arXiv subjects

K. Das

Publications and source records attributed to K. Das.

At least 19 recordsLinked to original sources

arXiv2025

Ferromagnetic Resonance in a Magnetically Dilute Percolating Ferromagnet: An Experimental and Theoretical Study

Ferromagnetic resonance (FMR) serves as a powerful probe of magnetization dynamics and anisotropy in percolating ferromagnets, where short-range interactions govern long-range magnetic order. We apply this approach to Ga$_{1-x}$Mn$_x$N ($x \simeq 8$\%), a dilute ferromagnetic semiconductor, combining FMR and superconducting quantum interference device magnetometry. Our results confirm the percolative nature of ferromagnetism in (Ga,Mn)N, with a Curie temperature $T_{\mathrm{C}} = 12$ K, and reveal that despite magnetic dilution, key features of conventional ferromagnets are retained. FMR measurements establish a robust uniaxial anisotropy, dictated by Mn$^{3+}$ single-ion anisotropy, with an easy-plane character at low Mn content. While excessive line broadening suppresses FMR signals below 9 K, they persist up to 70~K, indicating the presence of non-percolating ferromagnetic clusters well above $T_{\mathrm{C}}$. The temperature dependence of the FMR intensity follows that of the magnetization, underscoring the stability of these clusters. We quantitatively describe both FMR and SQUID observables using atomistic spin model operating on a common set of parameters. The level of agreement, achieved without tuning parameters between datasets, demonstrates the robustness and practical applicability of the approach in capturing the essential physics of spin-diluted, percolating ferromagnets. This study advances the understanding of percolating ferromagnetic systems, demonstrating that FMR is a key technique for probing their unique dynamic and anisotropic properties. Our findings contribute to the broader exploration of dilute ferromagnets and provide new insights into percolating ferromagnetic systems, which will be relevant for spintronic opportunities.

cond-mat.mtrl-sci↗

arXiv2024

Pair anisotropy in disordered magnetic systems

Accurate modelling of magnetism is pivotal for elucidating the microscopic origins of magnetic phenomena in functional materials. However, for a specified class of materials, such as random dilute ferromagnets or alloys, the reliance on simplifying assumptions, such as single-ion anisotropy, limits the accuracy of existing spin models. In such systems, there is a significant probability of the formation of nearest-neighbor magnetic ion pairs or higher order clusters, whose presence breaks the local symmetry of otherwise isolated magnetic species. Here, we introduce the concept of pair-induced uniaxial anisotropy and demonstrate how nearby atoms influence each other's anisotropic behavior. This effect is investigated in the dilute magnetic semiconductor Ga$_{1-x}$Mn$_x$N, by means of density functional theory calculations. The inclusion of pair anisotropy in the atomistic spin simulations significantly improves the agreement between simulated and experimental magnetization curves, in contrast to models that consider only single-ion anisotropy.

cond-mat.mtrl-sci↗

arXiv2024

Jet modification via $\pi^0$-hadron correlations in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

High-momentum two-particle correlations are a useful tool for studying jet-quenching effects in the quark-gluon plasma. Angular correlations between neutral-pion triggers and charged hadrons with transverse momenta in the range 4--12~GeV/$c$ and 0.5--7~GeV/$c$, respectively, have been measured by the PHENIX experiment in 2014 for Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$~GeV. Suppression is observed in the yield of high-momentum jet fragments opposite the trigger particle, which indicates jet suppression stemming from in-medium partonic energy loss, while enhancement is observed for low-momentum particles. The ratio and differences between the yield in Au$+$Au collisions and $p$$+$$p$ collisions, $I_{AA}$ and $\Delta_{AA}$, as a function of the trigger-hadron azimuthal separation, $\Delta\phi$, are measured for the first time at the Relativistic Heavy Ion Collider. These results better quantify how the yield of low-$p_T$ associated hadrons is enhanced at wide angle, which is crucial for studying energy loss as well as medium-response effects.

nucl-ex↗

arXiv2021

Systematic study of nuclear effects in $p$$+$Al, $p$$+$Au, $d$$+$Au, and $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV using $\pi^0$ production

The PHENIX collaboration presents a systematic study of $\pi^0$ production from $p$$+$$p$, $p$$+$Al, $p$$+$Au, $d$$+$Au, and $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Measurements were performed with different centrality selections as well as the total inelastic, 0%--100%, selection for all collision systems. For 0%--100% collisions, the nuclear modification factors, $R_{xA}$, are consistent with unity for $p_T$ above 8 GeV/$c$, but exhibit an enhancement in peripheral collisions and a suppression in central collisions. The enhancement and suppression characteristics are similar for all systems for the same centrality class. It is shown that for high-$p_T$-$\pi^0$ production, the nucleons in the $d$ and $^3$He interact mostly independently with the Au nucleus and that the counter intuitive centrality dependence is likely due to a physical correlation between multiplicity and the presence of a hard scattering process. These observations disfavor models where parton energy loss has a significant contribution to nuclear modifications in small systems. Nuclear modifications at lower $p_T$ resemble the Cronin effect -- an increase followed by a peak in central or inelastic collisions and a plateau in peripheral collisions. The peak height has a characteristic ordering by system size as $p$$+$Au $>$ $d$$+$Au $>$ $^{3}$He$+$Au $>$ $p$$+$Al. For collisions with Au ions, current calculations based on initial state cold nuclear matter effects result in the opposite order, suggesting the presence of other contributions to nuclear modifications, in particular at lower $p_T$.

nucl-ex↗

arXiv2020

Dynamical stability, Vibrational and optical properties of anti-perovskite A3BX (Ti3TlN, Ni3SnN and Co3AlC) phases: a first principles study

We have investigated various physical properties including phonon dispersion, thermodynamic parameters, optical constants, Fermi surface, Mulliken bond population, theoretical Vickers hardness and damage tolerance of anti-perovskite A3BX phases for the first time by employing density functional theory (DFT) methodology based on first principles method. Initially we assessed nine A3BX phases in total and found that only three phases (Ti3TlN, Ni3SnN and Co3AlC) are mechanically and dynamically stable based on analysis of computed elastic constants and phonon dispersion along with phonon density of states. We revisited the structural, elastic and electronic properties of the compounds to judge the reliability of our calculations. Absence of band gap at the Fermi level characterizes the phases under consideration as metallic in nature. The values of Pugh ratio, Poisson ratio and Cauchy factor have predicted the ductile nature associated with strong metallic bonding in these compounds. High temperature feasibility study of the phases has also been performed using the thermodynamic properties, such as the free energy, enthalpy, entropy, heat capacity and Debye temperature. The Vickers hardness of the compounds are estimated to be around 4 GPa which is comparable to many well-known MAX phases, indicating their reasonable hardness and easily machinable nature. The static refractive index n(zero) has been found around 8.0 for the phases under study that appeals as potential candidate to design optoelectronics appliances. The reflectivity is found above 44 percent for the Ti3TlN compound in the energy range of 0 to 14.8 eV demonstrating that this material holds significant promise as a coating agent to avoid solar heating.

cond-mat.mtrl-sci↗

arXiv2020

Measurement of jet-medium interactions via direct photon-hadron correlations in Au$+$Au and $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We present direct photon-hadron correlations in 200 GeV/A Au$+$Au, $d$$+$Au and $p$$+$$p$ collisions, for direct photon $p_T$ from 5--12 GeV/$c$, collected by the PHENIX Collaboration in the years from 2006 to 2011. We observe no significant modification of jet fragmentation in $d$$+$Au collisions, indicating that cold nuclear matter effects are small or absent. Hadrons carrying a large fraction of the quark's momentum are suppressed in Au$+$Au compared to $p$$+$$p$ and $d$$+$Au. As the momentum fraction decreases, the yield of hadrons in Au$+$Au increases to an excess over the yield in $p$$+$$p$ collisions. The excess is at large angles and at low hadron $p_T$ and is most pronounced for hadrons associated with lower momentum direct photons. Comparison to theoretical calculations suggests that the hadron excess arises from medium response to energy deposited by jets.

hep-ex↗

arXiv2019

A Cryogenic Interface for Controlling Many Qubits

A scaled-up quantum computer will require a highly efficient control interface that autonomously manipulates and reads out large numbers of qubits, which for solid-state implementations are usually held at millikelvin (mK) temperatures. Advanced CMOS technology, tightly integrated with the quantum system, would be ideal for implementing such a control interface but is generally discounted on the basis of its power dissipation that leads to heating of the fragile qubits. Here, we demonstrate an ultra low power, CMOS-based quantum control platform that takes digital commands as input and generates many parallel qubit control signals. Realized using 100,000 transistors operating near 100 mK, our platform alleviates the need for separate control lines to every qubit by exploiting the low leakage of transistors at cryogenic temperatures to store charge on floating gate structures that are used to tune-up quantum devices. This charge can then be rapidly shuffled between on-chip capacitors to generate the fast voltage pulses required for dynamic qubit control. We benchmark this architecture on a quantum dot test device, showing that the control of thousands of gate electrodes is feasible within the cooling power of commercially available dilution refrigerators.

quant-ph↗

arXiv2019

Characterising Quantum Devices at Scale with Custom Cryo-CMOS

We make use of a custom-designed cryo-CMOS multiplexer (MUX) to enable multiple quantum devices to be characterized in a single cool-down of a dilution refrigerator. Combined with a packaging approach that integrates cryo-CMOS chips and a hot-swappable, parallel device test platform, we describe how this setup takes a standard wiring configuration as input and expands the capability for batch-characterization of quantum devices at milli-Kelvin temperatures and high magnetic fields. The architecture of the cryo-CMOS multiplexer is discussed and performance benchmarked using few-electron quantum dots and Hall mobility-mapping measurements.

physics.app-ph↗

arXiv2018

Low-momentum direct photon measurement in Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We have measured direct photons for $p_T<5~$GeV/$c$ in minimum bias and 0\%--40\% most central events at midrapidity for Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The $e^{+}e^{-}$ contribution from quasi-real direct virtual photons has been determined as an excess over the known hadronic contributions in the $e^{+}e^{-}$ mass distribution. A clear enhancement of photons over the binary scaled $p$$+$$p$ fit is observed for $p_T<4$ GeV/$c$ in Cu$+$Cu data. The $p_T$ spectra are consistent with the Au$+$Au data covering a similar number of participants. The inverse slopes of the exponential fits to the excess after subtraction of the $p$$+$$p$ baseline are 285$\pm$53(stat)$\pm$57(syst)~MeV/$c$ and 333$\pm$72(stat)$\pm$45(syst)~MeV/$c$ for minimum bias and 0\%--40\% most central events, respectively. The rapidity density, $dN/dy$, of photons demonstrates the same power law as a function of $dN_{\rm ch}/d\eta$ observed in Au$+$Au at the same collision energy.

hep-ex↗

arXiv2018

Measurement of two-particle correlations with respect to second- and third-order event planes in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We present measurements of azimuthal correlations of charged hadron pairs in $\sqrt{s_{_{NN}}}=200$ GeV Au$+$Au collisions for the trigger and associated particle transverse-momentum ranges of $1 4$~GeV/$c$) correlations is suppressed compared to that of correlations measured in $p$$+$$p$ collisions. At the lowest associated particle $p_T$ ($0.5<p_T^a<1$ GeV/$c$), the away-side shape and yield are modified relative to those in $p$$+$$p$ collisions. These observations are consistent with the scenario of radiative-jet energy loss. For the low-$p_T$ trigger correlations ($2<p_T^t<4$ GeV/$c$), a finite away-side yield exists and we explore the dependence of the shape of the away-side within the context of an underlying-event model. Correlations are also studied differentially versus event-plane angle $\Psi_2$ and $\Psi_3$. The angular correlations show an asymmetry when selecting the sign of the difference between the trigger-particle azimuthal angle and the $\Psi_2$ event plane. This asymmetry and the measured suppression of the pair yield out of plane is consistent with a path-length-dependent energy loss. No $\Psi_3$ dependence can be resolved within experimental uncertainties.

hep-ex↗

arXiv2017

Measurement of emission angle anisotropy via long-range angular correlations with high $p_T$ hadrons in $d$$+$Au and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We present measurements of two-particle angular correlations between high-transverse-momentum ($2<p_T<11$ GeV/$c$) $\pi^0$ observed at midrapidity ($|\eta|<0.35$) and particles produced either at forward ($3.1<\eta<3.9$) or backward ($-3.7<\eta<-3.1$) rapidity in $d$$+$Au and $p$$+$$p$ collisions at $\sqrt{s_{_{NN}}}=200$ GeV. The azimuthal angle correlations for particle pairs with this large rapidity gap in the Au-going direction exhibit a ridge-like structure that persists up to $p_T{\approx}6$ GeV/$c$ and which strongly depends on collision centrality, which is a similar characteristic to the hydrodynamical particle flow in A+A collisions. The ridge-like structure is absent in the $d$-going direction as well as in $p$$+$$p$ collisions, in the transverse-momentum range studied. The results indicate that the ridge-like structure is shifted in the Au-going direction toward more central collisions, similar to the charged-particle pseudorapidity distributions.

hep-ex↗

arXiv2017

Measurements of $e^+e^-$ pairs from open heavy flavor in $p$+$p$ and $d$+$A$ collisions at $\sqrt{s_{NN}}=200$ GeV

We report a measurement of $e^+e^-$ pairs from semileptonic heavy-flavor decays in $p$+$p$ collisions at $\sqrt{s_{NN}}=200$~GeV. The $e^+e^-$ pair yield from $b\bar{b}$ and $c\bar{c}$ is separated by exploiting a double differential fit done simultaneously in dielectron invariant mass and $p_T$. We used three different event generators, {\sc pythia}, {\sc mc@nlo}, and {\sc powheg}, to simulate the $e^+e^-$ spectra from $c\bar{c}$ and $b\bar{b}$ production. The data can be well described by all three generators within the detector acceptance. However, when using the generators to extrapolate to $4\pi$, significant differences are observed for the total cross section. These difference are less pronounced for $b\bar{b}$ than for $c\bar{c}$. The same model dependence was observed in already published $d$+$A$ data. The $p$+$p$ data are also directly compared with $d$+$A$ data in mass and $p_T$, and within the statistical accuracy no nuclear modification is seen.

nucl-ex↗

arXiv2015

Azimuthally anisotropic emission of low-momentum direct photons in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

The PHENIX experiment at the Relativistic Heavy Ion Collider has measured 2nd and 3rd order Fourier coefficients of the azimuthal distributions of direct photons emitted at midrapidity in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV for various collision centralities. Combining two different analysis techniques, results were obtained in the transverse momentum range of $0.4<p_{T}<4.0$ GeV/$c$. At low $p_T$ the second-order coefficients, $v_2$, are similar to the ones observed in hadrons. Third order coefficients, $v_3$, are nonzero and almost independent of centrality. These new results on $v_2$ and $v_3$, combined with previously published results on yields, are compared to model calculations that provide yields and asymmetries in the same framework. Those models are challenged to explain simultaneously the observed large yield and large azimuthal anisotropies.

nucl-ex↗

arXiv2015

Scaling properties of fractional momentum loss of high-pT hadrons in nucleus-nucleus collisions at $\sqrt{s_{_{NN}}}$ from 62.4 GeV to 2.76 TeV

Measurements of the fractional momentum loss ($S_{\rm loss}\equiv{\delta}p_T/p_T$) of high-transverse-momentum-identified hadrons in heavy ion collisions are presented. Using $\pi^0$ in Au$+$Au and Cu$+$Cu collisions at $\sqrt{s_{_{NN}}}=62.4$ and 200 GeV measured by the PHENIX experiment at the Relativistic Heavy Ion Collider and and charged hadrons in Pb$+$Pb collisions measured by the ALICE experiment at the Large Hadron Collider, we studied the scaling properties of $S_{\rm loss}$ as a function of a number of variables: the number of participants, $N_{\rm part}$, the number of quark participants, $N_{\rm qp}$, the charged-particle density, $dN_{\rm ch}/d\eta$, and the Bjorken energy density times the equilibration time, $\varepsilon_{\rm Bj}\tau_{0}$. We find that the $p_T$ where $S_{\rm loss}$ has its maximum, varies both with centrality and collision energy. Above the maximum, $S_{\rm loss}$ tends to follow a power-law function with all four scaling variables. The data at $\sqrt{s_{_{NN}}}$=200 GeV and 2.76 TeV, for sufficiently high particle densities, have a common scaling of $S_{\rm loss}$ with $dN_{\rm ch}/d\eta$ and $\varepsilon_{\rm Bj}\tau_{0}$, lending insight on the physics of parton energy loss.

nucl-ex↗

arXiv2015

Centrality-dependent modification of jet-production rates in deuteron-gold collisions at $\sqrt{s_{NN}}$=200 GeV

Jet production rates are measured in $p$$+$$p$ and $d$$+$Au collisions at $\sqrt{s_{NN}}$=200 GeV recorded in 2008 with the PHENIX detector at the Relativistic Heavy Ion Collider. Jets are reconstructed using the $R=0.3$ anti-$k_{t}$ algorithm from energy deposits in the electromagnetic calorimeter and charged tracks in multi-wire proportional chambers, and the jet transverse momentum ($p_T$) spectra are corrected for the detector response. Spectra are reported for jets with $12<p_T<50$ GeV/$c$, within a pseudorapidity acceptance of $\left|\eta\right|<0.3$. The nuclear-modification factor ($R_{d{\rm Au}}$) values for 0\%--100\% $d$$+$Au events are found to be consistent with unity, constraining the role of initial state effects on jet production. However, the centrality-selected $R_{d{\rm Au}}$ values and central-to-peripheral ratios ($R_{\rm CP}$) show large, $p_T$-dependent deviations from unity, which challenge the conventional models that relate hard-process rates and soft-particle production in collisions involving nuclei.

nucl-ex↗

arXiv2015

Measurements of elliptic and triangular flow in high-multiplicity $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

We present the first measurement of elliptic ($v_2$) and triangular ($v_3$) flow in high-multiplicity $^{3}$He$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV. Two-particle correlations, where the particles have a large separation in pseudorapidity, are compared in $^{3}$He$+$Au and in $p$$+$$p$ collisions and indicate that collective effects dominate the second and third Fourier components for the correlations observed in the $^{3}$He$+$Au system. The collective behavior is quantified in terms of elliptic $v_2$ and triangular $v_3$ anisotropy coefficients measured with respect to their corresponding event planes. The $v_2$ values are comparable to those previously measured in $d$$+$Au collisions at the same nucleon-nucleon center-of-mass energy. Comparison with various theoretical predictions are made, including to models where the hot spots created by the impact of the three $^{3}$He nucleons on the Au nucleus expand hydrodynamically to generate the triangular flow. The agreement of these models with data may indicate the formation of low-viscosity quark-gluon plasma even in these small collision systems.

nucl-ex↗

arXiv2015

$\phi$ meson production in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

The PHENIX experiment has measured $\phi$ meson production in $d$$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV using the dimuon and dielectron decay channels. The $\phi$ meson is measured in the forward (backward) $d$-going (Au-going) direction, $1.2<y<2.2$ ($-2.2<y<-1.2$) in the transverse-momentum ($p_T$) range from 1--7 GeV/$c$, and at midrapidity $|y|<0.35$ in the $p_T$ range below 7 GeV/$c$. The $\phi$ meson invariant yields and nuclear-modification factors as a function of $p_T$, rapidity, and centrality are reported. An enhancement of $\phi$ meson production is observed in the Au-going direction, while suppression is seen in the $d$-going direction, and no modification is observed at midrapidity relative to the yield in $p$$+$$p$ collisions scaled by the number of binary collisions. Similar behavior was previously observed for inclusive charged hadrons and open heavy flavor indicating similar cold-nuclear-matter effects.

nucl-ex↗

arXiv2015

Measurement of higher cumulants of net-charge multiplicity distributions in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=7.7-200$ GeV

We report the measurement of cumulants ($C_n, n=1\ldots4$) of the net-charge distributions measured within pseudorapidity ($|\eta|<0.35$) in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=7.7-200$ GeV with the PHENIX experiment at the Relativistic Heavy Ion Collider. The ratios of cumulants (e.g. $C_1/C_2$, $C_3/C_1$) of the net-charge distributions, which can be related to volume independent susceptibility ratios, are studied as a function of centrality and energy. These quantities are important to understand the quantum-chromodynamics phase diagram and possible existence of a critical end point. The measured values are very well described by expectation from negative binomial distributions. We do not observe any nonmonotonic behavior in the ratios of the cumulants as a function of collision energy. The measured values of $C_1/C_2 = \mu/\sigma^2$ and $C_3/C_1 = S\sigma^3/\mu$ can be directly compared to lattice quantum-chromodynamics calculations and thus allow extraction of both the chemical freeze-out temperature and the baryon chemical potential at each center-of-mass energy.

nucl-ex↗