disorder created by defects in V$_2$O$_3$ alters the local magnetic moments and in turn reduces the energy cost of a transition between the insulating and conducting phases of the material. We also find self-trapped small polarons in V$_2$O$_3$, which has implications for transport properties in the insulating phase.

cond-mat.mtrl-sci↗

arXiv2018

Spin fluctuations in Sr2RuO4 from polarized neutron scattering: implications for superconductivity

Triplet pairing in Sr2RuO4 was initially suggested based on the hypothesis of strong ferromagnetic spin fluctuations. Using polarized inelastic neutron scattering, we accurately determine the full spectrum of spin fluctuations in Sr2RuO4. Besides the well-studied incommensurate magnetic fluctuations we do find a sizeable quasiferromagnetic signal, quantitatively consistent with all macroscopic and microscopic probes. We use this result to address the possibility of magnetically-driven triplet superconductivity in Sr2RuO4. We conclude that, even though the quasiferromagnetic signal is stronger and sharper than previously anticipated, spin fluctuations alone are not enough to generate a triplet state strengthening the need for additional interactions or an alternative pairing scenario.

cond-mat.supr-con↗